Effect of stacking faults and long period stacking order on mechanical properties for rare-earth magnesium alloy: As-cast versus as-solutioned

Abstract

In this paper, the microstructure and tensile mechanical properties of Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr rare-earth (RE) magnesium alloy in the as-cast state and its solid solution state were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), differential scanning calorimeter (DSC) and the first-principles simulation. The results show that the mechanical properties were improved after solution treatment. The CS520-12 alloy, treated at 520 °C for 12 h, exhibited excellent mechanical properties, possessing an ultimate tensile strength (UTS) of 343 ± 5 MPa and an elongation of 18.9 ± 0.8 %. These main reasons for the improving of mechanical properties was attributed the formation of I1 stacking faults (SFs) and accumulated-dislocations. The Y element, originating from the melt of the eutectic phase during solution treatment, resulted in a reduction in stacking faults energy (SFE) and facilitated the formation of I1 faults. The presence of numerous SFs hindered dislocation slip. The other key factors for elongation improving were that the weaken of texture, and geometrically necessary dislocation (GND) density decreased from 1.52 × 1013m−2 to 1.22 × 1013m−2. 14H-LPSO phase played an important role in the improving mechanical properties for CS480-12, CS480-24 and CS520-24 alloys. The higher elastic modulus of LPSO compared to Mg matrix, according to the first-principles calculated results, enabled its function as a short-fiber, thus strengthened the mechanical properties. Block-shaped 14H-LPSO in CS480-12 alloys, distributed at the grain boundaries (GBs), suppressed the grow of cracks. The lamellar 14H-LPSO within the grains of CS480-24 and CS520-24 alloys were parallel to the <a> dislocations, which compared with SFs of CS520-12 alloy, can't effectively inhibit the movement of dislocations.