Grain Boundary Processes in Strengthening, Weakening, and Superplasticity

Grain boundaries provide strength to materials at low temperatures by impeding slip transfer and they weaken materials at high temperatures by intergranular creep processes such as grain boundary sliding (GBS) and diffusion creep. At very fine nanocrystalline grain sizes of <10 nm, it is possible to observe grain boundary weakening rather than strengthening at room temperature. Superplastic flow is observed in a wide range of materials, but the common phenomenology of a high strain rate sensitivity and extensive GBS may arise from varying importance of GBS accommodated by dislocations, diffusion creep, and interface-controlled diffusion creep. Thus, superplasticity in metals and ceramics involves GBS with dislocation accommodation and interface-controlled diffusion creep, respectively. It is recognized that grains retain their equiaxed shapes during diffusion creep by grain switching and grain growth. Ultrafine-grained and nanocrystalline materials with a high strain rate sensitivity of >0.3 deform with a large GBS contribution to total strain of approximate to 40-80%. There are a limited number of studies showing superplasticity in fine-grained high-entropy alloys, involving GBS accommodated by dislocations