《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Effect of loading rate on the monotonic tensile behavior and tensile.pdf_大学文库

G Model ARTICLE IN PRESS Materials Science and Engineering A xxx(2008)xXX-XXX Contents lists available at Science Direct Materials Science and Engineering A ELSEVIER journalhomepagewww.elsevier.com/locate/msea Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide-oxide ceramic composite at 1200C* M.B. Ruggles-Wrenna, A T. Radzickia, S.S. Baek, K.A. Keller, 1 Technology, Wright-Patterson Air Force b Agency for Defense Development, Daejeon, Republic of Korea F UES, Inc, 4401 Dayton Xenia Road, Dayton, OH 45433-7817, USA ARTICLE INFO ABSTRACT The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide-oxide eceived 29 November 2007 ceramic composite was evaluated in laboratory air at composite consists of a porous alumina atrix reinforced with woven mullite/alu fibers. has no interface between the fiber vailable online xxx nd matrix, and relies on the porous matrix for flaw Tensile tests conducted at loading rates of 0.0025 and 25 MPa/ s revealed a strong effect of rate on the stress-strain behavior as well ultimate tensile strength (UTS). elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress Ceramic-matrix composites(CMCs) results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses 25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress-strain behavior was an artifact of incomplete processing of fibers Creep in the as-received material. The unusual material response in the 0-30 MPa stress range was further High-temperature properties investigated in creep tests conducted with the applied stresses <26 MPa. Negative creep(ie decrea in strain under constant stress)was observed. Porosity measurements indicate that a decrease in matrix orosity and matrix densification may be taking place in the n720/A composite exposed to 1200C at Published by Elsevier B V. 1. Introduction these applications require exposure to oxidizing environments, the thermodynamic stability and oxidation resistance of CMCs are vital Advances in power generation systems for aircraft engines, land- issues. based turbines, rockets, and, most recently, hypersonic missiles The main advantage of CMCs over monolithic ceramics is their and flight vehicles have raised the demand for structural materials superior toughness, tolerance to the presence of cracks and defects. that have superior long-term mechanical properties and retained and non-catastrophic mode of failure. It is widely accepted that in properties under high temperature, high pressure, and varying order to avoid brittle fracture behavior in CMCs and improve the environmental factors, such as moisture [1. Ceramic-matrix com- damage tolerance a weak fiber/matrix interface is needed, which posites (CMCs), capable of maintaining excellent strength and serves to deflect matrix cracks and to allow subsequent fiber pu fracture toughness at high temperatures are prime candidate mate- out [3-6]. Historically, following the development of Sic fibers, rials for such applications. Additionally, the lower densities of fiber coatings such as C or bn have been employed to promot CMCs and their higher use temperatures, together with a reduced the desired composite behavior. However, the non-oxide fiber/non- need for cooling air, allow for improved high-temperature perfor- oxide matrix composites generally show poor oxidation resistance mance when compared to conventional nickel-based superalloys 7, 8, particularly at intermediate temperatures(-800"C). These [2]. Advanced reusable space launch vehicles will likely incorporate systems are susceptible to embrittlement due to oxygen entering fiber-reinforced CMCs in critical propulsion components. Because through the matrix cracks and then reacting with the interphase and the fibers[9-12]. The degradation, which involves oxidation of fibers and fiber coatings, is typically accelerated by the presence of moisture[ 13-19]. Using oxide fiber/non-oxide matrix or non-oxide The views expressed are those of the authors and do not reflect the official pol- fiber/oxide matrix composites generally does not substantially Defense or the us improve the high-temperature oxidation resistance[20]. The need Corresponding author. Tel :+1 937 255 3636x 4641: fax: +1 937 6567053. for environmentally stable composites motivated the develop- E-mail address: marina. ruggles-wrenneafitedu(M.B. Ruggles-wrer ment of CMCs based on environmentally stable oxide constituents Under USAF Contract #F33615-01-C-5214 121-29 More recently it has been demonstrated that similar crack 0921-5093/S-see front matter. Published by Elsevier B.V. doi:10.1016msea2008.03.006 Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A(2008). doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 Materials Science and Engineering A xxx (2008) xxx–xxx Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 ◦C M.B. Ruggles-Wrenna,∗, A.T. Radzicki a, S.S. Baek b, K.A. Keller c,1 a Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, USA b Agency for Defense Development, Daejeon, Republic of Korea c UES, Inc., 4401 Dayton Xenia Road, Dayton, OH 45433-7817, USA article info Article history: Received 29 November 2007 Received in revised form 29 February 2008 Accepted 5 March 2008 Available online xxx Keywords: Ceramic-matrix composites (CMCs) Oxides Fibers Creep High-temperature properties Mechanical properties abstract The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 ◦C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (NextelTM720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses ∼25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 ◦C at stresses <30 MPa for prolonged periods of time. Published by Elsevier B.V. 1. Introduction Advances in power generation systems for aircraft engines, landbased turbines, rockets, and, most recently, hypersonic missiles and flight vehicles have raised the demand for structural materials that have superior long-term mechanical properties and retained properties under high temperature, high pressure, and varying environmental factors, such as moisture [1]. Ceramic-matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high temperatures are prime candidate materials for such applications. Additionally, the lower densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-temperature performance when compared to conventional nickel-based superalloys [2]. Advanced reusable space launch vehicles will likely incorporate fiber-reinforced CMCs in critical propulsion components. Because The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the U.S. Government. ∗ Corresponding author. Tel.: +1 937 255 3636x4641; fax: +1 937 656 7053. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles-Wrenn). 1 Under USAF Contract # F33615-01-C-5214. these applications require exposure to oxidizing environments, the thermodynamic stability and oxidation resistance of CMCs are vital issues. The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and defects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pullout [3–6]. Historically, following the development of SiC fibers, fiber coatings such as C or BN have been employed to promote the desired composite behavior. However, the non-oxide fiber/nonoxide matrix composites generally show poor oxidation resistance [7,8], particularly at intermediate temperatures (∼800 ◦C). These systems are susceptible to embrittlement due to oxygen entering through the matrix cracks and then reacting with the interphase and the fibers [9–12]. The degradation, which involves oxidation of fibers and fiber coatings, is typically accelerated by the presence of moisture [13–19]. Using oxide fiber/non-oxide matrix or non-oxide fiber/oxide matrix composites generally does not substantially improve the high-temperature oxidation resistance [20]. The need for environmentally stable composites motivated the development of CMCs based on environmentally stable oxide constituents [21–29]. More recently it has been demonstrated that similar crack- 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2008.03.006

