Plasticity Mechanisms in Laser Processed Al-Si Alloys with Heterogeneous Microstructures

A range of heterogeneous Al-Si microstructures, comprising of nanoscale Al-Si eutectic domains with nanotwinned Si fibers and sub-micron-scale primary Al, with or without Si nano-precipitates, were fabricated by processing as-cast Al-20wt.%Si alloy using laser rapid solidification. <i>In situ</i> tensile testing in a scanning electron microscope demonstrated high tensile strength, ≈600 MPa, and ductility, ≈10%, and high strain hardening rate, ≈7 GPa. Microstructural characterization revealed the plastic co-deformation mechanisms between soft Al grains and the surrounding relatively harder nanoscale Al-Si eutectic. The progression of plasticity in nanoscale Al-Si eutectic with increasing applied strain is accommodated by dislocation plasticity in the nano-Al channels and cracking in Si nanofibers. The propagation of nano-cracks is suppressed by surrounding Al, retaining good ductility of the sample. Cross-sectional scanning/transmission electron microscopy of nanoindents revealed a transition in morphology from high aspect ratio nano-fibrous Si to short nano-fibrous Si, preferentially located along triple junctions of the dynamically recovered sub-grains in Al. Molecular dynamics simulations of the interaction of glide dislocations in Al with the flat Si/Al interfaces revealed climb and cross-slip mechanisms but no direct slip transmission into Si. The role of the heterogeneous Al–Si microstructure in enhancing strain hardening rate and resulting in plasticity in micro-tensile sample even after fracture of nanoscale Si fibers is discussed by integrating experimental characterization with atomistic simulations and dislocation theory. This research is supported by DOE, Office of Science, Office of Basic Energy Sciences.