Experimental and Modeling Study of Microstructure and Interface Effects on Mechanical Behavior of Al-Si Alloys

Wenqian Wu, University of Nebraska-Lincoln

Copyright 2023, Wenqian Wu

Abstract

The strength–ductility trade-off is a persistent challenge in materials science. In metallic materials, the incorporation of hard phases or the refinement of characteristic sizes tends to enhance strength but often limits plasticity. This dissertation illustrates that refining the heterogeneous microstructure containing disparate soft/hard phases together with control of phase morphology and interface structure can achieve a synergistic enhancement of strength and plasticity, exemplified by aluminium-silicon (Al-Si) alloys. Cast Al-Si alloys with coarse Si flakes usually exhibit low strength (< 200 MPa) and limited ductility (< 5%) associated with interface cracking/decohesion and even brittle fracture of Si flakes. That limits their potential as structural materials. The weak strengthening effect of coarse Si flakes can be ascribed to the thermomechanical properties of Al/Si interfaces, as revealed by atomistic simulations in this work. Low shear resistance of the interfaces, coupled with low formation and migration energies of point defects in the interfaces, leads to readily happen of interface sliding or shear under mechanical loading or during dislocation-interface interactions. Laser rapid solidification (LRS) was applied to refine the microstructures of as-cast Al-Si alloys and modify the coarse Si flakes to nanofibers, yielding a remarkable enhancement in both strength and plasticity. In situ tension tests on the heterogeneous microstructure, comprising of sub-micron-scale Al dendrites and nano-fibrous Al-Si eutectic, exhibited high tensile strength (~600 MPa) and ductility (~10%) and high strain hardening rate (~7 GPa). Deformation mechanisms that account for the improved mechanical properties were explored by integrating electron microscopy characterization and atomistic modeling. Deformation incompatibility between soft Al and hard Si is accommodated by dislocation-mediated plasticity in Si nanofibers and formation of discrete nano-cracks, enabling continuous plastic deformation of the LRS Al-Si alloys. The fundamental understanding of plastic flow across interfaces separating disparate Al/Si phases can broadly apply to metallic composites reinforced with hard crystalline phases, offering insights into the design of ultra-high strength alloys (e.g., > 1 GPa for Al-alloys) with desired plasticity.