Study on the microstructure and creep behavior of squeeze-casted AZ91 magnesium alloy

Abstract

Magnesium alloys have received increasing attention in recent years for automotive and aerospace applications due to their lightweighting potential, which results in fuel efficiency and reduced environmental impact. However, widespread applications of Mg are limited by certain challenges, low creep resistance at elevated service temperature being one of the major challenges. In this study, AZ91 is prepared by squeeze casting in which the melt was solidified under an applied pressure. The squeeze casting parameters such as squeeze pressure, melt temperature, and mold temperature were fixed at 200MPa, 800°C and 200°C respectively. The microstructure of squeeze cast alloys was investigated using optical microscopy, and scanning electron microscopy while phase analysis was performed using X-ray diffraction. The microstructure of as-received sample showed lamellar discontinuous dendritic microstructure of -Mg17Al12 phase embedded in the -Mg matrix. The addition of Ca and Sr showed the significant refinement of dendritic size and reduction in the volume fraction of -Mg17Al12 by the formation of Al2Ca and Al4Sr phase along the grain boundaries. Creep behavior of squeeze casted AZ91 alloy was investigated in detail. Tensile Creep tests were performed at constant load in the nominal stress range of 50MPa-80MPa and the temperature range of 150°C-200°C. The Qc (activation n r y n n str ss xpon nt v lu s sso t w t r p orm t on w r st m t us n p nom nolo l r l t ons T ‗n‘ v lu w s oun to t ot C n C T t v t on n r y w s estimated to be 135.42±30.94 kJ/mol, 114.87±33.34 kJ/mol and 86.19±21.90 kJ/mol at 50MPa, 65MPa, and 80MPa stress respectively. 5-power law creep is found to be the governing creep mechanism where the dislocation climb is controlled by lattice diffusion. The dislocation density in the steady state creep m rostru tur s w r n lyz ollow n t -r y r t on l n pro l n lys s t n qu It w s o s rv t t slo t on ns ty n r s s y tor o w t n n r s n str ss rom M to M n t ot r hand, post creep microstructural study revealed that coarsening of thermally unstable -Mg17Al12 precipitate and -Mg grain size took place as a result of creep exposure. Fractography analysis suggested that the creep fracture initiated from brittle intergranular cavitation and then progressed by ductile growth and coalescence of cavity. The safe usage limit, threshold stress of the squeeze casted AZ91 alloy was estimated to be 23 MPa and 15 MPa at 150°C and 175°C, respectively.