Thermal Activation, Intermittency and Size Effects in the Plastic Deformation of Cast Aluminium Microwires

The microcasting process is a scaled-down investment casting process in which molten metal is pressure-infiltrated and directionally solidified in water-soluble moulds. It was previously developed to produce metallic microwires with a diameter (D) between 7 and 120 µm, characterized by a high surface quality and aspect ratio. In tension, the yield stress of aluminium (99.99%) microwires (initial dislocation density ~8*10^11 m-2) scales with the inverse of the microwire diameter and becomes highly stochastic at the smallest diameters. The plastic deformation behaviour is intermittent, suggesting that deformation progresses through the repeated activation and blockage of single-arm sources. The objective of this work is to contribute to the general understanding of plasticity size effects, using monocrystalline microcast wires. The focus is placed here on the contribution of thermal activation and initial dislocation content on the overall plastic deformation behaviour. In this frame aluminium (99.99% and 99.999%) and Al-2wt%Mg microwires with a diameter between 12-125 µm were produced and tested. Thermal activation was studied through (60 s) stress relaxations applied at different load levels throughout a tensile test. Single slip microwires (D