Emergence of localized plasticity and failure through shear banding during microcompression of a nanocrystalline alloy

Abstract

Microcompression testing is used to probe the uniaxial stress-strain response of a nanocrystalline alloy, with an emphasis on exploring how grain size and grain boundary relaxation state impact the complete flow curve and failure behavior. The yield strength, strain hardening, strain-to-failure, and failure mode of nanocrystalline Ni-W films with mean grain sizes of 5, 15, and 90 nm are studied using taper-free micropillars that are large enough to avoid extrinsic size effects. Strengthening is observed with grain refinement, but catastrophic failure through strain localization is found as well. Shear banding is found to cause failure, resembling the deformation of metallic glasses. Finally, we study the influence of grain boundary state by employing heat treatments that relax nonequilibrium boundary structure but leave grain size unchanged. A pronounced strengthening effect and increased strain localization is observed after relaxation in the finer grained samples.

Nanocrystalline grain boundary engineering: Increasing Σ3 boundary fraction in pure Ni with thermomechanical treatments

“…Hasnaoui et al [6] used molecular dynamics to show that shear strain can concentrate in the random boundaries, which resist sliding less strongly than low angle boundaries. Experimental studies support these observations, with boundary relaxation found to increase strength but also promote shear localization [11,12]. In addition to mechanical properties, it has become evident that grain boundary network characteristics are closely tied to the thermal stability of nanostructured materials.…”

Annealing effect on plastic flow in nanocrystalline CoCrFeMnNi high-entropy alloy: A nanomechanical analysis

“…9, all nc HEA samples (HPT, HPT+1A, and HPT+10A) exhibit strain softening behavior, which is opposite to the work hardening of the cg (as-cast) sample. Strain softening phenomena have been frequently observed in tensile and compressive tests of nc and ultrafine-grained (ufg) metals and alloys [40][41][42][43]. Recently, this softening behavior was also reported for HPT processed nc CoCuFeNi HEA [16].…”

Local Crystallographic Orientation Correlation Measurements Connecting the Processing and Properties of Face-Centered Cubic Metals

“…Hasnaoui et al [152] used molecular dynamics to show that shear strain can concentrate in the random boundaries, which resist sliding less strongly than low angle boundaries. Experimental studies support these observations, with boundary relaxation found to increase strength but also promote shear localization [233,234]. In addition to mechanical properties, it has become evident that grain boundary network characteristics are closely tied to the thermal stability of nanostructured materials.…”