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. 2008.03.006Please 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 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, land￾based 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 com￾posites (CMCs), capable of maintaining excellent strength and fracture toughness at high temperatures are prime candidate mate￾rials 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 perfor￾mance 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 pol￾icy 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 pull￾out [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/non￾oxide 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 develop￾ment 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