G Model MSA-24026: No of Pages 7 ARTICLE IN PRESS M.B. Ruggles-Wrenn et al Materials Science and Engineering A xxx(2008)xxx-Xxx deflecting behavior can also be achieved by means of a finely introduction of a short hold period at the maximum stress int distributed porosity in the matrix instead of a separate interface the fatigue cycle significantly degraded the fatigue performance between matrix and fibers 30. This concept has been success- of N720/A composite at 1200C in air. Ruggles-Wrenn et al.[43 fully demonstrated for oxide-oxide composites [21, 25, 29 31-33]. reported that the loading frequency had a strong effect on fatigue Resulting oxide/oxide CMcs exhibit damage tolerance combined rformance of n720 A composite at 1200C in steam. In addition, with inherent oxidation resistance. However, due to the strong it was shown that slow crack growth due to stress corrosion was bonding between the fiber and matrix, a minimum matrix porosity the governing failure mechanism in steam. These results suggest is needed for this concept to work [34 An extensive review of the that the loading rate may have a significant effect on mechanical mechanisms and mechanical properties of matrix CMCs is performance and durability of the N720/A CMC at elevated temper given in 35]. ature In many potential applications, CMCs will be subject to loading he objective of this study is to investigate the effects of load- ler a wide range of rates. Several studies examined the effect of ing rate on tensile stress-strain behavior and tensile properties of ing rate on tensile behavior and properties of CMCs. Sorensen N720/A, an oxide -oxide Cmc, at 1200C in laboratory air. Results nd Holmes [36 tested Nicalon Sic fiber-reinforced CMC over a reveal that the loading rate has a marked effect on monotonic wide range of loading rates at room temperature and reported that tensile behavior and on ultimate tensile strength, elastic modulus le monotonic stress-strain behavior as well as the microstructural and failure strain. The composite microstructure, changes in matrix damage was strongly dependent on the loading rate. These phe- porosity, as well as damage and failure mechanisms are discussed nomena were attributed partly to time-dependent matrix cracking (due to stress corrosion) and partly to the increase in interfacial 2. Material and experimental arrangements shear stress with increasing loading rate. More recently, Choi et al. [37-39] have shown that at elevated temperatures the ultimate The material studied was Nextel M720/Alumina(N720/A), an tensile strength and shear strength of several Nicalon Sic fiber- oxide-oxide CMC(manufactured by COl Ceramics, San Diego, CA) reinforced ceramic matrix composites are profoundly influenced consisting of a porous alumina matrix reinforced with Nextel M720 by the loading rate. The ultimate tensile strength of all composites fibers. There is no fiber coating. The damage tolerance of N720JA ivestigated decreased with decreasing loading rate. Likewise the is enabled by the porous matrix. The composite was supplied in shear strength degraded significantly as the loading rate decreased. a form of 2.8-mm thick plates comprised of 12 0/90 woven lay- Choi et al. suggested that the overall macroscopic failure mecha- ers, with a density of -2. g/cm, a fiber volume fraction of 0.45 nism was analogous to slow crack growth commonly observed in (or 45 volume% fibers), and porosity of -22%. The fiber fabric was advanced monolithic ceramics and was governed by a power-law infiltrated with the matrix in a sol-gel process. The laminate was type of damage evolution/ accumulation. dried with a"vacuum bag "technique under low pressure and low Porous-matrix oxide/oxide CMCs exhibit several behavior temperature and then pressureless sintered [44]. Representative trends that are distinctly different from those exhibited by tra- micrograph of the untested material is presented in Fig. 1(a).which ditional non-oxide CMCs with a fiber-matrix interface. For most shows 0o and 90% fiber tows as well as numerous matrix cracksIn lon-oxide CMCs, fatigue is significantly more damaging than creep. the case of the as-processed material, most are shrinkage cracks Zawada et al. [40] examined the high-temperature mechanical formed during processing rather than matrix cracks generated dur- behavior of a porous matrix Nextel610 Aluminosilicate composite ng loading Porous nature of the matrix is seen in Fig. 1(b). Results revealed excellent fatigue performance at 1000C.Con- A servocontrolled MTS mechanical testing machine equipped ersely, creep lives were short, indicating low creep resistance with hydraulic water-cooled collet grips, a compact two-zone and limiting the use of that CMC to temperatures below 1000 C. resistance-heated furnace and two temperature controllers was Ruggles-Wrenn et al. [41] showed that Nextel M720/Alumina used in all tests. An MTS TestStar ll digital controller was employed (N720/A)composite exhibits excellent fatigue resistance in labora- for input signal generation and data acquisition. Strain measure- it 1200C. The fatigue limit(based on a run-out condition ment was accomplished with an MTS high-temperature air-cooled of 10 cycles)was 170 MPa(88% UTS at 1200 C). Furthermore, the uniaxial extensometer. For elevated temperature testing, thermo- composite retained 100% of its tensile strength. However, creep couples were bonded to test specimens to calibrate the furnace oading was found to be considerably more damaging Creep run- on a periodic basis. The furnace controllers(using non-contacting out(defined as 100 h at creep stress )was achieved only at stress thermocouples exposed to the ambient environment near the test levels below 50% UTS. Mehrman et al. [42] demonstrated that specimen)were adjusted to determine the power setting needed 200pm Fig. 1. As-received material: (a)overview:(b) porous nature of the matrix is evident. Please cite this article in press as: M B. Ruggles-Wrenn, et al, Mater. Sci Eng. A(2008), doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 2 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx deflecting behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate interface between matrix and fibers [30]. This concept has been successfully demonstrated for oxide–oxide composites [21,25,29,31–33]. Resulting oxide/oxide CMCs exhibit damage tolerance combined with inherent oxidation resistance. However, due to the strong bonding between the fiber and matrix, a minimum matrix porosity is needed for this concept to