Effects Of Y On Microstructure And Mechanical Properties Of As-cast AZ31Magnesium Alloy

Because of less density, high specific strength and stiffness, well damping and superior electromagnetic shielding, good machining performance, excellent processing performance and good casting performance advantages, magnesium alloy is praised as "new green engineering material in21st century", has promising application prospect in the fields of auto-mobile industry, aero-space and medical appliances et. AZ31magnesium alloy is one of the most widely used in industry of magnesium alloy, but the mechanical property of magnesium alloys needs to be further improved. There is a close relationship between the microstructure and mechanical property of magnesium alloy, so we can change the microstructure of the Mg alloy to improve its mechanical property, then broaden the Mg alloy’s application areas.In this paper, the AZ31-xY experimental alloys were prepared by using as-cast forming technique. The effects of Y addition on the as-cast microstructure and mechanical properties of AZ31alloy were investigated and analyzed by optical microscopy(OM), field emission scanning electron microscopy(SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), hardness tests and tensile tests HV testing machines, electronic universal testing machine, et a series of analysis methods. Results show that:1) The addition of Y can significantly refine as-cast AZ31magnesium alloys, change the microstructure and phase constitute. When Y addition up to0.5%, the dendrite size of thick continuous net-like β-Mg17Al12phase and secondary dendrite spacing was significantly smaller. Furthermore, amount of β-Mg17Al12is refined to short strip or granules. The compounds with Y do not appears. The new phase A12Y appears when Y addition up to1.0%. The minimum grain size(47μm) occurs at Y content of1.0%, showing a decrease by77.4%from the original size (208μm) for Y-free sample. When Y addition up to1.5%, the block or needle A12Y phase have the trend of growing and gathering, then the microstructure of alloy become coarse.2) Effects of Y addition on the hardness of the test alloys have been investigated. Results show that:with the increase of Y content, the hardness of the alloy increased accordingly. When A12Y phase is changed to the main second phase, HV hardness of the alloy increases obviously. Due to structure of alloy refined and the second phase which is high hardness separated.3) The addition of Y can improve the room temperature tensile property of as-cast Mg alloy. The tensile strength, yield strength and elongation of the test alloy first increases and then decreases. The maximum tensile strength(225MPa), yield strength (159MPa) and elongation (8.2%) occur at Y content of1.0%, showing increase by30.8%,69.1%and70.8%, respectively, from their corresponding values for Y-free sample. The refined grain strengthening is the main factor improving the mechanical properties of as-cast Mg alloy.The test also show that the grains tend to be coarse due to the excessive segregation of A12Y phase, which causes the inhomogeneity of the structure and component of the alloys, even the stress concentration. In this case, the mechanical properties will decrease.4) Amount of Y can improve the mechanical properties of alloys in high temperature. With the increase in temperature, tensile strength and yield strength decrease, while elongation increases. The maximum tensile strength (128MPa), yield strength(112MPa) and elongation (11.3%) occurs at Y content of1.0%, showing increase by30.6%,47.4%and85.2%, respectively, from their corresponding values for Y-free sample. The main strengthening mechanism for improving the high temperature mechanical properties of as-cast AZ31-xY alloy is the thermal stability of second phase.5) Fracture analysis shows:With the addition of Y content, the fracture way of the alloy is slowly transforming from typical cleavage fracture into quasi-cleavage cleavage fracture; the fracture morphology gradually transformers into dimples and cleavage quasi-cleavage crack, from large cleavage plane. Y addition improves the tensile fracture surface, especially when the addition of Y is up to1.0%, the result is the most obvious.