MICROSTRUCTURAL EVOLUTION AND THERMAL STABILITY ANALYSIS OF AZ91 MAGNESIUM NANOCOMPOSITES UNDER CONTROLLED THERMAL EXPOSURE

Abstract

This study investigates the microstructural evolution and thermal stability of AZ91 magnesium nanocomposites under controlled thermal exposure conditions. The purpose of this work is to evaluate the effectiveness of nanoparticle reinforcement in enhancing microstructural stability, phase retention, and resistance to thermal degradation in AZ91 magnesium alloy for elevated-temperature applications.AZ91-based nanocomposites and unreinforced AZ91 alloy were subjected to controlled thermal exposure at elevated temperatures for different durations. Microstructural characterisation was carried out using optical microscopy, scanning electron microscopy, and transmission electron microscopy to analyse grain evolution, phase morphology, and interfacial stability, with particular emphasis on the thermally sensitive β-Mg₁₇Al₁₂ phase. The results demonstrate that the unreinforced AZ91 alloy undergoes significant grain growth and β-phase coarsening during thermal exposure. In contrast, the AZ91 nanocomposites exhibit refined microstructures, suppressed grain growth, and improved stability of the β-Mg₁₇Al₁₂ phase. These improvements are attributed to the grain boundary pinning effect and diffusion-restricting behaviour of nanoparticles, which delay microstructural degradation under thermal loading. In conclusion, nanoparticle reinforcement significantly enhances the thermal stability of AZ91 magnesium alloy by stabilising microstructural features and retarding phase degradation during prolonged thermal exposure. The findings confirm that AZ91 magnesium nanocomposites are promising candidates for applications requiring reliable performance in thermally demanding environments.