In situ studies of the transmission of strain across grain boundaries

In situ nanoindentation in a transmission electron microscope has been used to study the transmission of strain across grain boundaries in ultrafine- and nanograined metals. Several different mechanisms are revealed. In materials with mobile grain boundaries, strain is accommodated by grain boundary movement or, more interestingly, by strain-induced grain coalescence (strain-induced coarsening). In materials with fixed grain boundaries strain is transmitted by dislocation transmission across the boundary or, more commonly, by triggering deformation in adjacent grains. Dislocations are also observed to annihilate at the boundary, leading to grain boundary sliding. These mechanisms are illustrated by direct observation of the nanoindentation of nanograined Al, ultrafine grained aluminum, dislocated lath martensite and milled, ultrafine-grained Fe.