Effect of Li Addition on the Optimum Thermo-Mechanical Processing Parameters of the Mg Alloy AZ31

Abstract

The effect of minor additions of Li on the high-temperature workability of AZ31 Mg alloy is assessed using the processing maps approach. Data obtained from the uniaxial compression tests that were performed over the temperature, T, range of 523 K to 673 K (250 °C to 400 °C) and strain rate, $$ \dot{\varepsilon } $$ , range of 10−3 to 10+1 s−1 was utilized to construct the power dissipation and instability maps using modified dynamic material model proposed by Murty and Rao. These maps and complementary microstructural analyses were utilized to identify various deformation mechanisms that operate in different T- $$ \dot{\varepsilon } $$ regime. For the base 0Li alloy, two distinct deformation domains were identified, in both of which dynamic recrystallization (DRX) is the operating deformation mechanism. The addition of 1 wt pct Li extends Domain I to higher T and Domain II to higher $$ \dot{\varepsilon } $$ , both with enhanced power dissipation efficiency. On the other hand, higher Li-containing alloys exhibit one wide single domain over the entire T range of 523 K to 673 K (250 °C to 400 °C) and $$ \dot{\varepsilon } $$ of 10−2.5 to 10+0 s−1 with power dissipation efficiency varying from 20 to 70 pct. The optimum T- $$ \dot{\varepsilon } $$ combination for the high-temperature deformation of these alloys in homogenized condition is 623 K to 673 K (350 °C to 400 °C) and 10+0.5 to 10+0 s−1, where DRX occurs. Microstructural analysis indicates that Li addition in AZ31 suppresses DRX at low $$ \dot{\varepsilon } $$ and promotes dynamic recovery though cross-slip. Instability maps corroborated with microstructural analysis showed flow instabilities in the form of shear bands and cracking along grain boundaries.