Mesoscopic scale modeling of “superplastic†flow in geological and glacial materials

A viewpoint that suggests that grain/ interphase boundary sliding (GBS) that develops to a mesoscopic scale (“cooperative boundary sliding”) controls optimal superplastic (SP) deformation is able to explain superplasticity in metals and alloys, ceramics, intermetallics, composites and bulk metallic glasses of grain sizes ranging from a few microns down to a few nanometers. It is extended here to describe grain-size-sensitive (GSS) flow in minerals, rocks and ice within narrow experimental ranges. In this approach the accommodation processes of grain boundary diffusion, dislocation emission from sliding boundaries and/ or grain rotation accompanying boundary sliding are present over extremely short distances and are assumed to be faster than GBS. Analysis shows that GSS creep in geological and glacial materials can be accounted for in terms of four “universal”, mesoscopic scale constants of average values, γ0= 0.197, γB = 0.415 J.m-2, N = 8.9 and a = 0.166, where γ0 is the average shear strain associated with a basic boundary sliding event at the level of the atomistics, γB is the specific grain boundary energy (assumed to be isotropic),N is the number of boundaries that align to form a mesoscopic boundary glide plane and “a” is a constant that obeys the condition 0<a<0.5, whose magnitude depends on grain shape and size distribution in the material. It is demonstrated that with the help of these four constants and the Frost-Ashby equations for estimating the shear modulus, it is possible to predict steady state GSS creep flow in any geological or glacial material, including those whose mechanical response was not used to obtain the “universal” constants. Whether these observations are evidence for “superplasticity” in these materials can be known only if the findings are reproduced in tensile deformation also.