Influence of Heat Treatment on the Microstructure Evolution and Creep Performance of Squeezed Cast AZ91-Based Alloys

Abstract

The study systematically examines the effect of heat treatment on the microstructure, hardness, and creep performance of squeeze-cast AZ91 (alloy A), AZ91-1Sr (alloy B), and AZ91-1Ca-0.6Sr (alloy C). The addition of Ca and Sr to the AZ91 alloy introduced thermally stable Al<inf>4</inf>Sr and Al<inf>2</inf>Ca phases alongside the unstable β-Mg<inf>17</inf>Al<inf>12</inf> phase, enhancing creep performance. Heat treatment significantly alters the microstructure of the alloys, completely dissolving β-Mg<inf>17</inf>Al<inf>12</inf>, leading to solid-solution strengthening in the Mg matrix. Meanwhile, the undissolved Al<inf>2</inf>Ca and Al<inf>4</inf>Sr phases provide additional precipitation strengthening, improving hardness and creep performance of the alloys. The modified morphology of the Al<inf>4</inf>Sr phase on heat treatment further benefits creep resistance. In the heat-treated alloys, the β-Mg<inf>17</inf>Al<inf>12</inf> re-precipitates near the grain boundaries during the creep deformation, further improving creep performance. Despite grain growth during heat treatment, solid-solution strengthening in alloy A and combined solid-solution and precipitation strengthening in alloy B improve hardness and creep performance. However, in alloy C, the suppression of β-Mg<inf>17</inf>Al<inf>12</inf> during solidification prevents solid-solution strengthening upon heat treatment. Consequently, undissolved precipitates alone fail to counteract grain growth, leading to reduced hardness and creep performance in alloy C. © 2025 Elsevier B.V., All rights reserved.