Superplasticity grain boundaries

More distinct regularities observed in the Al-phase allow us to distinguish dislocation slips expressed by the movement of orientation peaks along the fibres from the grain boundary sliding responsible for the smoothing effects and thus for the superplasticity of the alloy... 

It was pointed out by Nieh and Wadsworth [5] that fine grain size is a necessary but insufficient condition for HSRS. This conclusion resulted from the observation that many fine-grained composites are not superplastic at high strain rates. Evidently, in addition to grain size, microstructural factors, such as detailed structure and chemical composition at the reinforcement-matrix interfaces and grain boundaries, may play important roles. 

There apparently exists a critical amount of liquid phase for the optimization of grain/interface boundary sliding during superplastic deformation. The optimum amount of liquid phase may depend upon the precise material composition and the precise nature of a grain boundary or interface, such as local chemistry (which determines the chemical interactions between atoms in the liquid phase and atoms in its neighboring grains) and misorientation. The existence of an equilibrium thickness of intergranular liquid phase in ceramics has been discussed [14]. This area of detailed study in metal alloys has not been addressed. 

T. G. Nieh, J. Wadsworth, and K. Higashi, "High Strain Rate Superplasticity in Metals and Composites," in Transaction of the Materials Research Society of Japan Vol 16B - Composites, Grain Boundaries and Nanophase Materials, pp. 1027-1032, M. Sakai, M. Kobayashi, T. Suga, R. Watanabe, Y. Ishida, and K. Niihara ed., Elsevier Science, Netherland, (1994). 

It is noted, however, that both of the above CaF2 and Ti02 nanoceramics had some amount of porosity. This may account for an apparent soft behavior related to the superplastic deformation at low temperature, which does not yet reveal the plastic deformation characteristics in nanoceramics. Localized superplastic deformation under cyclic tensile fatigue tests was observed by Yan et al. on 3Y-TZP nanoceramics at room temperature [25], The micromechanism behind this phenomenon is argued to be essentially governed by grain-boundary diffusion. The contribution of dislocation slip might be in operation as a parallel mechanism to develop slip band-like microfeatures. 

Several parameters can influence strongly the superplastic behaviour of ceramics, i.e. the strain rate at which the material can be superplastically deformed. Between them can be mentioned the grain size, second phases and segregation of impurities at the grain boundaries, etc. 

At high temperatures the glassy phase may become less viscous and even liquid and as a consequence may account for the plastic deformation. However, viscous flow creep is not regarded as a viable creep mechanism for superplasticity due to its limited deformation, which corresponds to the redistribution of the glassy phase and therefore to the squeeze of these secondary phases from grain boundaries subjected to compression.8... 

The diffusion coefficients of the process controlling superplasticity may be enhanced or retarded by the addition of impurities or solute atoms or by the addition of secondary phases, normally used as sintering aids, which distribute along the grain boundaries and triple-point junctions of the grains. 

Finally, the forthcoming comprehension of superplasticity will demand a well-settled justification of the basic equations for this phenomenon. In order to achieve this goal, first-principles calculations should be conducted. This is a very challenging task, because the mechanical behaviour of grain boundaries requires an understanding of the physics involved at many different scales. At this point, simulations at microscopic as well as mesoscopic levels can become a useful tool. 

Ishihara, S., etal., Stereographic analysis of grain boundary sliding in superplastic deformation of alumina-zirconia two phase ceramics , Mater. Trans. JIM, 1999, 40, 1158-65. 

Langdon, T.G., The significance of grain boundary sliding in creep and superplasticity , Metal Forum, 1981, 4, 14-23. 

Nakatani, K., Nagayama, H., Yoshida, H., Yamamoto, T., and Sakuma, T., The effect of grain boundary segregation on superplastic behavior in cation-doped 3Y-TZP , Scrlpta Mater., 2003, 49, 791-5. 

The absence of a lattice-based mechanism, such as slip planes, does not necessarily preclude aU deformation in brittle materials. Plastic flow can proceed in other modes. For example, at temperatures of about 40 percent to 50 percent of their melting points, grain-boundary shding can become important. Grain-boundary sliding is beheved to be the major contributor to the superplasticity observed in some polycrystal-hne ceramics. 

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