Effect of Ag addition on the characteristics of precipitates and improving age hardening response of Mg-6Bi alloy

The effect of Ag addition on the characteristics of precipitates and age hardening response of Mg-6Bi alloy were investigated. The results illustrate that the Ag addition in Mg-Bi alloy can significantly enhance the age hardening response of the alloy. The peak-aged hardness increases from 51.8 HV of the Mg-6Bi alloy to 59.1 HV of the Mg-Bi-Ag alloy after ageing at 160 degrees C for 40 h, which is increased by 14.1%. Using high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and STEM-EDS (energy dispersive X-ray spectroscopy), the crystal orientation relationships (ORs) between the Mg3Bi2 precipitate and Mg matrix were characterized to be [0 0 0 1]Mg3Bi2//[1 1 2 0]Mg, (1 1 0 0)Mg3Bi2//(1 1 0 0)Mg, (1 1 2 0)Mg3Bi2//(0 0 0 1)Mg, and [1 1 2 0]Mg3Bi2//[1 1 2 0]Mg, (0 0 0 1)Mg3Bi2//(1 1 0 2)Mg, (11 0 2)Mg3Bi2//(0 0 01)Mg. Furthermore, it was found that Ag atoms can be not only enriched in the Mg3Bi2 precipitates but also segregated at the interface between the Mg3Bi2 precipitate and Mg matrix. Thus, it promotes the nucleation rate and slows down the growth of the precipitates, resulting in the fined precipitates and the increased number density of precipitates. The roles of Ag in the improving ageing hardening response and characteristic of precipitates are discussed in the present paper.