work [34]. An extensive review of the mechanisms and mechanical properties of porous-matrix CMCs is given in [35]. In many potential applications, CMCs will be subject to loading under a wide range of rates. Several studies examined the effect of loading rate on tensile behavior and properties of CMCs. Sorensen and Holmes [36] tested Nicalon SiC fiber-reinforced CMC over a wide range of loading rates at room temperature and reported that the monotonic stress–strain behavior as well as the microstructural damage was strongly dependent on the loading rate. These phenomena were attributed partly to time-dependent matrix cracking (due to stress corrosion) and partly to the increase in interfacial shear stress with increasing loading rate. More recently, Choi et al. [37–39] have shown that at elevated temperatures the ultimate tensile strength and shear strength of several Nicalon SiC fiberreinforced ceramic matrix composites are profoundly influenced by the loading rate. The ultimate tensile strength of all composites investigated decreased with decreasing loading rate. Likewise the shear strength degraded significantly as the loading rate decreased. Choi et al. suggested that the overall macroscopic failure mechanism was analogous to slow crack growth commonly observed in advanced monolithic ceramics and was governed by a power-law type of damage evolution/accumulation. Porous-matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by traditional non-oxide CMCs with a fiber–matrix interface. For most non-oxide CMCs, fatigue is significantly more damaging than creep. Zawada et al. [40] examined the high-temperature mechanical behavior of a porous matrix Nextel610/Aluminosilicate composite. Results revealed excellent fatigue performance at 1000 ◦C. Conversely, creep lives were short, indicating low creep resistance and limiting the use of that CMC to temperatures below 1000 ◦C. Ruggles-Wrenn et al. [41] showed that NextelTM720/Alumina (N720/A) composite exhibits excellent fatigue resistance in laboratory air at 1200 ◦C. The fatigue limit (based on a run-out condition of 105 cycles) was 170 MPa (88% UTS at 1200 ◦C). Furthermore, the composite retained 100% of its tensile strength. However, creep loading was found to be considerably more damaging. Creep runout (defined as 100 h at creep stress) was achieved only at stress levels below 50% UTS. Mehrman et al. [42] demonstrated that introduction of a short hold period at the maximum stress into the fatigue cycle significantly degraded the fatigue performance of N720/A composite at 1200 ◦C in air. Ruggles-Wrenn et al. [43] reported that the loading frequency had a strong effect on fatigue performance of N720/A composite at 1200 ◦C in steam. In addition, it was shown that slow crack growth due to stress corrosion was the governing failure mechanism in steam. These results suggest that the loading rate may have a significant effect on mechanical performance and durability of the N720/A CMC at elevated temperature. The objective of this study is to investigate the effects of loading rate on tensile stress–strain behavior and tensile properties of N720/A, an oxide–oxide CMC, at 1200 ◦C in laboratory air. Results reveal that the loading rate has a marked effect on monotonic tensile behavior and on ultimate tensile strength, elastic modulus and failure strain. The composite microstructure, changes in matrix porosity, as well as damage and failure mechanisms are discussed. 2. Material and experimental arrangements The material studied was NextelTM720/Alumina (N720/A), an oxide–oxide CMC (manufactured by COI Ceramics, San Diego, CA) consisting of a porous alumina matrix reinforced with NextelTM720 fibers. There is no fiber coating. The damage tolerance of N720/A is enabled by the porous matrix. The composite was supplied in a form of 2.8-mm thick plates comprised of 12 0◦/90◦ woven layers, with a density of ∼2.85 g/cm3, a fiber volume fraction of 0.45 (or 45 volume% fibers), and porosity of ∼22%. The fiber fabric was infiltrated with the matrix in a sol–gel process. The laminate was dried with a “vacuum bag” technique under low pressure and low temperature, and then pressureless sintered [44]. Representative micrograph of the untested material is presented in Fig. 1(a), which shows 0◦ and 90◦ fiber tows as well as numerous matrix cracks. In the case of the as-processed material, most are shrinkage cracks formed during processing rather than matrix cracks generated during loading. Porous nature of the matrix is seen in Fig. 1(b). A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet grips, a compact two-zone resistance-heated furnace, and two temperature controllers was used in all tests. An MTS TestStar II digital controller was employed for input signal generation and data acquisition. Strain measurement was accomplished with an MTS high-temperature air-cooled uniaxial extensometer. For elevated temperature testing, thermocouples were bonded to test specimens to calibrate the furnace on a periodic basis. The furnace controllers (using non-contacting thermocouples exposed to the ambient environment near the test specimen) were adjusted to determine the power setting needed Fig. 1. As-received material: (a) overview; (b) porous nature of the matrix is evident

G Model ARTICLE IN PRESS to achieve the desired temperature of the test specimen. The deter- lined power settings were then used in actual tests. Fracture surfaces of failed specimens were examined us optical microscope( Zeiss Discovery V12) Density and porosity of the as-processed composite as well as of the specimens subjected to prior thermo-mechanical loading were measured using a Micro- metrics Auto Pore Ill Mercury Porosimeter. The porosimeter was calibrated using a standard blank in accordance with the proce- dures in ASTM Standard D 4284-03 and ASTM Standard D 4404-84 0.0025MPa/s ( Reapproved 2004). Both low and high pressure runs were carried out. The average volume of material analyzed using porosimetry was 0.5 cm. In the case of the specimens with prior therme mechanical loading history, the measurements were performed on the material cut from the gage section of the test specimen. Prior to measurements, the samples were heat treated at 100 C for 20 min STRAIN(%) in order to remove any excess moisture that may have accumulated in the material. The pore volume and the pore volume distribution Fig. 2. Tensile stress-strain curves for N720/A ceramic vere determined using procedures in the aforementioned ASTM 1200 C with the stress rates of 25 and 0.0025 MPa/ s. The it on stress-strain behavior Standards 3. Test procedures from 0. 