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. 2008.03.006Please 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 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 distri￾butions 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 as￾processed 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 investi￾gated 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 char￾acterized for stress levels in the 0–30 MPa range at 1200 ◦C in laboratory air. At applied stresses ≤26 MPa, the N720/A compos￾ite 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. McCullum, J.R. Hoenigman, H.M. Kim, A.P. Katz, H.A. Lipsitt, J. Mater. Sci. 19 (1984) 1191–1201. [9] J.J. Brennan, in: K.C. Masdayazni (Ed.), Fiber Reinforced Ceramic Composites, Noyes, New York, 1990 (Chapter 8). [10] F.E. Heredia, J.C. McNulty, F.W. Zok, A.G. Evans, J. Am. Ceram. Soc. 78 (8) (1995) 2097–2100. [11] R.S. Nutt, in: S.V. Nair, K. Jakus (Eds.), High-Temperature Mechanical Behavior of Ceramic Composites, Butterworth-Heineman, Boston, MA, 1995. [12] A.G. Evans, F.W. Zok, R.M. McMeeking, Z.Z. Du, J. Am. Ceram. Soc. 79 (1996) 2345–2352. [13] K.L. More, P.F. Tortorelli, M.K. Ferber, J.R. Keiser, J. Am. Ceram. Soc. 83 (1) (2000) 211–213. [14] K.L. More, P.F. Tortorelli, M.K. Ferber, L.R. Walker, J.R. Keiser, W.D. Brentnall, N. Miralya, J.B. Price, Proceedings of International Gas Turbine and Aerospace Congress, Paper No. 99-GT-292, 1999. [15] M.K. Ferber, H.T. Lin, J.R. Keiser, in: M.G. Jenkins, E. Lara-Curzio, S.T. Gonczy (Eds.), Mechanical, Thermal, and Environmental Testing and Performance of Ceramic Composites and Components, American Society for Testing and Mate￾rials, 2000, pp. 210–215 (ASTM STP 1392). [16] J.A. Haynes, M.J. Lance, K.M. Cooley, M.K. Ferber, R.A. Lowden, D.P. Stinton, J. Am. Ceram. Soc. 83 (3) (2000) 657–659. [17] E.J. Opila, R.E. Hann Jr, J. Am. Ceram. Soc. 80 (1) (1997) 197–205. [18] E.J. Opila, J. Am. Ceram. Soc. 77 (3) (1994) 730–736. [19] E.J. Opila, J. Am. Ceram. Soc. 82 (3) (1999) 625–636. [20] E.E. Hermes, R.J. Kerans, Material Research Society, Symposium Proceedings, vol. 125, 1988, pp. 73–78. [21] A. Szweda, M.L. Millard, M.G. Harrison, U.S. Patent No. 5,601,674 (1997). [22] S.M. Sim, R.J. Kerans, Ceram. Eng. Sci. Proc. 13 (9–10) (1992) 632–641. [23] E.H. Moore, T. Mah, K.A. Keller, Ceram. Eng. Sci. Proc. 15 (4) (1994) 113–120. [24] M.H. Lewis, M.G. Cain, P. Doleman, A.G. Razzell, J. Gent, in: A.G. Evans, R.G. Naslain (Eds.), High-Temperature Ceramic–Matrix Composites. II: Man￾ufacturing and Materials Development, American Ceramic Society, 1995, pp. 41–52. [25] F.F. Lange, W.C. Tu, A.G. Evans, Mater. Sci. Eng. A A195 (1995) 145–150. [26] R. Lunderberg, L. Eckerbom, in: A.G. Evans, R.G. Naslain (Eds.), High￾Temperature Ceramic–Matrix Composites. II: Manufacturing and Materials Development, American Ceramic Society, 1995, pp. 95–104. [27] E. Mouchon, P. Colomban, Composites 26 (1995) 175–182. [28] P.E.D. Morgan, D.B. Marshall, J. Am. Ceram. Soc. 78 (6) (1995) 1553–1563. [29] W.C. Tu, F.F. Lange, A.G. Evans, J. Am. Ceram. Soc. 79 (2) (1996) 417–424. [30] C.G. Levi, J.Y. Yang, B.J. Dalgleish, F.W. Zok, A.G. Evans, J. Am. Ceram. Soc. 81 (1998) 2077–2086. [31] A.G. Hegedus. U.S. Patent No. 50,177,522 (May 21, 1991). [32] T.J. Dunyak, D.R. Chang, M.L. Millard, in: J.D. Buckley (ed.), Proceedings of 17th Conference on Metal Matrix, Carbon, and Ceramic Matrix Com￾posites, NASA Conference Publication 3235, Part 2, NASA, 1993, pp. 675– 690. [33] L.P. Zawada, S.S. Lee, in: W.S. Coblenz (Ed.), Defense Advanced Projects Agency Conference Publication, ARPA Ceramic Technology Insertion Program (DARPA), Annapolis, MD, 1994, pp. 267–322. [34] T.J. Lu, J. Am. Ceram. Soc. 79 (1) (1996) 266–274. [35] F.W. Zok, C.G. Levi, Adv. Eng. Mater. 3 (1–2) (2001) 15–23. [36] B.F. Sorensen, J.W. Holmes, J. Am. Ceram. Soc. 79 (2) (1996) 313–320. [37] S.R. Choi, N.P. Bansal, J. Am. Ceram. Soc. 87 (10) (2004) 1912–1918. [38] S.R. Choi, J.P. Gyekenyesi, Int. J. Fatigue 27 (2005) 503–510. [39] S.R. Choi, N.P. Bansal, M.J. Verrilli, J. Eur. Ceram. Soc. 25 (2005) 1629–1636. [40] L.P. Zawada, R.S. Hay, S.S. Lee, J. Staehler, J. Am. Ceram. Soc. 86 (6) (2003) 981–990. [41] M.B. Ruggles-Wrenn, S. Mall, C.A. Eber, L.B. Harlan, Composites: Part A 37 (11) (2006) 2029–2040. [42] J.M. Mehrman, M.B. Ruggles-Wrenn, S.S. Baek, Comp. Sci. Technol. 67 (2007) 1425–1438. [43] M.B. Ruggles-Wrenn, G. Hetrick, S.S. Baek, Int. J. Fatigue 30 (3) (2008) 502–516. [44] R.A. Jurf, S.C. Butner, J. Eng. Gas Turbines Power, Trans. ASME 122 (2) (1999) 202–205.