Heat treatment of AZ91D Mg-Al-Zn alloy: Microstructural evolution and dynamic response

Magnesium alloys are attracting great interest from the automotive industry because of the potential for weight reduction. An AZ91D cast alloy was studied in the current work to understand the effect of heat treatment on the microstructure and dynamic compressive properties. The selected heat treatments include solution treatment (T4) and solution treatment followed by aging (T6). The as-cast alloy microstructure consists of intermetallic β-phase (Mg ₁₇Al₁₂) precipitates surrounded by α + β lamellar eutectic in α-Mg solid solution. The AZ91D-T4 specimens showed small β-phase precipitates along the grain boundaries and regions of eutectic mixture. The T6 heat treatment causes the β-phase platelets in the α + β eutectic to grow and develop into β-precipitates. The difference in the phase morphology reflects into the mechanical properties. The Vickers hardness of the T6 heat-treated specimens was 3.6% higher than the as-cast alloy. The compressive yield strengths of T4 and T6 treated specimens were 1.3% and 43.1% higher than those of as-cast specimens. The high strain rate compression testing resulted in increase in the strength with strain rate for the T4 and T6 specimens. A maximum increase of 42% was observed in T6 specimen tested at a strain rate of 4,000/s in comparison to the quasi-static compression. Under high strain rate compression testing, the T6 heat-treated specimens showed failure of the β-precipitates resulting in increased energy absorption in comparison to the quasi-static compression.