Achieving good tensile properties in ultrafine grained nickel by spark plasma sintering

Abstract

In this work, the tensile properties of ultrafine grain Ni samples processed by spark plasma sintering are investigated. To this aim, a high purity Ni powder was nanostructured by ball milling using different processing conditions, and consolidation of the powder was performed by spark plasma sintering. Milling parameters were varied to produce samples with different microstructures. Each sample was characterised in terms of grain size and grain boundary character distribution to study the influence of milling parameters on the microstructure. Results show that suitable ball milling and subsequent sintering can be employed to obtain specimens with grain sizes in the ultrafine grain range with a high fraction of Σ3 grain boundaries. All samples exhibit a low internal stress state, which is evaluated in terms of grain orientation spread computed from electron backscatter diffraction and dislocation observation by transmission electron microscopy. Uniaxial tensile testing shows an increase in yield strength with grain refinement with a pronounced deviation from the Hall-Petch relation, for samples with grain sizes below 1.2 μm. Heterogeneity in deformation at yielding seems responsible for the deviation due to differences in the strain hardening behaviour between ultrafine grains and conventional coarse grains. All samples display high ductility with values of elongation to fracture above 35% as well as good toughness. The combination of ball milling with spark plasma sintering is therefore a promising tool to produce ultrafine grain Ni with good mechanical properties.