B. Wilshire, F. Carrerio/ Materials Science and Engineering 4272(1999)38-44 1000m Fig. 6. Scanning electron micrograph showing crack development in Fig. 7. Scanning electron micrograph showing the zone characterized the alumina matrix of the 0/90 SiC Al,O, composite by oxidized fibres(marked A) and the final fibre pull-out zone on the creep fracture surface of a 0/90% SiC Al,O, crept to failure in even lower than that of the dense whisker-reinforced under a tensile stress of 79 MPa at 1573 K alumina, it appears that the matrix makes little contri- indicating that the product emtf increases as the strain bution to the strength of the fibre-reinforced CMC. In to failure increases with decreasing stress at each tem- contrast, stresses about five times higher must be ar perature(Figs. 10 and l1). Interestingly, by incorporat plied to the Nicalon TM fibres in order to achieve creep ing the tensile data quoted for the SIcw/AlO3 ceramic rates comparable with those recorded for the Nicalon [12] in Figs. 9-ll inclusive, similar trends in the rela fibre-reinforced alumina. The creep performance of the tionships between Em'tr and Er are revealed for the present 0/900 SiC, composite is therefore gov- whisker and fibre reinforced SiC/Al2O, materials erned by the longitudinal fibres, which occupy approxi tely one fifth of the testpiece cross-section With 0/90 CMCs, creep of the longitudinal fibres 4. Discussion transfers stress to the matrix, causing intergranular crack development, as shown in Fig. 6. Cracking re When power law approaches are used to describ duces matrix stiffness, reloading the fibres and inducing reep behaviour (Eq. (I)), it is common practice to further creep. As crack growth occurs, the developing approximate the curving log a/log im plots by a series of tangents, assuming that changes in n and @e signify cracks become bridged by the longitudinal fibres [21, 22] that different creep mechanisms become dominant but, as oxygen penetrates during tests in air, oxidation within different stress-temperature regimes. Conversely, promotes failure of the crack-bridging fibres. Zones showing oxidized fibres therefore extend over substan- tial fractions of the final fracture surfaces before the cracks grow to the length required to cause sudden failure of the testpieces by fibre pull out(Fig. 7) The rate of crack development and the time to 60 eventual failure of the SiC/Al,O, material depend on he rate of creep strain accumulation. As a result, the shapes of the log o/log tr plots in Fig. 8 seem to mirror 1473K he forms of the log o/log Em relationships in Fig 3 but, unlike the behaviour observed for many crystalline 1673K materials [23], the product(im' tr) of the minimum creep 031473K rate and the creep rupture life is not a constant. In 9 SiCw/Al203 1573K stead, for stresses giving the same creep rate, Emtr increases with increasing test temperature(Fig 9). Fur 103104105105 thermore, over the range of test conditions studied for time to fracture(s) the SICrAl2O3 composite, the time to fracture varies as Fig 8. The stress dependence of the time to fracture(td) for the 0/90 tr∝叫ln SiCdAlO3 composite from 1473 to 1673 K, with data also included for a SiCw Al2O3 ceramic tested in tension at 1473 and 1573K[12]B. Wilshire, F. Carren˜o / Materials Science and Engineering A272 (1999) 38–44 41 Fig. 6. Scanning electron micrograph showing crack development in the alumina matrix of the 0/90° SiCf /Al2O3 composite. Fig. 7. Scanning electron micrograph showing the zone characterized by oxidized fibres (marked A) and the final fibre pull-out zone on the creep fracture surface of a 0/90° SiCf /Al2O3 crept to failure in air under a tensile stress of 79 MPa at 1573 K. even lower than that of the dense whisker-reinforced alumina, it appears that the matrix makes little contri￾bution to the strength of the fibre-reinforced CMC. In contrast, stresses about five times higher must be ap￾plied to the Nicalon™ fibres in order to achieve creep rates comparable with those recorded for the Nicalon™ fibre-reinforced alumina. The creep performance of the present 0/90° SiCf /Al2O3 composite is therefore gov￾erned by the longitudinal fibres, which occupy approxi￾mately one fifth of the testpiece cross-section. With 0/90° CMCs, creep of the longitudinal fibres transfers stress to the matrix, causing intergranular crack development, as shown in Fig. 6. Cracking re￾duces matrix stiffness, reloading the fibres and inducing further creep. As crack growth occurs, the developing cracks become bridged by the longitudinal fibres [21,22] but, as oxygen penetrates during tests in air, oxidation promotes failure of the crack-bridging fibres. Zones showing oxidized fibres therefore extend over substan￾tial fractions of the final fracture surfaces before the cracks grow to the length required to cause sudden failure of the testpieces by fibre pull out (Fig. 7). The rate of crack development and the time to eventual failure of the SiCf /Al2O3 material depend on the rate of creep strain accumulation. As a result, the shapes of the log s/log tf plots in Fig. 8 seem to mirror the forms of the log s/log o; m relationships in Fig. 3 but, unlike the behaviour observed for many crystalline materials [23], the product (o; m·tf ) of the minimum creep rate and the creep rupture life is not a constant. In￾stead, for stresses giving the same creep rate, o; m·tf increases with increasing test temperature (Fig. 9). Fur￾thermore, over the range of test conditions studied for the SiCf /Al2O3 composite, the time to fracture varies as: tf8of /o; m (3) indicating that the product o; m·tf increases as the strain to failure increases with decreasing stress at each tem￾perature (Figs. 10 and 11). Interestingly, by incorporat￾ing the tensile data quoted for the SiCw/Al2O3 ceramic [12] in Figs. 9–11 inclusive, similar trends in the rela￾tionships between o; m·tf and of are revealed for the whisker and fibre reinforced SiC/Al2O3 materials. 4. Discussion When power law approaches are used to describe creep behaviour (Eq. (1)), it is common practice to approximate the curving log s/log o; m plots by a series of tangents, assuming that changes in n and Qc signify that different creep mechanisms become dominant within different stress-temperature regimes. Conversely, Fig. 8. The stress dependence of the time to fracture (tf ) for the 0/90° SiCf /Al2O3 composite from 1473 to 1673 K, with data also included for a SiCw/Al2O3 ceramic tested in tension at 1473 and 1573 K [12]