Evolution of β Mg₁₇Al₁₂ in Mg–Al–Zn–Ag alloy over time

In this study, the microstructure evolution of β-Mg₁₇Al₁₂ in Mg–9Al–0.5Zn–0.12Ag alloys aged at 175 °C for 18, 36, and 72 h was investigated. The results indicate that β-Mg₁₇Al₁₂ precipitates contain discontinuous precipitates and continuous precipitates. The interface between a discontinuous precipitation phase and the matrix appears with either a straight or a stepped morphology. Existing misorientation dislocations at a straight interface disappear with increasing aging time. For continuous precipitates, a [115¯]_β//[21¯1¯0]_α, (411¯)_β//(1011¯)_α orientation relationship (OR) was observed, and a stereogram of this new OR was calculated, the results of which are discussed herein. As aging time is prolonged, the continuous precipitates grow, and their OR develops toward a common Burgers OR. The yield strengths of the alloys aged for 18 and 36 h exhibit no significant differences owing to a better strengthening effect achieved by the new OR in the 18-h aged samples than that of the Burgers and Potter OR. As the aging time approaches 72 h, a larger grain size reduces the yield strength of the alloy.