S. gi Joumal of the European Ceramic Society 29(2009 )539-550 of as-fabricated and creep tested specimens were character- zed by scanning and transmission electron microscopy (SEM. mull e TEM), and particular attention was paid to the grain bound 106 ary regions and the location and size distribution of the Sic particles. The creep tests, and theoretical modelling for the prediction of the creep behaviour of these ceramics, have uFau 1400° previously been carried out by Clegg and co-workers. 8, I9 Results from that work, relevant to the electron microscopy ivestigation presented in this paper, are reviewed shortly 1300 2. Review of creep test and modelling results 1010 STRESS(MPa) Polycrystalline mullite, and mullite reinforced with 5 vol %o SiC nanoparticles (in the following termed"the nanocompos- mu‖tesc ite"), have been subjected to tensile creep tests in air at 1300 and 1400C under stresses between 10 and 50 MPa. 18, 19 The oretical modelling of diffusion-controlled creep deformation of these two materials was also carried out. 18,19 u 2.1. Polycrystalline mullite 1300°C Creep tests of the polycrystalline mullite material at 1300C showed a stress exponent of n=1. 2 which implies that diffu sion processes(n=1)are controlling the creep deformation at this temperature, see Fig. la Creep tests performed at 1400C resulted in a higher stress exponent of n=2(Fig. la), which STRESS(MPa suggests that, in addition to diffusion processes, other creep Fig. 1. Experimentally determined steady-state creep rates of(a)the polycrys mechanisms become active at this temperature. talline mullite and(b) the mullite/5 vol %o SiC at 1300 and 1400C plotted The experimentally determined creep rates were compared as function of stress. The testing conditions for the specimens subjected to with creep rates expected for diffusion-controlled creep as the microstructural characterization are marked by circles. Diffusional creep experimental materials, are also shown plotted. Data taken from Pitchtordto 1402 from mullite creep data presented in the literature. 9, 12, 13, 20These where o is the applied stress, s2 the volume of the rate- values were then used in an estimate of the creep rate interval of ontrolling diffusing species, k the Boltzmann constant and d is the mullite ceramic in the present investigation(d=1.5 um). 18 the grain size. Deff is the effective diffusion coefficient. related The plots in Fig. la, based on data taken from Pitchford, show to the diffusion coefficients for lattice diffusion Di and grain that the experimentally determined creep rates at 1300 C, and at boundary diffusion Db according to Deff= DI (2) 2.2. Mullite reinforced with 5 vol %o SiC nanoparticles where 8 is the grain boundary width. Maximum and minimum The experimental creep rates of the nanocomposite at 1300 values of Defm S2 at the two test temperatures were calculated and 1400C are plotted in Fig. Ib. The creep tests at 1400C Table I The as-sintered and creep tested materials Material Test temperature(°C) Stress(MPa) Steady-state creep rate(s-) Grain size(um) Polycrystalline mullite 15 15x 1400 1.2×10 5 1300 9.5×10-9 Mullite/SiC nanocomposite As-sintered 1.9x 1400 2.9×10 3.2×10 0.8540 S. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 of as-fabricated and creep tested specimens were character￾ized by scanning and transmission electron microscopy (SEM, TEM), and particular attention was paid to the grain bound￾ary regions and the location and size distribution of the SiC particles. The creep tests, and theoretical modelling for the prediction of the creep behaviour of these ceramics, have previously been carried out by Clegg and co-workers.18,19 Results from that work, relevant to the electron microscopy investigation presented in this paper, are reviewed shortly below. 2. Review of creep test and modelling results Polycrystalline mullite, and mullite reinforced with 5 vol.% SiC nanoparticles (in the following termed “the nanocompos￾ite”), have been subjected to tensile creep tests in air at 1300 and 1400 ◦C under stresses between 10 and 50 MPa.18,19 The￾oretical modelling of diffusion-controlled creep deformation of these two materials was also carried out.18,19 2.1. Polycrystalline mullite Creep tests of the polycrystalline mullite material at 1300 ◦C showed a stress exponent of n = 1.2 which implies that diffu￾sion processes (n = 1) are controlling the creep deformation at this temperature, see Fig. 1a. Creep tests performed at 1400 ◦C resulted in a higher stress exponent of n =2 (Fig. 1a), which suggests that, in addition to diffusion processes, other creep mechanisms become active at this temperature. The experimentally determined creep rates were compared with creep rates expected for diffusion-controlled creep as given by ε˙ = 14σΩ kTd2 Deff (1) where σ is the applied stress, Ω the volume of the rate￾controlling diffusing species, k the Boltzmann constant and d is the grain size. Deff is the effective diffusion coefficient, related to the diffusion coefficients for lattice diffusion Dl and grain boundary diffusion Db according to Deff = Dl + πδ d Db (2) where δ is the grain boundary width. Maximum and minimum values of DeffΩ at the two test temperatures were calculated Fig. 1. Experimentally determined steady-state creep rates of (a) the polycrys￾talline mullite and (b) the mullite/5 vol.% SiC at 1300 and 1400 ◦C plotted as function of stress. The testing conditions for the specimens subjected to the microstructural characterization are marked by circles. Diffusional creep rate intervals of polycrystalline mullite, predicted for the grain sizes of the two experimental materials, are also shown plotted. Data taken from Pitchford18. from mullite creep data presented in the literature.9,12,13,20 These values were then used in an estimate of the creep rate interval of the mullite ceramic in the present investigation (d = 1.5m).18 The plots in Fig. 1a, based on data taken from Pitchford,18 show that the experimentally determined creep rates at 1300 ◦C, and at higher stresses at 1400 ◦C, were higher than the predicted values. 2.2. Mullite reinforced with 5 vol.% SiC nanoparticles The experimental creep rates of the nanocomposite at 1300 and 1400 ◦C are plotted in Fig. 1b. The creep tests at 1400 ◦C Table 1 The as-sintered and creep tested materials Material Test temperature (◦C) Stress (MPa) Steady-state creep rate (s−1) Grain size (m) Polycrystalline mullite As-sintered 1.5 1400 48.6 1.5 × 10−6 1.3 1400 13.0 1.2 × 10−7 1.5 1300 14.9 9.5 × 10−9 1.5 Mullite/SiC nanocomposite As-sintered 0.7 1400 50.0 1.9 × 10−6 0.7 1400 12.1 2.9 × 10−8 0.8 1300 14.4 3.2 × 10−9 0.8