Q. Tai. A. Mocellin/Ceramics International 25(1999)395-408 defects may control the creep deformation. This is also been proposed by a number of authors but are not termed interface reaction-controlled diffusion creep. to be reviewed here Because this process involves diffusion of vacancies, the The lattice mechanisms can be briefly subdivided into grains are also elongated along the tensile axis In grain two categories [12]: dislocation climb and glide con- boundary sliding and grain rearrangement, there are trolled by climb on the one hand and dislocation climb several different models(Lifshitz sliding, Rachinger on the other(Fig 4). For the dislocation climb and glide sliding, Ashby and Verrall model, Gifkins model, etc. ) controlled by climb mechanism, in ceramics, the anion Lifshitz sliding occurs naturally as part of diffusion cation ratio ra/rc is < 2, and the ceramics must have five creep, to maintain grain contiguity, the grains elongate independent slip systems. While for the dislocation along the tensile axis and maintain their adjacent climb from Bardeen-Herring sources, the ra/rc ratio is neighbours, so there is no increase in the number of >2, and the ceramics are either lacking some slip sys grains lying along the tensile axis. In contrast, during tems, or if five independent systems are available, not all Rachinger sliding, the grains slide, rearrange and retain may be active simultaneously [12]. The dislocation creep there is an increase in number of grains along the tensile tensile axis as weave the elongation of grains along the their original shapes, but exchange their neighbours, so mechanisms involve axis. In Ashby and Verrall model, which is two-dimen sional, during the deformation process, grains suffer a 2.3. Methods to identify deformation mechanism transient but complex shape change by diffusional transport Fig. 2). While in the Gifkins model, which In general, several mechanisms may contribute to the may be viewed as three-dimensional, during the defor- creep deformation at elevated temperature, but creep is mation process, grains move apart by grain boundary usually controlled by only one of these mechanisms. To sliding caused by the motion of grain boundary dis- identify which mechanism is dominant, there are several locations, resulting in a gap between the grains. When methods the gap is large enough, it is filled by an emerging grain Ashby [13]constructed deformation-mechanism maps from one layer to the next(Fig. 3)[10]. In these two by using rate-equations and sufficient data on the models, the grains almost retain their original shapes, materials. These maps for example show the fields of and there is an increase in number of grains along the stress and temperature in which each independent tensile axis. In the cavitation creep and microcracking, mechanism for plastic deformation is dominant. Knowl- extensive cavities form. They grow and link up forming edge of any two of the three variables(stress, temperature microcrack by grain boundary sliding. The principal and strain-rate) locates a point on the map, identifies mode of deformation is a damage mechanism. Varia- the dominant mechanism or mechanisms and gives the tions or refinements of the previous basic models have value of the third variable. Mohamed and Langdon [14] constructed deformation-mechanism maps with grain size as a variable. based on the deformation -mechanism maps constructed by Ashby and Mohamed et al., Heuer et al. [9], for example, have devised a stress-grain size deformation mechanism map for MgO-doped Al2O3 at 1500C(Fig. 5). They suggested that diffusional defor mation dominated for most grain sizes of interest, but Fig. 2. Grain boundary sliding and grain rearrangement by diffusion shby and verrall model)[10] 双 Fig 3. Grain boundary sliding and grain rearrangement. A gap forms between four grains and is filled by an emerging grain(Gifkins model) Fig 4. Dislocations move by (a)climb, (b)climb and glide.defects may control the creep deformation. This is termed interface reaction-controlled di€usion creep. Because this process involves di€usion of vacancies, the grains are also elongated along the tensile axis. In grain boundary sliding and grain rearrangement, there are several di€erent models (Lifshitz sliding, Rachinger sliding, Ashby and Verrall model, Gifkins model, etc.). Lifshitz sliding occurs naturally as part of di€usion creep, to maintain grain contiguity, the grains elongate along the tensile axis and maintain their adjacent neighbours, so there is no increase in the number of grains lying along the tensile axis. In contrast, during Rachinger sliding, the grains slide, rearrange and retain their original shapes, but exchange their neighbours, so there is an increase in number of grains along the tensile axis. In Ashby and Verrall model, which is two-dimen￾sional, during the deformation process, grains su€er a transient but complex shape change by di€usional transport Fig. 2). While in the Gifkins model, which may be viewed as three-dimensional, during the defor￾mation process, grains move apart by grain boundary sliding caused by the motion of grain boundary dis￾locations, resulting in a gap between the grains. When the gap is large enough, it is ®lled by an emerging grain from one layer to the next (Fig. 3) [10]. In these two models, the grains almost retain their original shapes, and there is an increase in number of grains along the tensile axis. In the cavitation creep and microcracking, extensive cavities form. They grow and link up forming microcrack by grain boundary sliding. The principal mode of deformation is a damage mechanism. Varia￾tions or re®nements of the previous basic models have also been proposed by a number of authors but are not to be reviewed here. The lattice mechanisms can be brie¯y subdivided into two categories [12]: dislocation climb and glide con￾trolled by climb on the one hand and dislocation climb on the other (Fig. 4). For the dislocation climb and glide controlled by climb mechanism, in ceramics, the anion/ cation ratio ra/rc is<2, and the ceramics must have ®ve independent slip systems. While for the dislocation climb from Bardeen±Herring sources, the ra/rc ratio is >2, and the ceramics are either lacking some slip sys￾tems, or if ®ve independent systems are available, not all may be active simultaneously [12]. The dislocation creep mechanisms involve the elongation of grains along the tensile axis as well. 2.3. Methods to identify deformation mechanism In general, several mechanisms may contribute to the creep deformation at elevated temperature, but creep is usually controlled by only one of these mechanisms. To identify which mechanism is dominant, there are several methods. Ashby [13] constructed deformation-mechanism maps by using rate-equations and sucient data on the materials. These maps for example show the ®elds of stress and temperature in which each independent mechanism for plastic deformation is dominant. Knowl￾edge of any two of the three variables (stress, temperature and strain-rate) locates a point on the map, identi®es the dominant mechanism or mechanisms and gives the value of the third variable. Mohamed and Langdon [14] constructed deformation-mechanism maps with grain size as a variable. Based on the deformation-mechanism maps constructed by Ashby and Mohamed et al., Heuer et al. [9], for example, have devised a stress-grain size deformation mechanism map for MgO-doped Al2O3 at 1500C (Fig. 5). They suggested that di€usional defor￾mation dominated for most grain sizes of interest, but Fig. 2. Grain boundary sliding and grain rearrangement by di€usion (Ashby and Verrall model) [10]. Fig. 3. Grain boundary sliding and grain rearrangement. A gap forms between four grains and is ®lled by an emerging grain (Gifkins model) [10]. Fig. 4. Dislocations move by (a) climb, (b) climb and glide. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 397