Investigation on the microstructure of a τ-Mg₃₂(Al, Zn)₄₉ strengthened Mg–Zn–Al alloy with relatively low Zn content

Abstract

Microstructures of an as-cast and heat-treated Mg–6Zn–5Al (wt.%) alloy have been studied using optical microscopy, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The second phase of the as-cast alloy is ϕ-Mg₂₁(Zn, Al)₁₇. After solution heat-treatment, the alloy exhibits a prominent hardening effect when aged at 180°C. This is attributed to nanosized τ-Mg₃₂(Al, Zn)₄₉ precipitates finely distributed in Mg matrix. A new orientation relationship between τ precipitates and Mg matrix has been identified, i.e., (1 0 0)_τ//(0 1 −1 0)_α, (0 2 −1)_τ//(−2 1 1 2)_α, [0 1 2]_τ//[2 −1 −1 3]_α. MgZn₂ precipitates are found in over-aged samples.

Keywords:

• magnesium alloys
• microstructure
• phase transitions
• mechanical properties

Acknowledgments

The experimental alloy was prepared by the group of Professor R.-S. Chen from the Institute of Metal Research, Chinese Academy of Science. Valuable discussions from Professor R.-S. Chen and Mr L. Gao are gratefully acknowledged. Analysis of HRTEM results was helped by Dr Y. Shao from the Key Laboratory for Advanced Manufacturing by Materials Processing Technology, Tsinghua University. This work made use of the resources of the Beijing National Center for Electron Microscopy and was supported by National Basic Research Program of China (no. 2007CB613704) and National Nature Science Foundation of China (no. 50971076). The authors wish to thank Mr C. Ocier for helpful proof reading.