Researches on the formation of precipitate-free zones with different characteristics and corresponding correlation with creep fracture for the peak-aged Mg-10Gd-0.4Zr casting alloy

In this paper, the precipitate-free zones (PFZs) formed during creep testing in the peak-aged Mg-10Gd-0.4Zr casting alloy were carefully distinguished by different characteristics, the corresponding formation mechanisms and relationship with creep fracture were systematically investigated. The results showed that the PFZs approximately perpendicular to the stress direction can be classified into type I PFZs with one precipitate array and type Ⅱ PFZs with two precipitate arrays around grain boundary. Type I PFZs can be further classified into type I_A and type I_B according to the width difference between adjacent grains. The adjacent grains of type I_A PFZs had a big difference in the plastic deformability, while type I_B PFZs were with the similar plastic deformability. The different plastic deformability influenced the degrees of dislocation pile-up and accumulation on the two sides of precipitate array, which led to the formation of type I_A PFZs with significant width difference. The type Ⅱ PFZs formed in the tertiary creep stage and had serrated grain boundary, which may be attributed to the strain-induced grain boundaries migration of type I_B PFZs caused by localized stored energy difference between adjacent grains. Although the width of PFZs with different characteristics was roughly the same, the micro-crack almost initiated and propagated along the type I_A PFZs, which was resulted from the large stress concentration. By contrast, type Ⅱ PFZs retarded micro-void formation and lowered micro-crack propagation rate due to the serrated grain boundary.