Plastic strain triggers structural instabilities upon cyclic loading in ultrafine-grained nickel

Abstract

Grain growth accompanied by shear band formation shortens lifetime of nanostructured metals upon cyclic loading. Although the occurrence of structural instabilities was reported frequently, the difficulty to detect and track their initiation and evolution using standard testing routines prevents an in-depth understanding of the underlying mechanisms. Usage of samples from different synthesis routes tested under varying conditions further complicates this issue. Here, cyclic high pressure torsion is presented as an alternative method to mimic low cycle fatigue. It allows to study initiation and evolution of structural instabilities reproducibly up to enormous accumulated strains, not accessible in conventional fatigue tests. It enabled a general understanding of the processes causing structural instabilities in nanostructured nickel tested with different parameters such as strain amplitude or temperature. Grain coarsening starts from the very first cycles and initiates strain localization in shear bands. Accumulation of cyclic strain induces progressive growth of the shear band thickness accompanied by further grain growth within these bands. Clearly, cyclic strain amplifies grain coarsening suggesting that not the applied stress alone forces boundary motion. This is emphasized further as preferential texture components which facilitate cyclic slip evolve. Although the imposed cyclic strain drives grain growth it stagnates at certain grain sizes. Experiments at 77 K revealed identical instabilities, proving that for nickel boundary migration occurred predominantly mechanically driven.