Strain rate sensitivity of bulk multi-phase nanocrystalline Al-W-based alloy

Abstract

High-energy ball milling of conventional coarse-grained aluminium and nanocrystalline W in an Al-10 at.%W composition results in the formation of a two-phase mixture of Al and W with nanocrystalline features. Subsequent compaction of these powders using spark plasma sintering (SPS) at 748 K resulted in the formation of an Al12W phase in the nanocrystalline aluminium matrix. It is suggested that the mere attainment of nanocrystallinity was not enough to trigger a reaction between Al and W to form Al12W but that sufficient thermal activation was also required, as supplied during SPS. The second-phase particles (~175 nm in size) are uniformly distributed in the nanocrystalline Al matrix having a grain size of ~40 nm. The nanocomposite possessed a high hardness of 5.42 ± 0.33 GPa and an elastic modulus of 145 ± 5 GPa, both measured using depth-sensing nanoindentation. At room temperature, this novel nanocomposite exhibited a strain rate sensitivity (SRS) of 0.024 ± 0.001 and an activation volume in the range of 3.78-3.88 b3. Interfacial regions, viz. grain boundaries and triple junctions in the matrix and the reinforcement, matrix/particle boundaries, etc. could be influential factors in deciding the SRS and the activation volume. A scanning probe microscope image of the nanoindent shows a plastic flow region around the periphery of the indent.