Critical role of dynamic precipitation on enhanced creep resistance in Mg–Zn alloy by Mn substitution for Zr

Abstract

Mg alloys with excellent creep resistance are able to withstand fatigue and damage under high temperature and complex load. Adding alloying elements rather than rare earth elements, such as Mn, can further reduce the cost and thus facilities the widespread applications of Mg alloys. In this paper, the individual effect of Mn and Zr elements on the dynamic microstructural evolution of Mg–Zn alloys during creep is evaluated, on the basis of identical initial microstructure before creep. And the essential origin of Mn substitution for Zr improving the creep resistance of Mg–Zn alloys by one order of magnitude is revealed. Experimental investigations and first principle calculations reveal that prior-precipitation of α-Mn particles serve as the heterogeneous nucleation sites for the precipitation of β1′ phases, and Mn partitioning in MgZn2 phases promote the nucleation and thermal stability of β1′ precipitates during creep. High number density of fine precipitates can effectively inhibit the occurrence of continuous dynamical recrystallization (CDRX), pin dislocations and impede the activation of cross-slip, which are the main reasons for the enhanced creep resistance in ZM60 alloy. As Zr does not participate in the formation of precipitates in ZK60 alloy, the MgZn2 phases with low thermal stability are prone to growth and coarsening during long-term creep. Large number of fine CDRXed grains and sparse coarse precipitates in ZK60 alloy cannot effectively impede the dislocation movement, which contributes to the occurrence of cross-slip and deteriorate the creep resistance. This work provides new insights for designing and fabricating outstanding heat-resistance Mg alloys.

Keywords

• Mg–Zn alloy
• Mn element
• Creep resistance
• Dynamic precipitation