1 to 100 MPa/s. A linear stress-strain behavior was produced only at the fastest loading rate of 1000 MPa/s. These results indicate all mechanical tests were performed at 1200C in laboratory that at stress rates <1000 MPa/s the tensile behavior and tensile air. Each test specimen was heated to 1200 C in 25 min, and held strength ov The nonlinear stress-strain behavior of the N720/A pecimens of 152-mm total length with a 10-mm-wide gage sectio composite produced at 25 MPa/s in the present study likewise sug- were used in all tests. Monotonic tension tests were performed in gests that creep deformation and damage mechanisms affect the load control with the constant rates of 0.0025 and 25 MPa/s Creep- monotonic tensile behavior of this CMC at 1200oC. rupture tests were conducted in load control in accordance with As seen in Fig. 2, the tensile stress-strain behavior produced ASTM Standard C 1337. Specimens were loaded to the creep stress at the stress rate of 0.0025 MPa/s is fundamentally different from 25 MPa s. The stress-strain curves level at the rate of 25 MPa)s In each test, stress-strain data were obtained at 0.0025 MPa/s are markedly nonlinear.Furthermore,a creep period. Thus both total strain and creep strain could be calcu- non-monotonic change in strain is observed in all 0.0025 MPa/s tests As the stress increases the strain first decreases and then lated and examined. Creep run-out was defined as 100 h at a given begins to increase. The corresponding rate of change of strain is creep stress, which is consistent with the service life expected n first negative and then becomes positive. Such strain rate reversal etained strength and modulus, specimens that achieved run-out occurs as the stress reaches 20-25 MPa. as the stress continues to were subjected to tensile tests to failure at 25 MPa/s at 1200 C In rease, appreciable inelastic strains develop The failure strains some ca anging from 0.73 to 1.06%, are two to three times those obtained at ne specimen was tested per test condition. The authors 25 MPa/s. Conversely, the average UTS of 154 MPa is considerably recognize that this is a limited set of data. However, extreme care lower than the strength values obtained at 25 MPa/ s. was taken in generating the data. Selective duplicate tests have demonstrated the data to bevery repeatable. This exploratory effort at 25 and 0. 0025 MPa/s are shown in Fig 3(a)and(b),respectively serves to identify the behavioral trends and to determine whethe a more rigorous investigation should be undertaken. The fracture planes of both specimens are not well defined. The 0 fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The fibers in the 0 tows 4. Results and discussion in each cloth layer exhibit random failure producing brushy fracture surfaces. Note that the specimen tested at 0.0025 MPa/s has a con- 4.1. Monotonic tension-effect of loading rate siderably longer damage zone (8 mm) than the en tested at 25 MPa/s(5 mm), which accounts for larger strain accumulated The effect of loading rate on tensile stress-strain behavior of at the slower stress rate. the n720/A ceramic composite at 1200Cis typified in Fig. 2. At the The strong dependence of tensile strength on loading rate exhib- stress rate of 25 MPa/s, the average ultimate tensile strength was ited by this composite at 1200 C is similar to that observed at 181 MPa, the average elastic modulus, 70 GPa, and the average fail- elevated temperatures for advanced ceramics 38, 39, 49-51 as well ure strain,0.36%. These results are consistent with the data reported as for the Nextel M720 fibers [52, 47, 48. For composites with a glass earlier [41, 46]. The tensile stress-strain curves obtained at 25 MPa s or glass-ceramic matrix, the degradation of strength with decreas- appear to be linear to failure. Yet, upon closer examination it is seen ing loading rate is a consequence of slow crack growth process due at the stress-strain behavior becomes nonlinear as the stress to stress corrosion 36. However, in the case of oxide ceramics creep reaches 70 MPa, although the nonlinearity is not strongly pro- deformation can have a significant influence on the monotonic ten- nounced. Because of the inherent nature of an exceptionally weak sile behavior. Wilson et al. [52] suggested that the deformation porous matrix, much of the stress-strain behavior of the composite mechanism operat N720 fibers at elevated temperatures was is controlled by the fibers. Material exhibits typical fiber-dominated diffusion creep controlled by interface reaction. As the diffusion composite behavior. Milzet al [47 and Goering and Schneider 48 creep is a time-dependent process, the loading rate dependence studied mechanical behavior of Nextel M720 fibers at temperatures of the N720/A tensile behavior and tensile strength at 1200.C is in the 900-1200 Crange In tensile tests conducted at 1200 C non- readily explained. At slower loading rates, more time is allowed linear stress-strain behavior was observed at stress rates ranging for diffusion creep to develop, leading to extension of existing crit Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A(2008). doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx 3 to achieve the desired temperature of the test specimen. The determined power settings were then used in actual tests. Fracture surfaces of failed specimens were examined using an optical microscope (Zeiss Discovery V12). Density and porosity of the as-processed composite as well as of the specimens subjected to prior thermo-mechanical loading were measured using a Micrometrics Auto Pore III Mercury Porosimeter. The porosimeter was calibrated using a standard blank in accordance with the procedures in ASTM Standard D 4284-03 and ASTM Standard D 4404-84 (Reapproved 2004). Both low and high pressure runs were carried out. The average volume of material analyzed using porosimetry was 0.5 cm3. In the case of the specimens with prior thermomechanical loading history, the measurements were performed on the material cut from the gage section of the test specimen. Prior to measurements, the samples were heat treated at 100 ◦C for 20 min in order to remove any excess moisture that may have accumulated in the material. The pore volume and the pore volume distribution were determined using procedures in the aforementioned ASTM Standards. 3. Test procedures All mechanical tests were performed at 1200 ◦C in laboratory air. Each test specimen was heated to 1200 ◦C in 25 min, and held at 1200 ◦C for additional 15 min prior to testing. Dog bone shaped specimens of 152-mm total length with a 10-mm-wide gage section were used in all tests. Monotonic tension tests were performed in load control with the constant rates of 0.0025 and 25 MPa/s. Creeprupture tests were conducted in load control in accordance with ASTM Standard C 1337. Specimens were loaded to the creep stress level at the rate of 25 MPa/s. In each test, stress–strain data were recorded during the loading to the creep stress level and the actual creep period. Thus both total strain and creep strain could be calculated and examined. Creep run-out was defined as 100 h at a given creep stress, which is consistent with the service life expected in aerospace applications at that temperature [45]. To determine the retained strength and modulus, specimens that achieved run-out were subjected to tensile tests to failure at 25 MPa/s at 1200 ◦C. In some cases one specimen was tested per test condition. The authors recognize that this is a limited set of data. However, extreme care was taken in generating the data. Selective duplicate tests have demonstrated the data to be very repeatable. This exploratory effort serves to identify the behavioral trends and to determine whether a more rigorous investigation should be undertaken. 