Researches On Formation Mechanism Of The LPSO Structure,Microstructures And Mechanical Properties Of The Mg-Gd-Y-Zn-Zr Alloys

As a high-strength heat-resistant magnesium alloy, Mg-RE (rare earth, RE) alloys has an extensive application prospects in automotive and aerospace industries. Adding Zn into the Mg-RE alloys can regulate the aging microstructures of alloys, and under appropriate conditions, a new structure, Long Period Stacking Ordered Structure (LPSO Structure), will form in the Mg-RE-Zn alloys. The alloys containing LPSO Structure exhibit excellent yield strength and elongation at the room and elevated temperature. However, the current researches on the Mg-REi-RE2-Zn alloys with LPSO structure are still in its infancy, and the formation mechanism of LPSO structure in different alloys is still not well understood. Especially which on how to control the size and quantity of LPSO phases is still waiting be clarified.Based on the early researches of LPSO structure phase in the Mg-Gd-Zn and Mg-Y-Zn, this paper studies the formation ruler and control methods of LPSO structure by using X-ray diffractometer (XRD-6000), optical microscope (OM, MEF-4A), transmission electron microscope (TEM, TG220S-Twin JEM-100CX Ⅱ), electron probe microanalyse (EPMA-1600), scanning electron microscope (SEM, JSM-5600LV), vickers hardness tester (HXD-1000) and electronic universal tensile testing (WD-10A). According to the results, this paper further studies the rolling deformation characteristics of Mg-Gd-Y-Zn-Zr alloys containing LPSO structure.The results of the study are showed bellow:Y/Zn weight ratio can represent the formation condition and range of LPSO phase. When the Y/Zn weight ratio is2.3or less, alloys are mainly contributed by the precipitation of phase with LPSO structure; the ratio adjusting to4.5or more results in the disappearance of Mgi2(Y, Gd)Zn phase. Y/Zn weight ratio can remarkably influence the formation and transformation of LPSO phases during high temperature heat-treatment. When rare earth content is higher in the alloys, with the increase of the ratio, LPSO structure Phases can be significantlyt refined.By adjusting the alloy composition and Y/Zn weight ratio, two novel low RE magnesium alloys, Mg-5Gd-2Y-2Zn-0.5Zr and Mg-6Gd-3Y-2Zn-0.5Zr alloys, have been designed. The Mg-5Gd-2Y-2Zn-0.5Zr alloy containing only a-Mg and LPSO structure phase obtain excellent high temperature mechanical properties. The LPSO structure phases on the grain boundary can effectively limit grain boundary sliding, and ensure the strength at room and elevated temperatures, respectively. Because of it, the alloy can still remain as the UTS and TYS in the range of200-275℃as room temperature and also have a good elongation. The tensile strength at250℃is240MPa, nearly30MPa higher than that of WE43alloy.Y and Nd atoms can respectively promote and prevent the ordering process during the solidification and the heat-treatment in the Mg-Gd-Zn alloys, and the relationship between the secondary phases and RE is Mg5(Gd,Zn)(?)Mg12(Y,Gd)Zn(?)Mg5(Gd,Y,Zn). By this relationship, the number and size of LPSO structure can be effectively controlled.During the solid solution treatment, the change of diffusion rate will lead to the change of the actual concentration of solute atoms, and affect the formation and growth of the intracrystalline LPSO structure. By controlling the solute diffusion, the formation location, number and morphosis of LPSO structure can be adjusted effectively. The resolution of the LPSO structure phases at the grain boundary during the heat-treatment can adjust the the intracrystalline solid solubility. For the intracrystalline LPSO structure, the selection of heat-treatment regime need be determined according to the alloy composition, type and number of secondary phases.The results of rolling process of Mg-Gd-Y-Zn-Zr alloys containing LPSO structure phases show that, when the rolling temperature is450℃, rolled sheets are prone to appear shear cracks in45°direction, and with the increasing of rolling temperature, the number of these cracks significantly become fewer. The rolling temperature of Mg-6Gd-3Y-2Zn-0.5Zr and Mg-5Gd-2Y-2Zn-0.5Zr alloys is450℃, and that of Mg-8Gd-5Y-2Zn-0.5Zr need be500℃. The LPSO structure phases can significantly absorb the dislocations, greatly impair the dislocation block in the grain, and hider the formation of dynamic recrystallization grains. The broken LPSO structure phases at higher rolling temperature reduce the absorption of dislocations the alloys appear the different degree of recrystallization organization.