Microstructure evolution and high density of nanotwinned ultrafine Si in hypereutectic Al-Si alloy by laser surface remelting

Abstract

The microstructures of hypereutectic Al-Si alloy that were fabricated by laser surface remelting (LSR) have been systematically characterized for four melt pools created at different scanning speeds using scanning electron microscopy (SEM) and scanning/transmission electron microscopy (S/TEM). The composition of the Al-Si alloy was modified from Al-20 wt%Si in as-cast sample to approximately Al-16 wt%Si in the LSR region. Three distinct microstructures were observed in LSR Al-Si alloy: (i) ultrafine-scale fully eutectic; (ii) heterogeneous (bimodal) structure with ultrafine-scale Al-Si eutectic embedded in coarse, primary Al matrix; (iii) the same structure as (ii) but with nanoscale faceted Si precipitates embedded in Al matrix. The volume fraction of fully eutectic region increases as scanning speed increases. The Si fibers have diameters ranging from 45 to 65 nm, much finer than Si flake in the as-cast sample (1.3 ± 0.4 μm). More surprisingly, a high density of nanoscale twins formed in the Si fibers at all scanning speeds in laser-refined eutectic, in contrast to the rare occurrence of twinned Si in as-cast alloy. The key differences in the LSR microstructures and crystallographic orientation relationships compared to traditional casting techniques are discussed.