Plastic deformation induced microstructure transition in nano-fibrous Al-Si eutectics

Abstract

Indentation plastic zone of Al-20 wt% Si laser rapid solidified (LRS) alloy with nanoscale, fibrous, fully eutectic microstructure is characterized using a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM) and scanning/transmission electron microscopy (S/TEM). In contrast to cracking around the indents in the as-cast Al-Si alloy, the LRS nano-fibrous eutectic microstructure demonstrated higher strength, strain-hardening, and uniformly distributed plasticity around indents with no cracks. A transition in morphology from long nano-fibrous Si to short nano-fibrous Si is observed underneath the indent in LRS eutectics. The mechanism for this morphological evolution with increasing plastic strain is postulated to involve three stages of the deformation process: First, the formation of closely-spaced dislocation arrays in the nano-channels of Al matrix confined by Si fibers. Second, fiber segmentation when the stress field of the dislocation array exceeds the critical stress for fracture in Si nano-fibers. Third, recovery of dislocation substructures and sub-grain formation in Al leading to a rotation of fractured Si segments. The final microstructure consisted of short Si nano-fibers along triple junctions of the subgrains in Al. The implication of this unusual microstructure evolution with increasing plastic strain on the plastic co-deformation in high-strength LRS nano-eutectic microstructures was discussed in this report.