Compressive Deformation Behavior And Microstructure Evolution Of Electrodeposited Bulk Nanocrystalline Ni

In the late-1980s, Prof. Gleiter made the visionary argument that metals and alloys, ifmade nanocrystalline (NC), would have a number of appealing mechanical characteristics ofpotential significance for structural applications. This provocative thought has opened up awhole new area of the research of metallic materials. The past two decades have witnessed asurge in research effects through the whole materials research community addressed uponNC metals and alloys. This is mainly because NC metals and alloys are highly desirablenot only for technological applications exploiting their compelling mechanical properties,such as ultra-high yield and fracture strength, superior wear resistance and low-temperaturesuperplastic formability; but also for scientific research as they offer new opportunities toinvestigate deformation behaviors at an extremely fine microstructural scale. Despite someuseful from both experimental and computational investigations, some critical issues remaincontroversial and need to be further explored. First of all, due to the synthesis limitations,most of the mechanical test on NC metals so far carried are limited to tensile test with verythin samples. The early necking and subsequent limited ductility revealed by NC metals intensile tests severely restrict the experimentally accessible parameter space. As a result, theeffect of large deformation on NC metals has not been explored to develop a lucid under-standing. More than that, in comparison with the inspiring progress that has been made inboth achieving extraordinary mechanical properties and uncovering unique deformation be-haviors in nano-materials, their fracture behaviors are only beginning to be understood. Inparticular, such understanding should inevitably include comprehensive knowledge of initia-tion, growth and coalescence of voids, which dominates the ductile fracture ofcoarse-grained (CG) metals and has been proven to also play a critical role in controlling thefailure of NC metals and alloys. Nevertheless, how such voids initiate and evolve in NCmetals seems a problem being laid dormant and few experimental studies have been ad-dressed upon this issue.In this work, to address upon these fundamental issues, we chose bulk NC Ni asmodel materials and systematically investigate their deformation behavior and microstruc-ture evolution during large-strain compression. The morphologies and microstructures of the NC Ni samples before and after deformation were extensively studied by X-ray diffractmeter(XRD), transmission electron microscope (TEM) and high-resolution TEM (HRTEM), scan-ning electron microscope (SEM) and electron backscattered diffraction-transmission Kiku-chi diffraction technology (EBSD-TKD) etc.A surfactant-assistant direct-current electrodeposition technique was proposed and ex-ploited to synthesize bulk NC Ni used in the present study. By reasonably controlling thecontent of additives and current density, high-purity, fully-dense bulk NC Ni sheets withmaximum dimension of up to7~8mm in thickness were successfully fabricated. The NC Niconsisted of roughly equiaxed grains with random orientations and the average grain sizewas estimated to be20±6nm. Most of the nano-grains were found to be separated byhigh-angle GBs whereas no GB phases (e.g. amorphous layers) were detected. A high purityof the as-deposited NC Ni (99.92%, weight percent) was revealed by the chemical analysis.Room-temperature uni-axial quasi-static compression tests were carried out on a MTS-810mechanical testing system in both unconfined and confined manners to introduced largeplastic strain in the range of0.36~1.2, which enables systematic exploration of the deforma-tion mechanisms and microstructure evolution over a relatively wide range of strain.Over the strain rate range from10-5s-1to10-1s-1(but excluding10-3s-1), the NC Ni exhi-bited remarkably enhanced plasticity: the compressive strain to failure reaches~0.3at allstrain rates, far beyond that can be obtained in tension. Also, the NC Ni exhibited ultra-highstrength with the maximum compressive strength ranging from2197MPa to2490MPa. Wedemonstrated, for the first time, significant intra-grain dislocation accumulation, as well aspropensity of deformation twinning and faulting in a NC Ni deformed at room temperatureand under conventional uniaxial loading. The equilibrium to non-equilibrium transforma-tions of GBs and TBs, induced by large plastic deformation, were also observed. Such trans-formations on one hand promote PDMPs; on the other hand, causes switch of the dominantmicroscopic thermal-activated process at different stages of deformation, which was mani-fested as the varying activation volume and strain rate sensitivity index at the different stagesduring deformation.We present experimental evidence of localized solid-state amorphization in bulk nano-crystalline nickel introduced by quasi-static compression at room temperature.High-resolution electron microscope observations illustrate that nano-scale amorphousstructures present at the regions where severe deformation occurred, e.g. along crack paths orsurrounding nano-voids. High densities of GBs and dislocations, both of which are intro- duced by localized heavy plastic deformation, contribute significantly to destabilizing thecrystalline structure and to driving the solid-state amorphization. Upon such a scenario, na-nocrystalline structures facilitate the c-a transformation both thermodynamically and kineti-cally. It is noteworthy that the grain-refinement-induced and disloca-tion-accumulation-induced amorphization operate concomitantly throughout the sample, butit is possible that, for a given region in the sample, only one of the two mechanisms domi-nates the c-a transformation, probably depending on the local stress state and strain condition.Amorphization might be the intrinsic structural feature in NC metals and alloys that weredeformed to large strains.Extensive formation of nano-voids, which was primarily attributed to the severe plasticincompatibility in NC metals, was identified during large-strain compression of NC Ni. De-tailed postmortem TEM and HRTEM observations, together with MD simulation results,have drawn a conclusive picture concerning nano-void evolution in NC metals: the void in-itiation preferentially take place and TJs and GBs, and then dislocation emission and absorp-tion are responsible for the outward flux of matter, promoting their growth, until they coa-lescence into larger ones. The local stress/strain conditions were identified to significant in-fluence the nucleation and evolution of the nano-voids. Moreover, the impurities introducedduring electrodeposition, in particular the hydrogen, also play a crucial role in “catalyzing”the nucleation of the nano-voids. In such a case, vacancy diffusion may also be in part re-sponsible for the void expansion.We provide compelling experimental evidence for a novel failure mode in NC Ni in-volving deformation-induced amorphization during large strain plastic deformation. Thefailure of the NC Ni was found to be somewhat strain-dependent. When deformation is rela-tively small (ε=0.3), the sample fractured into two parts by sliding one part relative to anoth-er along the plane inclined approximately45°relative to the compression axis. Signatures ofmicro-void coalescence fracture, intergranular fracture and shear-slipped fracture can beidentified during TEM and SEM investigations. But when large plastic deformation was im-posed (ε=1.2), the generation of large amount of relatively “weak” amorphous structuresprovide preferable sites for crack initiation and easy means for cleavage fracture, giving riseto the change in the fracture morphology and thereby fragmentation-type failure.