Stabilizing Al-Mg-Si-Cu alloy by precipitation nano-phase control

The thermal stability of Al-Mg-Si-Cu quaternary alloys with different Mg/Si ratios (2.0 and 1.0) at high temperature has been investigated by creep test, Vickers microhardness measurement, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results show that the thermal stability of the two alloys designed in this study is comparable to the Al-Cu-Mg-(Ag) alloys used for high temperature applications. The alloy with a Mg/Si ratio of 2 exhibits a creep life nearly two times longer than the alloy with a ratio of 1 under the same conditions. TEM observations reveal the formation of rod-like Q' phase and lath-like C phase in the crept alloys, while the proportion of the latter one is significantly larger in the alloy with a higher Mg/Si ratio. The applied stress has a softening effect and a hardening effect in the alloy with a Mg/Si ratio of 1 and 2, respectively, relative to their stress-free counterparts. This phenomenon can be explained by the accelerated coarsening of rod-like Q' phase by creep-induced dislocations. The creep fracture of the two alloys occurs in a ductile mode. A constitutive model based on the continuous damage mechanics that can accurately describe the observed three-stage creep behaviors is developed. Our results suggest that tuning stable nano-precipitates is essential to develop aluminum alloy with excellent thermal stability.