Microstructural characterization of intermetallic phases in a solution-treated Mg-5.0Sm-0.6Zn-0.5Zr (wt%) alloy

Although Zn-Zr intermetallic phases have been frequently reported in solution-treated Mg-RE-Zn-Zr systems (RE presents rare earth), the types, crystal structures and influence on precipitation remain to be systematically investigated. In this work, the intermetallic phases formed during solution treatment in a Mg-Sm-Zn-Zr alloy were studied using transmission electron microscopy. Four types, namely the plate phase, the spindle-like phase, the nano-scale granuliform phase and the rod-like phase were revealed. Except the first one, the other three ones are Zn-Zr phase. In addition, the granuliform phase presents quadrate-shaped or lath-shaped and was identified as Zn2Zr3. However, these two Zn2Zr3 phases with different shapes follow quite different orientation relationships with Mg matrix. Furthermore, three kinds of rod-like phases were observed: one across the grain boundary but only coherent with one grain, one covered by jagged Mg3Sm precipitates and one with no surficial phase. The former twos are Zn2Zr3 while the later one is Zn2Zr. Finally, this work indicates that only the plate MgZn2 phase and the rod-like Zn2Zr3 phase in the grain interior will act as heterogeneous nucleation sites for the Mg3Sm precipitates, thus influencing the precipitation.