Study On The Microstructure And Mechanical Properties Of Semi-continuous Cast Mg-8Gd-3Y-Zr Alloy

The Mg-8.0Gd-3.0Y-0.5Zr(wt.%, GW83K) alloy ingot with a diameter of 110 mm and the length of 800 mm was successfully fabricated by semi-continuous casting. In this paper, microstructure in as-cast alloy, the optimum heat treatment process and the properties of sample in different heat treatment conditions such as as-cast, solid solution treated and peak-aged in different position were compared by using optical microscopy, X-ray diffraction equipment, scanning electron microscopy equiped with EDX, and mechanical test of tension and creep. At last, we also studied the properties of compression creep and indentation creep properties of the cast-T6 alloy.The results show that: The grain size in the edge part of ingot is smallest and the second phase also distribution scatterly. The microstructure evolution of the GW83 K alloy from as-cast to cast-T4 involved α-Mg matrix+Mg24(Gd,Y)5 eutectic compounds and supersaturated solid solution + cubic-shaped Mg5(Gd,Y) compounds, respectively. While the microstructure of peak-aged alloy involved solid solution + cubic γ phase +β precipitates. Integrate solid solution process and the mechanical properties of samples in different parts, solution process of the entire ingot is determined as 500oC×4h. The specimen of edge part exhibits the best mechnical properties, The corresponding optimum heat treatment condition for the this part alloy was as follows: 480oC×4h+225oC×12h, it’s ultimate tensile strength(UTS) and elongation was 372 MPa and 6.6 %, respectively.The GW83 K alloy under different conditions show different fracture behaviors: in as-cast alloy, quasi-cleavage pattern is observed; after solution treatment, the alloy exhibits a trans-granular quasi-cleavage fracture; after peaked-aged at 200°C and 225°C, the fracture mode is a mixed mode of trans-granular and inter-granular fracture, in which inter-granular mode is dominant in alloy peak-aged at 200°C. When the temperature increase to 300 oC, the fracture mode also change to intergranular- fracture mode.Further study of the creep behavior indicate that temperature and stress affect the compression creep and indentation creep behavior of of the cast-T6 alloy. The steady stage creep rate increases with increasing temperature and stress. For the cast-T6 alloy compression creep tested at 250oC-300 oC for 80MPa-120 MPa range, the creep mechanism is cross slide When the range change to 50MPa-80 MPa,the creep mechanism is related to dislocation climb. At 200oC/120 MPa range the creep mechanism is cross slip. At 200oC/50MPa-80 MPa range grain boundary slide may be the main mechanism. When GW83 K alloy indentation creep tested at 250oC-300 oC for 270-360 MPa range the creep mechanism is cross slide When the range change to 120MPa-360 MPa,the creep mechanism also change to dislocation climb. At 200oC/360 MPa range the creep mechanism is cross slip. At 200oC/120MPa-270 MPa range the main mechanism. may related to grain boundary slide.