4. Results and discussion 4.1. Monotonic tension—effect of loading rate The effect of loading rate on tensile stress–strain behavior of the N720/A ceramic composite at 1200 ◦C is typified in Fig. 2. At the stress rate of 25 MPa/s, the average ultimate tensile strength was 181 MPa, the average elastic modulus, 70 GPa, and the average failure strain, 0.36%. These results are consistent with the data reported earlier [41,46]. The tensile stress–strain curves obtained at 25 MPa/s appear to be linear to failure. Yet, upon closer examination it is seen that the stress–strain behavior becomes nonlinear as the stress reaches ∼70 MPa, although the nonlinearity is not strongly pronounced. Because of the inherent nature of an exceptionally weak porous matrix, much of the stress–strain behavior of the composite is controlled by the fibers.Material exhibits typical fiber-dominated composite behavior. Milz et al. [47] and Goering and Schneider [48] studied mechanical behavior of NextelTM720 fibers at temperatures in the 900–1200 ◦C range. In tensile tests conducted at 1200 ◦C nonlinear stress–strain behavior was observed at stress rates ranging Fig. 2. Tensile stress–strain curves for N720/A ceramic composite obtained at 1200 ◦C with the stress rates of 25 and 0.0025 MPa/s. The influence of loading rate on stress–strain behavior is evident. from 0.1 to 100 MPa/s. A linear stress–strain behavior was produced only at the fastest loading rate of 1000 MPa/s. These results indicate that at stress rates <1000 MPa/s the tensile behavior and tensile strength of the N720 fibers are strongly influenced by creep damage mechanisms. The nonlinear stress–strain behavior of the N720/A composite produced at 25 MPa/s in the present study likewise suggests that creep deformation and damage mechanisms affect the monotonic tensile behavior of this CMC at 1200 ◦C. As seen in Fig. 2, the tensile stress–strain behavior produced at the stress rate of 0.0025 MPa/s is fundamentally different from the nearly linear response at 25 MPa/s. The stress–strain curves obtained at 0.0025 MPa/s are markedly nonlinear. Furthermore, a non-monotonic change in strain is observed in all 0.0025 MPa/s tests. As the stress increases, the strain first decreases and then begins to increase. The corresponding rate of change of strain is first negative and then becomes positive. Such strain rate reversal occurs as the stress reaches 20–25 MPa. As the stress continues to increase, appreciable inelastic strains develop. The failure strains, ranging from 0.73 to 1.06%, are two to three times those obtained at 25 MPa/s. Conversely, the average UTS of 154 MPa is considerably lower than the strength values obtained at 25 MPa/s. Optical micrographs of fracture surfaces obtained in tensile tests at 25 and 0.0025 MPa/s are shown in Fig. 3(a) and (b), respectively. The fracture planes of both specimens are not well defined. The 0◦ fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The fibers in the 0◦ tows in each cloth layer exhibit random failure producing brushy fracture surfaces. Note that the specimen tested at 0.0025 MPa/s has a considerably longer damage zone (∼8 mm) than the specimen tested at 25 MPa/s (∼5 mm), which accounts for larger strain accumulated at the slower stress rate. The strong dependence of tensile strength on loading rate exhibited by this composite at 1200 ◦C is similar to that observed at elevated temperatures for advanced ceramics [38,39,49–51] as well as for the NextelTM720 fibers [52,47,48]. For composites with a glass or glass-ceramic matrix, the degradation of strength with decreasing loading rate is a consequence of slow crack growth process due to stress corrosion[36]. However, in the case of oxide ceramics creep deformation can have a significant influence on the monotonic tensile behavior. Wilson et al. [52] suggested that the deformation mechanism operating in N720 fibers at elevated temperatures was diffusion creep controlled by interface reaction. As the diffusion creep is a time-dependent process, the loading rate dependence of the N720/A tensile behavior and tensile strength at 1200 ◦C is readily explained. At slower loading rates, more time is allowed for diffusion creep to develop, leading to extension of existing crit-

G Model MSA-24026: No of Pages 7 ARTICLE IN PRESS M.B. Ruggles-Wrenn et al Materials Science and Engineering A xxx(2008)xxx-Xxx -10 mm 10 mm Fig 3. Fracture surfaces of the N720 / A specimens obtained in tensile tests conducted at 1200C with the stress rate of: (a)25 MPa/s and(b)0.0025 MPa/s. ical flaws and formation of new cracks and flaws, finally causing reduction in tensile strength and an increase in failure strain. At Stress Rate =0.0025 MPa/s T=1200° fast loading rates, the time-dependent diffusion creep is minimal and has little influence on tensile behavior and tensile strength Because tensile behavior of the N720/A composite is dominated by the tensile response of the N720 fibers, the uts of the compos- g ite at 1200C is likely to be strongly influenced by the diffusion creep damage mechanism operating in N720 fibers at loading rates ≤100MPa/s Additional tests were carried out in order to determine t 12hat1200° observed at 0.0025 MPa/s is an artifact of the incomplete proces 0100hat1200° ng of the n720 fibers, which could be eliminated by additional heat treatment. Several specimens were heat-treated at 1200 Cfor 12 and 100 h and then tested at 0.0025 MPa/s. Results presented STRAIN (% in Fig. 4 demonstrate that the stress-strain response of the spe ens subjected to additional sintering was essentially the same Fig 4. Tensile stress-strain cur treatments obtained at 1200oc with the stress 0025 MPa/. The influence s of the specimens subjected to additional sintering(see Fig 5) have the same appearance as those of the specimens tested in the as-processed condition. Apparently, the N720 /A composite stresses in the 0-30 MPa range. Because in several potential appli- exhibits shrinking when exposed to low and slowly varying load at cations this Mc could be subjected to sustained loading at such 1200Cat the beginning of the 0.0025 MPa/s test. Note that all spec- fairly low stress levels, tensile creep behavior for applied stresses imens tested at 0.0025 MPa/s, regardless of prior heat treatment. in this range was investigated. Results are summarized in Table 1 produced longer damage zones (8-9 mm)than the specimens where creep strain accumulation and rupture time are shown sted at 25 MPa/s. As expected the specimens which produced each creep stress level. Creep curves are shown in Fig. 6. larger strains also exhibit longer damage zones. It is noteworthy that at applied stresses up to 26 MPa at 1200.C the N720/A composite exhibited only negative deformation. The negative creep strain accumulations ranged from-0 23% at 1 MPa 4.2. Creep-rupture at stress levels in 0-30 MPa range to-0.11% at 26 MPa. As expected, creep strain increases with applied stress Creep curves presented in Fig. 6 exhibit primary 10m Fig. 5. Fracture surfaces obtained in tensile tests conducted at 0.0025 MPa/s at 1200 C on N720 /A specimens with different prior heat treatment: (a)12 h at 1200.C and (b 0hat1200° Please cite this article in press as: M B. Ruggles-Wrenn, et al, Mater. Sci Eng. A(2008), doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 4 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx Fig. 3. Fracture surfaces of the N720/A specimens obtained in tensile tests conducted at 1200 ◦C with the stress rate of: (a) 25 MPa/s and (b) 0.0025 MPa/s. ical flaws and formation