Mesoscopic Scale Modeling of 'Superplastic' Flow in Geological and Glacial Materials

摘要

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, γ_0= 0.197, γ_B =0.415 J.m-2, N = 8.9 and a = 0.166, where γ_0 is the average shear strain associated with a basic boundary sliding event at the level of the atomistics, γ_B 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