Influence of rare earth addition on microstructures and mechanical properties of high-pressure die casting Mg–8Zn–6Al alloy

Abstract

Effects of 3 wt% RE (rare earth) addition on microstructures and mechanical properties of a high-pressure die casting Mg–8Zn–6Al (ZA86) alloy before and after aging treatment was thoroughly studied in this work. Microstructural characterizations present that RE addition has indiscernible influence on the well-defined dendrite structure and grain size, but would additionally introduce Al-RE intermetallic phases, which were identified as Al3RE and Al11RE. Simultaneously, two groups of Mg–Al–Zn ternary phases were detected in alloys with/without RE addition, while their volume fraction was clearly reduced by RE addition. After aging at 90 °C for 320 h, a large density of precipitates were observed particularly near grain/cell boundaries, and were identified as MgZn2, Mg4Zn7, and I-phase. Although RE addition seems to have no influence on the components of precipitates, it obviously improves their volume fraction and slightly increases their size. Finally, the yield strength of the ZA86 alloy after aging was improved by ∼33 MPa by RE addition. Therefore, the die cat ZA86 alloy exhibits high-strength and certain precipitation strengthening ability, and RE addition can significantly enhance the precipitation strengthening, accomplishing the highest yield strength in the reported HPDC magnesium alloys, being ∼267 MPa.