of new cracks and flaws, finally causing reduction in tensile strength and an increase in failure strain. At fast loading rates, the time-dependent diffusion creep is minimal and has little influence on tensile behavior and tensile strength. Because tensile behavior of the N720/A composite is dominated by the tensile response of the N720 fibers, the UTS of the composite at 1200 ◦C is likely to be strongly influenced by the diffusion creep damage mechanism operating in N720 fibers at loading rates ≤100 MPa/s. Additional tests were carried out in order to determine whether the atypical stress–strain behavior (i.e. strain rate reversal) observed at 0.0025 MPa/s is an artifact of the incomplete processing of the N720 fibers, which could be eliminated by additional heat treatment. Several specimens were heat-treated at 1200 ◦C for 12 and 100 h and then tested at 0.0025 MPa/s. Results presented in Fig. 4 demonstrate that the stress–strain response of the specimens subjected to additional sintering was essentially the same as that of the as-processed specimens. Furthermore, the fracture surfaces of the specimens subjected to additional sintering (see Fig. 5) have the same appearance as those of the specimens tested in the as-processed condition. Apparently, the N720/A composite exhibits shrinking when exposed to low and slowly varying load at 1200 ◦C at the beginning of the 0.0025 MPa/s test. Note that all specimens tested at 0.0025 MPa/s, regardless of prior heat treatment, produced longer damage zones (∼8–9 mm) than the specimens tested at 25 MPa/s. As expected, the specimens which produced larger strains also exhibit longer damage zones. 4.2. Creep-rupture at stress levels in 0–30 MPa range The negative strains observed in the 0.0025 MPa/s tensile tests at stresses <30 MPa suggest that negative creep may occur for creep Fig. 4. Tensile stress–strain curves for N720/A specimens with different prior heat treatments obtained at 1200 ◦C with the stress rate of 0.0025 MPa/s. The influence of prior heat treatment on stress–strain behavior is negligible. stresses in the 0–30 MPa range. Because in several potential applications this CMC could be subjected to sustained loading at such fairly low stress levels, tensile creep behavior for applied stresses in this range was investigated. Results are summarized in Table 1, where creep strain accumulation and rupture time are shown for each creep stress level. Creep curves are shown in Fig. 6. It is noteworthy that at applied stresses up to 26 MPa at 1200 ◦C the N720/A composite exhibited only negative deformation. The negative creep strain accumulations ranged from −0.23% at 1 MPa to −0.11% at 26 MPa. As expected, creep strain increases with applied stress. Creep curves presented in Fig. 6 exhibit primary and secondary creep regimes. Unlike in the case of tensile creep at stresses ≥80 MPa where primary creep rapidly transitions into Fig. 5. Fracture surfaces obtained in tensile tests conducted at 0.0025 MPa/s at 1200 ◦C on N720/A specimens with different prior heat treatment: (a) 12 h at 1200 ◦C and (b) 100 h at 1200 ◦C

G Model ARTICLE IN PRESS M.B. Ruggles-Wrenn er al/ Materials Science and Engineering A xxx(2008)xxx-XXx Summary of creep-rupture results for the N720 A ceramic composite at 1200.C in T=1200° Creep stress(MPa) Creep strain (%) Time to rupture(s) 100 h at 1 MPa a Run-out Ennu2 As-Processed T=1200° 0.1 STRAIN (% e0.1 Fig. 7. Effect of prior creep at stress levels in the Ooc ge on tensile (shrinkage )for the Nextel M720 fibers subjected to creep at 1200oC at applied stresses up to 80 MPa(equivalent to the stress of 18 MPa or a crossply composite with the fiber volume of 45%). In addition, recent studies 54-56]of the effects of thermal aging Time(h) on the physical and mechanical properties of a composite consist- ing of NextelM720 fibers and a porous alumina matrix, reported Fig.6. Creep strain vs time curves for N720/A ceramic composite at 1200.C Creep a porosity reduction of -6% after a 10-min exposure at 1200C. stress=1.6. 20 and 26 MPa. The loss of porosity was attributed to additional sintering of the Al2O3 matrix during the aging treatments. It is likely that additional secondary creep(41). for creep stresses in the MPa range tran- sintering of the alumina matrix occurred during the 100-h creep sition from primary to secondary creep occurs much later in creep tests performed in this effort. Shrinkage of the n720 fibers occur- life. Primary creep persists during the first 30-50 h of the creep test. ring simultaneously with the loss of porosity in the alumina matrix The creep curves in Fig. b indicate that secondary creep is likely to tion exhibited by the n720/ A composite at applied stresses up to reached in all tests. All steady-state creep rate magnitudes were below 10-95-1. Creep run-out of 100 h was reached in all tests Retained tensile strength and modulus of the run-out specimens 43. Composite porosity and density were measured in tensile tests conducted at 25 MPa s at 1200C. The retained properties are summarized in Table 2. Representative The density as well as the porosity and average pore size of tensile stress-strain curves obtained for the specimens subjected t the specimens subjected to prior creep at 1, 20 and 26 MPa was rior creep at 1200 C are presented in Fig. 7 together with the measured. The results are summarized in table 3 where the mea- sile stress-strain curves for the as-processed material. The run-out surements obtained for the as-processed material are included for pecimens retained 100% of their tensile strength. A slight increase comparison. Differential plots of pore size distribution for the as- in tensile modulus was also observed. Tensile stress-strain behav- processed material and specimens subjected to prior creep at 1, 20, lor of the specimens subjected to prior creep remained qualitatively and 25 MPa are shown in Fig 8. An increase in bulk density and a milar to that of the as-processed material. decrease in porosity observed for the pre-crept specimens are con- The results of this effort are consistent with those found for the sistent with the negative creep deformation(shrinkage)produced Nextel M720 fibers alone [52, 53]. Wilson et al. [52]reported that at 1093Cthe Nextel720 fibers exhibited negative creep at applied Table 3 stresses below 138 MPa(equivalent to the stress of 31 MPa for a Density and porosity of the N720/A specimens subjected to prior thermo- crossply composite with the fiber volume of 45%). Fiber shrink- mechanical loading age continued at a decreasing rate for the entire duration of these Bulk density (g/mL Porosity (% tests (257 h at 69 MPa and 130 h at 138 MPa). At 69 MPa(equivalent to the stress of 15. 5 MPa for a crossply composite with the fiber As-proces volume of 45%), the total shrinkage was 0. 12%. The shrinkage of As-processee 22.0375 the Nextel M720 fibers during creep tests was attributed to crys- 100h at 1 MPa at tallization of a-Al2O3 from mullite supersaturated with alumina. 