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. 2008.03.006Please 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 success￾fully 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 phe￾nomena 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 fiber￾reinforced 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 mecha￾nism 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 tra￾ditional 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. Con￾versely, 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 labora￾tory 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 run￾out (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 temper￾ature. The objective of this study is to investigate the effects of load￾ing 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 lay￾ers, 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 dur￾ing 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 measure￾ment was accomplished with an MTS high-temperature air-cooled uniaxial extensometer. For elevated temperature testing, thermo￾couples 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