Unexpected effects on creep resistance of an extruded Mg-Bi alloy by Zn and Ca co-addition: Experimental studies and first-principles calculations

In the present work, a new Mg-Bi based alloy is developed by the addition of Zn and Ca in equivalent atom fraction with Bi. Mg-Bi and Mg-Bi-Zn-Ca alloys were prepared by extrusion at a ram speed of 20 mm/s. Room temperature mechanical properties and creep behaviors at 423 K were investigated. The results show that Zn and Ca co-addition shows little influence on average grain size and texture intensity but changes the dispersive Mg3Bi2 into Mg2Bi2Ca particles in different sizes and a lower density. Twinning is largely activated during room-temperature deformation. Consequently, a slightly decreased proof strength but tripled elongation is shown at room temperature. Unexpectedly, large enhancement in creep resistance is detected after the co-alloying of Zn and Ca and the minimum creep rate is reduced by 10 to 20 times in the BZX621 alloy. Stress exponent n = 4-5 indicates that the creep is a dislocation-climb controlled type. Post-mortem characterization on microstructure shows slip of dislocation < c + a > are also largely found in B6 as well as BZX621 alloy and cross-slip is detected more severe in B6 alloy. Dynamic segregation and precipitation are also seen in both alloys. Bi-clusters are seen dispersive across the grains in B6 and so did the PFZs that could undermine creep resistance at the grain boundaries. By contrast, Zn-rich needle-like precipitates are developed at most "ends" of < c + a > dislocations, which would hinder the further dislocation motions and thus improve the creep resistance. First-principles calculations were adopted and the results show that the thermal stability and thermomechanical properties of Mg2Bi2Ca are much better than that of Mg3Bi2. Stacking faults energy is lowered down with the co-addition of Ca and Zn, which could inhibit the rate of dislocation climb and cross-slip. As a result, the improved creep resistance is obtained in the Mg-Bi-Zn-Ca alloys. Microstructural and controlling mechanism changes by thermal activation result in the unexpected enhancement in creep resistance with decreased room-temperature proof strength after co-addition. These findings could contribute to the development and optimization of creep-resistant Mg alloys in the future.