| Due to the strong demands for weight reduction of transportation vehicles for better fuel efficiency, magnesium alloys recently receive strong research interest for applications to various structural components of automobiles and aircrafts, as a lightest structural material.However, applications of magnesium alloys are still limited because of poor strength. In order to overcome this disadvantage, efforts are devoted to develop new magnesium alloys with good mechanical properties.Addition of rare-earth elements in Mg alloys has been known to improve its properties, and thus many commercial magnesium alloys contain RE elements, e.g.WE54 commercial alloy. Since the solubility of the RE elements decreases with decreasing temperature, these alloys show precipitation hardening by aging. Recently, it has been reported that magnesium alloys containing gadolinium and yttrium were found to exhibit higher strength than conventional magnesium alloys at room and high temperature.Mg-Gd-Y-Zr alloy shows an excellent combination of strength, elongation and creep resistance in the peak ageing condition.As the mechanical properties obtained from this alloy are much superior to those of commercial WE type alloy, it has a potential to replace conventional Mg alloys.The present work has investigated the effects of different pre-deformation and aging parameters on the microstructure and mechanical properties of Mg-9Gd-3Y-0.5Zr alloy by such experimental methods as optical microscopy (OM),scanning electron microscopy (SEM), transition electron microscopy(TEM), tensile test and microhardness test. The aim of the present work is to provide theoretical and practical results for the development of Mg-Gd-Y-Zr alloy.The microstructure of as-cast Mg-8.42Gd-213Y-0.42Zr alloy consist of a-Mg solid solution,coarse cutectic Mg-RE phases at the grain boundaries. After homogenized at 510℃for 8 hours, the cutectic Mg-RE phases solved to the magnesium matrix.Meanwhile, the grains grow up with the size of 100μm. The grain refined after hot-extrude at 400℃, because the effect of recrystallization. The mechanical properties increase obviously,with the UTS,YS and elongation attained 295MPa,186MPa and 26%, respectively.The as-extrude Mg-Gd-Y-Zr alloy aged at 190℃,220℃,and 250℃,respectively.The heat-treatment at 220℃is the most effective which can be seen from the ageing hardenning curve. After ageing at 220℃for 18 hours, the microhardness reached the peak point(105HV).With the UTS,YS and elongation attained 335MPa,225MPa and 16%, respectively.After the pre-deformation, the YS of the as-extrude alloy increased obviously, which can reach at 305MPa, with the increased rate of 64%. The elongation rate declined while the deformation increased.The mechanical properties and hardness test results show that the pre-deformation increases the peak hardness, and reduces the peak aged time by a margin of 2～3 hours at 220℃.The strength of peak aged tensile specimens, with 8% pre-deformation, increased obviously, the UTS and YS attain 410 MPa and 365 MPa, increased 20% and 60% respectively; However, the plasticity of the specimens decreased.It was found from this study that the dominant precipitate phase in peak-ageing alloy is the metastableβphase(Mg5X,X:Gd+Y),which plays an important role in the age hardening, has a shape of convex lens.TEM shows that theβprecipitates have the orientation relationship with the a-Mg matrix, i.e. [001]β//[0001]α,(020)β'//(1010)a, d{020)β=4d(1010)α·The precipitates are very effective in obstructing the motion of dislocation gliding on the basal plane because they have a habit plane of{1120},which is vertical to the basal plane.The{1012}type twinning can be seen after the pre-deformation,with the further deformation, the sedondary twinning appeared, which was also the{1012}type twinning. Meanwhile the basal slip was observed in the alloy. With the increase of pre-deformation, the density of the dislocation was increased.Subsequent ageing promotes precipitation at the dislocation and the boundary of twinning, because the dislocation can accelerate the ageing process; The dominant precipitation at the peak-ageing was also theβphase with its size become smaller. But overmuch deformation may cause stress concentration, which is very harmful to mechanical properties of the alloy.The optimum pre-deformation rate for Mg-8.42Gd-2.13Y-0.42Zr alloy is proved to be 8.0 percent, at which the best tensile strength and plasticity were obtained.
|
PDF Full Text Request