First Principles Investigation Of ?? And ?? In Mg–Gd Alloy

Magnesium has the advantage of low density,high specific strength and good castability.However,low absolute mechanical strength and poor heat resistance have prevented more widespread use of magnesium and its alloys.By adding rare earth elements,we can increase the working temperature of magnesium alloys substantially.Usually,age hardening process can be used to strengthen Mg-RE alloys.The precipitation sequence of Mg-RE alloys is G.P.zone,??,?? and ?.At present,some disagreement on the nature of the precipitates still exists.The disagreement centers around whether or not ?? forms in Mg-RE alloys.Thus,it's necessary to use simulation methods to investigate the precipitates of Mg-RE alloys.In this research,we summed up several methods of calculating the strain energy as well as the interfacial energy of supercells containing both matrix and precipitates based on first principles calculations.By using these methods,we calculated the strain energy and interfacial energy of ?? and ?? forming coherent interfaces with Mg matrix in Mg–Gd alloys.We then used these data to explain why ??-long is more inclined to form during agehardening process,even though these two structures have almost the same formation energy.Calculation results of ?? in Mg–Gd alloys show that ?? has the lowest strain energy for supercells containing(1010)prismatic interface,while interfacial energies for both prismatic and basal interfaces are negative.Such negative interfacial energy will lead to ?? forming in small clusters with short range order.Calculation results of ??-short and ??-long in Mg–Gd alloys show that ??-short and ??-long will have(100)prismatic habit plane as the strain energy of this interface is the lowest.Due to positive interfacial energy for the prismatic habit plane,(100)interface is also energetically stable.However,as Strain energy of Mg/??-long supercell containing(100)interface is almost half as that of Mg/??-short supercell containing(100)interface,while interfacial energies of these two supercells are almost the same,??-long will be more inclined to form during the age-hardening process in Mg–Gd alloys.