Microstructure And Mechanical Behavior Of Electrodeposited Bulk Nanocrystalline Ni And Ni-Co Alloy

Nanocrystalline (nc) materials have been a focus of research in the field of materials science due to their special physical, chemical and mechanical properties different from the conventional coarse-grained (CG) counterparts. In term of the mechanical property, nc materials hold extreme high strength and hardness, resulting in a promising structural application. Unfortunately, nc materials often exhibits a very low ductility (typically less than 4%), which in a way limits their valuable application. Except for their low ductility, nc materials also exhibits a special phenomena, such as high strain rate sensitivity and inverse Hall-Petch relation, which implies nc materials can hold a deformation mechanism different from CG counterparts. However, to now, the deformation mechanism of nc materials is not well understood. So, optimizing the mechanical properties of nc materials and understanding their deformation mechanical become very important for actual application and scientific study.Experimental studies demonstrates some extrinsic factors including the immature technology, impurity and the nonstandard dimension can disturb the true evaluation on the mechanical properties of nc materials. To avoid these effects, in present work, we fabricate some high purity and bulk nc Nis with different mean grain size and a Ni-Co alloy in an electrolyte that is scientifically purified by reasonably controlling the content of additives and current density. The max dimension of specimen is up to about 7～8 mm in thickness. Microstructures and mechanical properties of these nc Ni and Ni-Co alloy were extensively studied by X-ray diffractmeter (XRD), transmission electron microscope (TEM), MTS tensile testing system and scanning electron microscope (SEM) etc. The main results are shown as follows:1. An electrodeposited nc Ni with a broad grain size distribution from 10-160 nm and slightly (200) crystallography texture was made. The tensile tests revealed that nc Ni holds high strength of 1440～1916 MPa in ultimate tensile strength (σUTS) and good ductility of 6.2%～11.3% in elongation to failure (δETF), which strongly depended on the strain rate. With decreasing the strain rate, the strength decreases and the ductility increases. This nc Ni exhibits two distinguished strain rate sensitivity (m) and activity volume (v). Compared with the normal and high strain rates, nc Ni holds a higher m and lower v at the low strain rates, which implies a change from the single dislocation-based deformation to the dislocation and grain boundary (GB)-mediated deformation. The fracture surface morphology changes from the dimple structure at high strain rate to the hunch structure at low strain rate, which further confirm the change of deformation mechanism. At the low strain rate, the high ductility of nc Ni is attributed to the dislocation and GB-based harmonious action.2. Six electrodeposited Nis with a different average grain sizes from 256 nm to 16 nm were fabricated by adjusting the content of saccharine and density. With decreasing grain size, the growth orientation of the deposits changed from a preferred (200) crystallographic texture to the random. There exists an optimized grain size region of 28～41 nm where nc Ni holds highσUTS of 1440～1916 MPa and goodδETF of 6.2%～11.3%. In such a grain size area, the good ductility is attributed to the intrinsic dislocation-based deformation and themselves inhomogeneous microstructure, which induces some extra strain hardening ability. When the grain size of nc Ni is reduced to 22 nm and 16 nm, the strength and ductility both decrease, which is followed by the change in the fracture surface morphology from the typical dimples to the"cup"and"cone"structure. The low ductility is attributed to the restrained dislocation activity and insufficient GB activity in such a small size area.3. An electrodeposited nc Ni-24.7%Co alloy with a mean grain size of 15 nm and a face centered cubic (fcc) structure is fabricated. Unlike pure nc Nis whose grain size locates the critical region, tensile tests shows the present nc Ni-Co exhibits not only high strength of 1813～2232 MPa inσUTS but also good ductility of 6～9.6% inδETF. The addition of element Co decreases the stacking fault energy (SFE) of nc Ni, which improves the strain hardening ability and thus the ductility. The high strength is attributed to the fine small grain size and solid solution hardening effect. Based on the attained m (0.029) and v (13b3), the dislocation motion should be responsible fro the plastic deformation of the nc Ni-Co alloy. 4. Bulk electrodeposited nc Ni with a thick of about 7～8 mm is fabricated. XRD analysis and TEM observation imply nc Ni holds an evident anisotropy in grain growth. Compressive tests demonstrate the grain orientation markedly affect the mechanical property of nc Ni. When the loading axis is perpendicular to the growth direction, nc Ni exhibits a high yield strength (σY) of 1875～2315 MPa and aδETF of 3.5%～7%. However, when the load was applied along the growth direction, the totalδETF is increased to 7%～18.5% and theσY is increased to 1974～2545 MPa. The improved ductility and strength is attributed to the active GB sliding and efficient dislocations motion, which accords with the attained higher m and lower v when the load axis is parallel to the growth direction. Under two loading modes, the nc Ni specimens fail approximately 45°to the loading direction and their fracture surface morphology are consist of the dimples area and lacerated area. When the load was applied along the growth direction, the proportion of dimples area is evidently improved, which is relative to the active GB sliding and efficient dislocations motion.5. The continuous tensile (CT) test and intermittent tensile-relaxation cycle (TRC) test were performed on nc Ni (27 nm) and a CG Ni (2μm) to explore the deformation behavior of nc Ni. When deformed by the TRC test, the ductility of nc Ni is markedly increased from the 6.5%～10% attained under the CT test to 10.7～12% and its strength almost keep consistent. While for CG Ni, under the same two tensile modes, its strength and ductility both have no changes. The large time-dependent decay in flow stress during relaxation and the reloading tensile track deviating from the origin one demonstrates the nc Ni has not a permanent dislocation network. The improved ductility for nc Ni is attributed to the release of large internal stress and the regained new strain hardening ability upon reloading.