Evaluating the orientation relationship of prismatic precipitates generated by detwinning in Mg alloys

It is commonly accepted that prismatic precipitates in Mg alloys are more potent strengtheners than the ones formed on basal planes. However, in most rare-earth (RE) free Mg alloys, precipitation commonly occurs on basal planes. Recent results showed that coupling {102} twinning, aging and detwinning (a process termed TAD) promotes prismatic precipitation in Mg-Al alloys by a mechanism that remains unclear. The present work aims to theoretically and experimentally evaluate possible crystallographic orientation relationships (ORs) between the prismatic precipitates and the detwinned matrix in TAD processed samples in order to understand their formation mechanism. A crystallography-based algorithm is proposed to predict such ORs and the predictions are subsequently validated through experimental observation in a transmission electron microscopy (TEM). Three new ORs between Mg17Al12 precipitates and the α-Mg-Al matrix are experimentally determined, which agree well with the predictions of the proposed algorithm after considering the 3.69o rotation of precipitates at twin boundaries. Additionally, the proposed algorithm is validated in other RE free Mg alloys produced via the TAD process, including Mg-Sn-Zn, Mg-Al-Ca, Mg-Zn and Mg-Ca-Zn systems. This work provides a clearer understanding of the ORs between prismatic precipitates and the matrices of Mg alloys that is deemed critical to fully exploit their potential for precipitation hardening.