Research On Mechanical And Creep Properties In High Temperature Of Extruded Mg-6Gd-4Y-Nd-0.7Zr Alloy

In current study of heat-resistant magnesium alloy, adding rare earth elements into the traditional magnesium alloy to obtain high strength and heat resistance has become a major direction. The object of this paper is about the extruded Mg-6Gd-4Y-Nd-0.7Zr alloys. However, most of researches were about to discuss focused on the microstructure and properties of the certain alloy. But few studies on the contact between microstructure evolution and strengthening mechanism are found. In addition, the research about heat and creep resistance in high temperature environment of the Mg-RE alloy is still limited.In this paper, at first, the microstructure of the Mg-6Gd-4Y-Nd-0.7Zr alloy(referred GWN641K) is indicated. And then the mechanical performance test in high temperature and the creep tests are conducted on the extruded GWN641 K. And at the same time, the aging treatment of GWN641 K is studied and the optimal process parameters are obtained. Finally mechanical performance test in high temperature was conducted on the GWN641 K alloy after aging, and analysis and discussion are accomplished about the strengthening mechanism in high temperature by researching on the microstructure.Results showed that: the extruded GWN641 K alloy is mainly consist of the elongatd deformed grains along the extrusion direction, a lot of tequiaxed grains in different sizes and the β′phase on the grain boundaries, and the average grain size is 5.2 um. The yield strength, tensile strength and elongation of the extruded GWN641 K alloy at room temperature are: 260.31 MPa,318.70 MPa and 6.30%, and the extruded alloy can work stably under the temperature 150℃-200℃. And the optimal aging parameters is 225℃-72 h, the GWN641 K alloy after aging could obtain the hardness with 128.4HV, and the yield strength, tensile strength and elongation at room temperature are: 336.28MPa、371.77 MPa and 1.44%, and the yield strength increased by 29%, and the tensile strength increased by 17%, but the elongation decreased.In 200 ℃ tensile process, GWN641 K alloy after aging has amounts of high-strength fiber textures, serious dislocation pile and precipitated phases. When the dislocations bypass the phases, dislocation loop will be remained, precipitation strengthening effect is obvious. And when the stretching temperature is at 250 ℃, texture intensity decreases, the number of precipitation significantly reduced, dislocation climb begins, stress concentrations to is released and the strength decreased.With increasing temperature creep or creep stress, the extruded GWN641 K alloy, size of the grain and the second phase on the boundary increased, the strip β ’phase gradually precipitated, which is parallel distributed or distributed into 60 ° angle. In the creep at 250℃/60MPa-120 MPa, the creep stress exponent is 3.27, with dislocation slipping as the main creep mechanism. And in the creep at 60MPa/200℃-300℃, a sudden change of the creep activation energy occurs at 250℃. At temperatures below 250 ℃, the creep activation energy QA=86.5k J/mol, with grain boundary sliding as the main creep mechanism; but at temperatures above 250 ℃, the higher creep activation energy is obtained, QA=395.4k J/mol, and the main mechanism was changed into cross-slip mechanism.