Quantitative tracking of grain structure evolution in a nanocrystalline metal during cyclic loading

Abstract

Molecular dynamics simulations were used to quantify mechanically-induced structural evolution in nanocrystalline Al with an average grain size of 5 nm. A polycrystalline sample was cyclically strained at different temperatures, while a recently developed grain tracking algorithm was used to measure the relative contributions of novel deformation mechanisms such as grain rotation and grain sliding. Sample texture and grain size were also tracked during cycling, to show how nanocrystalline plasticity rearranges overall grain structure and alters the grain boundary network. While no obvious texture is developing during cycling, the processes responsible for plasticity act collectively to alter the interfacial network. Cyclic loading led to the formation of many twin boundaries throughout the sample as well as the occasional coalescence of neighboring grains, with higher temperatures causing more evolution. A temperature-dependent cyclic strengthening effect was observed, demonstrating that both the structure and properties of nanocrystalline metals can be dynamic during loading.2

Pronounced grain boundary network evolution in nanocrystalline Cu subjected to large cyclic strains

“…However, in a nanocrystalline metal, it has been hypothesized that even modest plastic deformation can alter boundary character rather dramatically. For example, Panzarino et al showed that even small cyclic strains can cause nanocrystalline grains to rotate relative to their neighbors and create new twin boundaries [15]. Other proposed nanocrystalline deformation mechanisms could be associated with even more dramatic changes to the boundary network.…”

Concurrent transitions in wear rate and surface microstructure in nanocrystalline Ni-W

“…In addition, nanocrystalline metals have been reported to coarsen through stress-driven grain boundary migration [31,48,49], with such migration usually resulting in discontinuous grain growth [29]. Recent molecular dynamics simulations by Panzarino et al [50] also verify that grain rotation and coalescence can contribute to discontinuous grain growth during cyclic loading and that the magnitude of such coarsening increases with increasing number of applied loading cycles. Experimental observation of grain growth and its dependence on shear stress was also reported by Rupert et al [31], with higher stresses leading to more evolution.…”

“…However, grain growth can be accompanied by relaxation of nonequilibrium grain boundary structure, which significantly increases hardness [43]. Panzarino et al [50] also showed that strengthening can occur during cyclic deformation as a result of the formation of lower energy boundary configurations. For the gradient nanograined structure, only the coarsened grains near the surface have boundaries with relaxed structure, while the grains below remain close to the as-deposited state.…”

Fatigue Deformation of Polycrystalline Cu Using Molecular Dynamics Simulations

“…Recently, Panzarino et al [6] characterized the grain structure evolution during the cyclic deformation of polycrystalline Al having grain size of 5 nm for 10 cycles. The cyclic strengthening was associated with the grain rotation, grain growth and the formation of many twin boundaries [6]. Like polycrystalline Al and Ni, MD simulations on the polycrystalline Cu with 5.5 nm grain size revealed the occurrence of grain coarsening during the cyclic deformation [7].…”