Nickel with multimodal grain size distribution achieved by SPS: microstructure and mechanical properties

Using spark plasma sintering, bulk polycrystalline nickel samples are created from high-purity multimodal nickel powder. The resulting compacts yield relative mass densities of 95-97 %, depending on the sintering temperature. Microstructural investigations of the processed samples show a composite-like microstructure made of the following elements: multicrystalline agglomerates (5 < d < 120 mu m), isolated single crystalline and small aggregates (1 < d < 5 mu m) and an ultrafine-grained (UFG) component (d < 1 mu m). Adjusting the processing parameters allows full control of the microstructural characteristics, such as the average grain size and the volume fraction of the UFG component. The mechanical properties of the processed samples are investigated by compression tests conducted at room temperature at a strain rate of 2 x 10(-4) s(-1). The flow stress of the multimodal micrometre-sized nickel increases from similar to 100 to 400 MPa, depending on the microstructural characteristics, whereas the monomodal nickel has a flow stress of 395 MPa. Stress-strain plots reveal that there is a strong but brief strain hardening, followed by a plateau and/or a strain softening. The evolution of grain misorientation across boundaries after straining suggests the following: in the UFG components, deformation occurs mainly via a grain rotation mechanism, while the coarser microstructures experience deformation by a classical dislocation-based plasticity. Strain softening is more pronounced for samples with a large fraction of UFG components because the rotations of the grains may induce deformation incompatibilities, leading to microcracking at grain boundaries.