00h at 20 MPa at Likewise, Deleglise et al. [53] reported only negative deformation 100h at 26 MPa at 1200C 20259 Table 2 Retained properties of the n720/A specimens subjected to prior creep at 1200C Creep stress(MPa) Retained strength(MPa) Strain at failure (% 198 0.36 0.34 Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A(2008). doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx 5 Table 1 Summary of creep-rupture results for the N720/A ceramic composite at 1200 ◦C in laboratory air Creep stress (MPa) Creep strain (%) Time to rupture (s) 1 −0.23 360,000a 6 −0.20 360,000a 20 −0.17 360,000a 26 −0.11 360,000a a Run-out. Fig. 6. Creep strain vs. time curves for N720/A ceramic composite at 1200 ◦C. Creep stress = 1, 6, 20 and 26 MPa. secondary creep [41], for creep stresses in the 0–30 MPa range transition from primary to secondary creep occurs much later in creep life. Primary creep persists during the first 30–50 h of the creep test. The creep curves in Fig. 6 indicate that secondary creep is likely to continue for the duration of the creep life. Minimum creep rate was reached in all tests. All steady-state creep rate magnitudes were below 10−9 s−1. Creep run-out of 100 h was reached in all tests. Retained tensile strength and modulus of the run-out specimens were measured in tensile tests conducted at 25 MPa/s at 1200 ◦C. The retained properties are summarized in Table 2. Representative tensile stress–strain curves obtained for the specimens subjected to prior creep at 1200 ◦C are presented in Fig. 7 together with the tensile stress–strain curves for the as-processed material. The run-out specimens retained 100% of their tensile strength. A slight increase in tensile modulus was also observed. Tensile stress–strain behavior of the specimens subjected to prior creep remained qualitatively similar to that of the as-processed material. The results of this effort are consistent with those found for the NextelTM720 fibers alone [52,53]. Wilson et al. [52] reported that at 1093 ◦C the NextelTM720 fibers exhibited negative creep at applied stresses below 138 MPa (equivalent to the stress of 31 MPa for a crossply composite with the fiber volume of 45%). Fiber shrinkage continued at a decreasing rate for the entire duration of these tests (257 h at 69 MPa and 130 h at 138 MPa). At 69 MPa (equivalent to the stress of 15.5 MPa for a crossply composite with the fiber volume of 45%), the total shrinkage was 0.12%. The shrinkage of the NextelTM720 fibers during creep tests was attributed to crystallization of -Al2O3 from mullite supersaturated with alumina. Likewise, Deleglise et al. [53] reported only negative deformation Fig. 7. Effect of prior creep at stress levels in the 0–30 MPa range on tensile stress–strain behavior of N720/A ceramic composite at 1200 ◦C. (shrinkage) for the NextelTM720 fibers subjected to creep at 1200 ◦C at applied stresses up to 80 MPa (equivalent to the stress of 18 MPa for a crossply composite with the fiber volume of 45%). In addition, recent studies [54–56] of the effects of thermal aging on the physical and mechanical properties of a composite consisting of NextelTM720 fibers and a porous alumina matrix, reported a porosity reduction of ∼6% after a 10-min exposure at 1200 ◦C. The loss of porosity was attributed to additional sintering of the Al2O3 matrix during the aging treatments. It is likely that additional sintering of the alumina matrix occurred during the 100-h creep tests performed in this effort. Shrinkage of the N720 fibers occurring simultaneously with the loss of porosity in the alumina matrix are the probable mechanisms behind the negative creep deformation exhibited by the N720/A composite at applied stresses up to 26 MPa. 4.3. Composite porosity and density The density as well as the porosity and average pore size of the specimens subjected to prior creep at 1, 20 and 26 MPa was measured. The results are summarized in Table 3 where the measurements obtained for the as-processed material are included for comparison. Differential plots of pore size distribution for the asprocessed material and specimens subjected to prior creep at 1, 20, and 25 MPa are shown in Fig. 8. An increase in bulk density and a decrease in porosity observed for the pre-crept specimens are consistent with the negative creep deformation (shrinkage) produced Table 3 Density and porosity of the N720/A specimens subjected to prior thermomechanical loading Specimen Bulk density (g/mL) Porosity (%) As-processed 2.8544 21.8081 As-processed 2.8432 21.8431 As-processed 2.8544 22.0375 100 h at 1 MPa at 1200 ◦C 2.8952 20.0421 100 h at 20 MPa at 1200 ◦C 2.9237 20.0412 100 h at 26 MPa at 1200 ◦C 2.8843 20.2592 Table 2 Retained properties of the N720/A specimens subjected to prior creep at 1200 ◦C Creep stress (MPa) Retained strength (MPa) Retained modulus (GPa) Strain at failure (%) 1 198 76 0.36 6 196 72 0.34 20 201 74 0.35 26 202 72 0.36

G Model MSA-24026: No of Pages 7 ARTICLE IN PRESS M.B. Ruggles-Wrenn et al Materials Science and Engineering A xxx(2008)xxx-Xxx e Acknowledgements "E As-Processed thank Dr. R.A. Kerans and dr. t 505050 -e- 100h at 1 MPa Parthasarathy for many discussions. The financial support -e-100 h at 20 MPa of Dr r. sikorski and Dr Propulsion Directorate, Air Force - h at 26 MPa Research Laboratory is. preciated References E.zok,」 Am Ceram.Soc.89(11)(2006)3309-3324 3IRJ. Kerans, R.S. Hay, N. Pagano, T.A. Parthasarathy, Am. Ceram Soc. Bull. 68(2) 0.0010.01 0 10 100 1000 i5jRJ Kerans TA Parthasarathy, Compo Pore Size(um) [6]R Kerans, R Hay, T. Parthasarathy, M. Cinibulk, J- Am. Ceram Soc. 85(11)(2002) 2599-2632. Figs 8. Effect of prior creep at 1200 C on pore size distribution for N720/A ceramic KM. Prewo, JA. Batt, J Mater. Sci. 23(1988)523-527. T Mah. N.L. Hecht, D.E. gman, H M. Kim, A.P. Katz, H.A. Lipsitt, J Mater. Sci. 19(1984)1191-1201. [91 J-J. Brennan, in: K.C. Masdayazni(Ed. Fiber Reinforced Ceramic Composites. in mechanical tests. The bi-modal nature of the pore size distri- Noyes, New York, 1990(cha butions in Fig 8 indicates that all specimens contain two majo [10] F.E. Heredia, I.C. McNulty, k, A.G. Evans, Am Ceram Soc. 78(8)(1995) classes of pores: those with a characteristic dimension of -O1 um (111RS Nutt, in: S.V. Nair, K Jakus(Eds ) High-Temperature Mechanical Behavior and those with a characteristic dimension of 100 um. The as of Ceramic Composites, Butterworth-Heineman, Boston, MA, 1995. processed material has the largest population of the 100-um pores 2 2345=235:. F.W. Zok, R.M. McMeeking, Z Z Du, J Am. Ceram. Soc. 79(1996) and the smallest population of the 0. 1-Wm pores Conversely, the [13] KL More, P.F. Tortorelli, M.K. Ferber, J.R. Keiser, J Am Ceram Soc. 83(1)(2000) pre-crept specimens show a decrease in the pore population with the characteristic dimension of 100 um and an increased presence [14 kL More, PF. Tortorelll, M.K. Ferber, LR Walker, J.R. Keiser, w D Brentnall of pores with the characteristic dimension of 0.1 um, suggesting Congress, Paper No. 99-Gr-292, 1999. that the matrix has densified during creep tests. Such changes in [15] M.K. Ferber, H.T. Lin, J.R. Keiser, in: M.G. Jenkins, E. Lara-Curzio, S.T. Gonczy matrix porosity are also consistent with negative strains measured (Eds ) Mechanical, Thermal, and Environmental Testing and Perform In creep tests. rials, 2000, pp 210-215(ASTM STP 1392). [161 J-A. Haynes, M J. Lance, K.M. M.K. Ferber, R.A. Lowden, D P Stinton, I 5. Concluding remarks [17. Opil The effect of loading rate on tel ensile stress-strain behavior and 120jEE. Hermes R). Kerans, Material Research Society, Symposium proceedings. nsile properties of the N720/ A ceramic composite was investi- d at 1200C in laboratory air. The stress rates were 0.0025 and [211 A. Szweda, M L Millard, M.G. Harrison, U.S. Patent No 5, 601, 674(1997). 