Microstructural evolution and its implications on the mechanical properties during age hardening in Mg-1Al-0.28Ca-0.13Mn-0.05Si alloy

Abstract

A cast Mg-1Al-0.28Ca-0.13Mn-0.5Si (wt%) alloy has been subjected to solution-annealing at 723 K followed by aging treatment at 473 K. The specimens aged for 15 min, 60 min, and 900 min were considered as the under-aged, peak-aged, and over-aged specimens based on the microhardness variations. In comparison to the solution-annealed condition, the peak-aged specimen exhibits excellent strengthening i.e. ~87% increase in the yield strength and ~37% rise in the ultimate tensile strength, at the expense of ~41% decrease in the ductility. The evolution of nano-sized Al-Mn particles and coarse Ca-rich phases takes place with the aging of the alloy specimens. The Al-Mn phase has been identified as the Al8Mn5 phase with the aid of X-ray diffraction, scanning transmission electron microscopy, high-resolution transmission electron microscopy, and 3D atom probe tomography. The changes in the size, fraction, and nature of precipitate-dislocation interactions depending on the coherency of the nano-sized Al8Mn5 precipitates, are accountable for strengthening in the different aging stages of the developed alloy. Precipitate-dislocation interactions involving precipitate-shearing mechanism leads to the excellent strengthening in the peak-aged specimen. Contrarily, the evolution of the crack-initiating coarse (Mg,Al)2Ca phase leads to the loss of ductility in the peak-aged and over-aged specimens.