<p>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 Al₄Sr and Al₂Ca phases alongside the unstable β-Mg₁₇Al₁₂ phase, enhancing creep performance. Heat treatment significantly alters the microstructure of the alloys, completely dissolving β-Mg₁₇Al₁₂, leading to solid-solution strengthening in the Mg matrix. Meanwhile, the undissolved Al₂Ca and Al₄Sr phases provide additional precipitation strengthening, improving hardness and creep performance of the alloys. The modified morphology of the Al₄Sr phase on heat treatment further benefits creep resistance. In the heat-treated alloys, the β-Mg₁₇Al₁₂ 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-Mg₁₇Al₁₂ 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.</p>