25 MPa/ s he elevated-temperature tensile properties of N720/A exhibit 15(4)(1994)113-120 I M.H. Lewis, M.G. Cain, P. Doleman, A.G. Razzell, J. Gent, in: A.G. Eva a strong dependence on loading rate. As the loading rate decreases. the ultimate tensile strength decreases and the failure strain ufacturing and Materials Development, American Ceramic Society, 1995, pp increases. At 25 MPa/s the Uts was 181 MPa and the failure strain was 0.36%. At 0.0025 MPa/s the average UTS was 154 MPa (26iR. Lunderberg, L. Eckerbom, in: A.G. Ev and the failure strains ranged from 0.73 to 1.06%. The strong dependence of tensile strength on loading rate is attributed to Development, American Ceramic Society, 1995. pp 95-104. diffusion creep controlling the deformation of the N720 fibers at (281 P.E. D Morgan, D.B. Marshall, _ Am. Ceram. Soc. 78(6)(1995)1553-1563 1200°C [291 W.C.Tu, F.F. Lange, A.G. Evans, ) Am Ceram Soc. 79(2)(1996)417-424. At 25 MPa/s the stress-strain behavior is nearly linear to failure. Dalgleish, F W. Zok, A.G. Evans, Am Ceram Soc. 81 stresses <15 MPa. As the stress increases, the strain first decreases j32jTl Hegedus u.S. Patent No 50, 17752(May 21, 199\ay(ed).Proceedings and then increases Such reversal of strain rate occurs at the stress of 17th Conference on Metal Matrix, Carbon, and Ceramic Matrix Com of 20-25 MPa. Significant inelastic strains develop prior to failure. osites, NASA Conference Publication 3235. Part 2, NASA, 1993 Due to the irregular stress-strain behavior, the linear elastic region is difficult to identify and the elastic modulus cannot be readily Conference Publication, ARPA Ceramic Technology Insertion Program(DARP determined Annapolis, MD, 1994, pp. 267-322. The creep-rupture behavior of the n720/A composite was char- I E.W. Zok, C.G. Levi, Adv Eng Mater. 3(1-2)(2001)15-23. acterized for stress levels in the 0-30 MPa range at 1200C in I B F Sorensen. ]. w. Holmes, J Am Ceram Soc. 79(2)(1996)313-320. 37]SR Choi, N.P. Bansal, J Am Ceram Soc. 87(10)(2004)1912-1918 aboratory air. At applied stresses <26 MPa, the N720/A compos- 38 S.R. Choi, Le. Gyeke te exhibits only negative creep deformation. Both primary and nt). Fatigue27(2005)503-510 nsal, M. verrilli, ]. Eur. Ceram 25(2005) observed. Creep strain accumulations [40] LP. Zawada, R.S. Hay. S.S. Lee, J. Staehler. J. Am. Ceram Soc. 86(6)(2003) ranged from -0 23% at 1 MPa to-0. 11% at 26 MPa. All steady-state reep rate magnitudes were less than 10-95-l Creep run-out of 41] M.B. Ruggles-Wrenn, S Mall, C.A. Eber, LB. Harlan, Composites: Part A 37(1 100 h was achieved in all tests. The run-out specimens retained [421 J.M. Mehrman, M.B. Ruggles-Wrenn, S.S. Baek, Comp. Sci. TechnoL. 67(2007) 100% of their tensile strength. Shrinkage of the N720 fibers and 25-1438. loss of matrix porosity due to additional sintering are the probable 141 M.B. Ruggles-Wrenn, G Hetrick, S.S.Baek, Int ]. Fatigue 30 (3)(2008)502-516. RA. Jurf, S.C. Bu J Eng. Gas Turbines Power, Trans. ASME 122(2)(1999) mechanisms behind the negative creep deformation 202-205 Please cite this article in press as: M B. Ruggles-Wrenn, et al, Mater. Sci Eng. A(2008), doi: 10. 1016/j. msea. 2008.03.006

Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 6 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx Fig. 8. Effect of prior creep at 1200 ◦C on pore size distribution for N720/A ceramic composite. in mechanical tests. The bi-modal nature of the pore size distributions in Fig. 8 indicates that all specimens contain two major classes of pores: those with a characteristic dimension of ∼0.1 m and those with a characteristic dimension of ∼100 m. The asprocessed material has the largest population of the 100-m pores and the smallest population of the 0.1-m pores. Conversely, the pre-crept specimens show a decrease in the pore population with the characteristic dimension of 100 m and an increased presence of pores with the characteristic dimension of 0.1 m, suggesting that the matrix has densified during creep tests. Such changes in matrix porosity are also consistent with negative strains measured in creep tests. 5. Concluding remarks The effect of loading rate on tensile stress–strain behavior and tensile properties of the N720/A ceramic composite was investigated at 1200 ◦C in laboratory air. The stress rates were 0.0025 and 25 MPa/s. The elevated-temperature tensile properties of N720/A exhibit a strong dependence on loading rate. As the loading rate decreases, the ultimate tensile strength decreases and the failure strain increases. At 25 MPa/s the UTS was 181 MPa and the failure strain was 0.36%. At 0.0025 MPa/s the average UTS was 154 MPa and the failure strains ranged from 0.73 to 1.06%. The strong dependence of tensile strength on loading rate is attributed to diffusion creep controlling the deformation of the N720 fibers at 1200 ◦C. At 25 MPa/s the stress–strain behavior is nearly linear to failure. At 0.0025 MPa/s the stress–strain behavior departs from linearity at stresses ≈15 MPa. As the stress increases, the strain first decreases and then increases. Such reversal of strain rate occurs at the stress of 20–25 MPa. Significant inelastic strains develop prior to failure. Due to the irregular stress–strain behavior, the linear elastic region is difficult to identify and the elastic modulus cannot be readily determined. The creep-rupture behavior of the N720/A composite was characterized for stress levels in the 0–30 MPa range at 1200 ◦C in laboratory air. At applied stresses ≤26 MPa, the N720/A composite exhibits only negative creep deformation. Both primary and secondary creep regimes are observed. Creep strain accumulations ranged from −0.23% at 1 MPa to −0.11% at 26 MPa. All steady-state creep rate magnitudes were less than 10−9 s−1. Creep run-out of 100 h was achieved in all tests. The run-out specimens retained 100% of their tensile strength. Shrinkage of the N720 fibers and loss of matrix porosity due to additional sintering are the probable mechanisms behind the negative creep deformation. Acknowledgements The authors would like to thank Dr. R.A. Kerans and Dr. T. Parthasarathy for many valuable discussions. The financial support of Dr. R. Sikorski and Dr. J. Zelina, Propulsion Directorate, Air Force Research Laboratory is highly appreciated. References [1] F. Zok, J. Am. Ceram. Soc. 89 (11) (2006) 3309–3324. [2] L.P. Zawada, J. Staehler, S. Steel, J. Am. Ceram. Soc. 86 (8) (2003) 1282–1291. [3] R.J. Kerans, R.S. Hay, N.J. Pagano, T.A. Parthasarathy, Am. Ceram. Soc. Bull. 68 (2) (1989) 429–442. [4] A.G. Evans, F.W. Zok, J. Mater. Sci. 29 (1994) 3857–3896. [5] R.J. Kerans, T.A. Parthasarathy, Composites: Part A 30 (1999) 521–524. [6] R. Kerans, R. Hay, T. Parthasarathy, M. Cinibulk, J. Am. Ceram. Soc. 85 (11) (2002) 2599–2632. [7] K.M. Prewo, J.A. Batt, J. Mater. Sci. 23 (1988) 523–527. [8] T. Mah, N.L. Hecht, D.E. 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《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Effect of loading rate on the monotonic tensile behavior and tensile

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Effect of loading rate on the monotonic tensile behavior and tensile