Effect of heat treatment on the precipitation of Mg17Al12 in Mg-7Al alloy

Abstract

Mg–7Al alloy is synthesized using a protective atmosphere electrical resistance furnace in which both the melting and casting compartments have provision for operation under a protective atmosphere. The maximum solid solubility of Al in Mg is around 12 wt% at 437 °C and is decreased to 0.1 wt% at room temperature as dictated by the phase diagram. Microstructural observation of the cast structure confirmed that most of the aluminium present in the alloy forms a eutectic β phase segregated around the grain boundaries of α-magnesium and hampered the mechanical properties significantly. To understand the precipitation behaviour, the Mg alloy is heat treated employing three sets of heat treatment cycles. The cast Mg alloy is solutionized at 420 °C for 24 hrs and then quenched in ice water at 7 °C to achieve a supersaturated solid solution (SSSS). The quenched samples are aged at 350 °C, 227 °C, and 150 °C for 8, 10, and 12 hrs respectively. The microstructure confirms that the alloy aged at 350 °C, forms continuous precipitation of β-phase whereas the alloy aged at low temperature i.e., at 150 °C forms discontinuous precipitation, while the sample aged at intermediate temperature i.e., 227 °C both the continuous and discontinuous precipitation are observed. Vickers hardness values of the solutionized sample are 48 ± 3 HV and aged at 350 °C, it is 55 ± 5 HV. At low-temperature ageing i.e., at 150 °C the hardness value is decreased to 44 ± 5 HV, whereas ageing at 227 °C, the hardness value is found 53 ± 5 HV.