Study on Microstructural Evolution and GP Zone Strengthening Mechanism by Solution and Aging Treatment in Non-dendritic Mg-6Zn-5Al Magnesium Alloy

The Guinier–Preston (GP) zones have an important influence on the improvement of the age-hardening level of alloy. We here prepare six kinds of Mg-6Zn-5Al (ZA65) magnesium alloy samples to investigate the microstructural evolution, precipitation behavior, and strengthening mechanism using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and micro-hardness tests. The finding reveals that the GP zones are observed in non-dendritic alloys experiencing pre-aging at 80°C for 24 h, whose formation creates more nucleation sites and accelerate kinetics for precipitated phases. Consequently, the introduced pre-aging has been successful in promoting the increase of the age-hardening level of non-dendritic alloys. The double-aged non-dendritic alloys show a significantly changed microstructure with a high density of long rod-shaped β₁′ precipitations, making the age-hardening level of double-aging much better than that of single-aging, and the peak value 90 HV was obtained 4 h in advance, which is an increase of 4 HV compared with that of the direct aged non-dendritic alloy. The strengthening mechanism of non-dendritic alloys is mainly summarized as grain boundary strengthening, GP zone strengthening, and the shearing strengthening of β₁′ and β₂′ precipitations. Among these, the GP zone strengthening is a fundamental contributor to improving the age-hardening level.