Research On High Temperature Mechanical Behavior Of Mg-Gd-Y Alloys

Mg-Gd-Y alloy is a newly developed high-strength and heat-resistant magnesium rare earth alloy in recent years.It has a wide application prospect in the field of aeronautics and astronautics with great demand for its lightweight.Since the aerospace vehicles during the flight will be subjected to the friction and heat,the high temperature mechanical performance is one of the technical issues for these applications.However,the mechanical behavior of Mg-Gd-Y alloy is not yet systematically studied and the relationship between composition,microstructures and high temperature mechanical properties of Mg-Gd-Y alloy is not well understood yet,especially the mechanism of the influence of precipitation on the relative high temperature properties.In this paper,Mg-Gd-Y(Gd:3-12,Y:3-13 wt.%)alloys with different contents of Gd and Y were prepared.Effect of composition,heat treatment and thermomechanical treatment on the microstructure,high temperature tensile,high temperature fatigue properties,wear resistance and fracture behavior of Mg-xGd-yY-0.5Zr alloys were investigated.The quantitative relationship between the composition,tensile temperature and mechanical properties of the alloy was established for the first time.The results show that the alloy has superior high temperature strength(yield strength,tensile strength,high cycle fatigue),and the mechanism of high temperature strengthening and the anomalous behavior of high temperature strength were revealed,the research results have great significance on the development of high-performance heat-resistant magnesium rare earth alloys.Firstly,the effect of Gd content on the microstructure,high temperature tensile properties and high-temperature wear behavior of Mg-xGd-3Y-0.5Zr alloys were studied.The microstructure of the alloys at as-cast,T4 and T6 conditions were revealed and effect of Gd on the phases and?'phase discussed.The results showed that,among all the alloys with different Gd content,Mg-10Gd-3Y-0.5Zr alloy shows the highest UTS and YS of 375and 238 MPa at room temperature,respectively.With further increasing the Gd content from 10 wt.%to 12 wt.%,the ultimate tensile strength at room temperature decreased to360MPa and the yield strength reached the maximum value of 244 MPa.The Mg-12Gd-3Y-0.5Zr alloy exhibited the highest tensile strength at high temperatures.The yield strength and ultimate tensile strength of the Mg-12Gd-3Y-0.5Zr alloy increased with the test temperature and showed its maximum value of 375 and 259 MPa at 150°C,and then decreased as the temperature increased further.The alloy showed significant anomalous high temperature strength.The wear rate of the tested alloys increased with increasing the sliding load.With increasing the wear temperature to 200°C,the wear rate of the Mg-6Gd-3Y-0.5Zr alloy decreased by about 24%.At higher wear speeds,wear rate of the Mg-6Gd-3Y-0.5Zr and the Mg-12Gd-3Y-0.5Zr alloys decreased by about 68%and50%,respectively,due to the formation of stable oxide layers on the worn surfaces.The alloy containing 12 wt.%Gd exhibited higher wear resistance compared with the alloys containing lower Gd contents under the same conditions.Then,the effect of Gd and Y content on high temperature mechanical properties of Mg-Gd-Y alloys in order to optimize the best high temperature mechanical properties were studied in detail.The results showed that among Mg-xGd-yY-0.5Zr alloys,the Mg-10Gd-5Y-0.5Zr alloy shows the optimum strength at room and high temperature.The alloy shows the UTS and YS value of 331 and 241 MPa at room temperature,however,the UTS and YS increased to 350 and 261 MPa with increasing the test temperature from room temperature to 150°C,respectively.Even under the tensile test at 300°C,the ultimate tensile strength of the alloy can reach the value of 225 MPa.For the first time,it is found that with increase of x+y,the Mg-xGd-yY-0.5Zr alloys show the anomalous temperature dependence of tensile properties,especially for x+y>13 wt.%,the anomalous temperature dependence of tensile properties is significant.For further study,the effect of heat treatment and pre-deformation on high temperature tensile properties of the Mg-10Gd-5Y-0.5Zr alloy were investigated.The results showed that under-aged,peak aged and over-aged Mg-10Gd-5Y-0.5Zr alloy shows similar strength at room temperature and three aged alloys showed obvious anomalous temperature behavior of yield and ultimate tensile strength,i.e.both tensile yield strength and ultimate tensile strength of the studied alloy are first increased with the increasing of temperature,and then decreased as the temperature is increased further.The anomalous behavior of the alloy become more pronounce with prolonging the aging time.The density of the?'precipitate in the alloy with pre-deformation is higher than that in the alloy without pre-deformation,which is attributed to the fact that twin boundaries and dislocations in pre-deformed alloy act as a nucleation sites for the?'precipitates.The pre-deformed Mg-10Gd-5Y-0.5Zr alloy showed the highest strength and lowest elongation under all experimental conditions,the UTS of the pre-deformed Mg-10Gd-5Y-0.5Zr alloy at room temperature is 366 MPa,which is almost 9%higher than that of peak-aged alloy.For the first time in this thesis the high temperature high cycle fatigue behavior of Mg-Gd-Y-Zr alloys were studied and the fatigue crack initiation and propagation mechanisms of Mg-Gd-Y alloy at different temperatures were discussed.The result showed that alloy exhibits a significant anomalous temperature dependence for fatigue properties at the temperature range of room temperature to 250°C,the fatigue life increased with increasing the test temperature and reached its maximum at 200°C and then decreased with further increasing the temperature.In addition to,the high cycle fatigue results from S-N curves at room temperature and 200°C after 5×10~6 cycles show that strength of the tested alloy at 200°C(110 MPa)is significantly superior to that at room temperature(90 MPa).The anomalous temperature dependence of fatigue properties for the alloy is likely to be related to the different crack initiation mechanism,cracks were generated at the twin boundaries at room temperature,whilst crack initiated from the persistent slip band at high temperature.The anomalous temperature dependence of the strength behavior of magnesium alloys containing rare earth elements studied and discussed in detail.The results showed that anomalous temperature dependence of the tensile properties of Mg-Gd-Y alloys was attributed to the shape,and orientation of the?'precipitate.The orientation of the?'precipitate during tensile test at higher temperatures changed and it seems that this orientation is favorable to suppress the basal slip and casue strengthening of the alloys at higher temperature.At the same time,twinning growth increased at higher test temperatures.The growth effect can be rationalized in terms of the relaxation of the misfit back-stresses by increased dislocation activity.Twin introduces additional barriers to dislocation movement,?1012?twinning in Mg reorients the lattice to‘‘hard''crystallographic orientations and the transformation of dislocations as they pass through the twinning front can lead to the establishment of high hardening rates within the twin interior.In addition,when drawing temperature exceeds 250°C,?'precipitates significantly grew and softened,and phase transformation occurs,thereby this caused reducing the strength of the alloys.