How do grain boundaries strain? New insights from ultrahigh temperature in situ TEM

Polycrystals exhibit high temperature interface mediated strain in response to externally applied stress during processes such as creep, superplasticity, or hot-press sintering, or an internal stress such as stress relaxation at interfaces during thermal cycling, oxide scale growth, or densification during sintering.  These processes strongly influence both materials processing and performance in extreme environments. Such problems have almost universally been analyzed and treated in the context of purely diffusional models. The diffusional flux, however, is one of only three necessary steps or conditions required for diffusional dependent interfacial strain, the other two include interfacial dislocation nucleation and the emission and absorption of point defects at the interfacial dislocations.  The prevalence of diffusional rate limited models results from somewhat unsubstantiated assumptions within the early literature along with the non-uniqueness of the various rate limiting kinetic models, i.e., the disparate models have non-unique solutions in many cases.   

Our group developed a laser heating-based approach for ultrahigh temperature in situ transmission electron microscopy (TEM) during small-scale mechanical testing.  This approach enables more direct characterization of interfacial strain kinetics and thermodynamics at individual grain boundaries, which provides an improved basis for evaluating the high temperature deformation mechanisms. This talk will present experiments that reveal grain boundary dislocation nucleation limits interfacial strain kinetics in many systems up to relatively large stresses, i.e., ≈108 Pa.  Based on the experimental observations, new models for sintering and grain boundary mediated creep are developed to account for the appropriate mechanism. These are demonstrated to fit experimental data well, predict broad trends in the literature, and provide explanations for several poorly understood phenomena within the sintering and creep literature.  The talk will conclude by discussing broader implications of this new paradigm for understanding interfacial strain kinetics.