Controlling factors in tensile deformation of nanocrystalline cobalt and nickel

Abstract

In an effort to understand and enhance the tensile ductility of truly nanocrystalline metals, we have investigated and compared the mechanical behavior, especially the tensile behavior, of hcp nanocrystalline cobalt (~20 nm) and fcc nanocrystalline nickel (~28 nm). Although both materials exhibit obvious plasticity in tension, their uniform tensile ductility, tensile elongationto-failure, and fracture behavior are drastically different. In-situ synchrotron x-ray diffraction and ultra-small angle x-ray scattering reveal distinct deformation disparity in terms of residual strain development, texture evolution, nanovoid formation, and subsequent strain hardening and strain rate hardening behavior. The dependence of tensile property on the strain rate and temperature is examined and discussed. Factors that influence the strength and ductility of nanocrystalline metals are considered and prioritized according to the current findings. A new Hall-petch relationship is proposed for nanocrystalline nickel.

Twinning/detwinning-mediated grain growth and mechanical properties of free-standing nanotwinned Ni foils: Grain size and strain rate effects

“…5(c). A comparison of the present results with the reported yield strength collected from the literatures [54][55][56][57][58][59][60][61][62] demonstrates that the embedding of nanotwins into UFG metals indeed enhances the strength significantly at a similar testing condition, see Fig. 5(d).…”

Compressive behavior and deformation kinetics of ultrafine grained aluminum processed by equal channel angular pressing

“…Similarly, the grainboundary shear could also be ruled out since it bears the same order of activation volume with Coble creep [56,63]. More importantly, recent analysis presented evidence that such diffusion-assisted GB processes are more likely operate at very low rates and high temperatures, especially in metals with grain size below tens of nanometers [3,14,64,65]. That is the grain size of current UFG-Al seems not small enough to activate these diffusion-assisted GB processes.…”

Geometric effects on the mechanical strengths of strong nanocrystalline rhodium sub-micron structures

“…Smallscale mechanical testing specimens fabricated via this 'touch-less' approach are free of mechanical-or ion-induced damage to the pillar surfaces. It has been suggested by Wang et al [36] that fabrication-induced mechanical damage to the specimen surface plays a critical role in the measured mechanical properties of nanocrystalline metals. As well, ion-induced damage in small-scale metallic pillars fabricated by conventional focused ion beam (FIB) milling has routinely been shown to influence the observed mechanical behaviour [37][38][39][40][41].…”

“…These flow stress values are significantly greater than cold rolled bulk scale rhodium specimens, which display mechanical strengths between 1.38 and 2.07 GPa [32]. One possible explanation is the 'touch-less' fabrication method used for these rhodium specimens may also reduce mechanical damage to the pillar surfaces, which can influence the measured strength of nanocrystalline specimens [36]. Another explanation may be the dominance of the Hall-Petch strengthening mechanism due to the nanocrystalline microstructure of these pillar specimens.…”