Effect Of Gd And Y On Twinning Type And Annealing Strengthening Of Magnesium Alloy

As the lightest metallic structural material with low density,high specific stiffness and strength,as well as weak anisotropy,Mg alloy has broad applications in the fields of aerospace,national defense,military,automotive and electronics industry.However,because of the low strength of traditional rare earth magnesium alloy,the use of magnesium alloy has been limited.Extensive research has shown that formability and annealing treatment generally result in improved strength.In this paper,Gd and Y elements are introduced into the existing Mg-0.3Zr system alloys to further research their microstructures and improve annealing strengthening.Optical Microscope(OM),Scanning Electron Microscope(SEM),Transmission Electron Micro scope(TEM),Electron Backscattered Diffraction(EBSD)and compression tests are used to study the microstructures,mechanical properties,deformation and annealing processing parameters of Mg-0.3Zr based alloys with different compositions after solution treatment.Firstly,the effect of the solution treatment on microstructures and mechanical properties has been studied and the mechanism for segregation by Gd and Y elements additions was revealed.Secondly,the effect of deformation rate and rare earth element addition on the microstructures evolution of Mg-0.3Zr system alloys were investigated,and the formation mechanism of the style and lamellae of twins was discussed.Finally,the annealing strength behavior of alloys was investigated and the strengthening mechanisms of deformation rate and annealing temperature were revealed.From these investigations,the following conclusions has been obtained.A lot of second phases in the cast rare earth magnesium alloys,containing Gd and Y,has dissevered the matrix of the alloy and has deteriorated the mechanical property.Hence the microstructure has been optimized using solution treatment.Y element had an obvious effect on grain refinement,but Gd element had a great effect on segregation.Some of the second phaseswere difficult to dissolve during the solid solution process,and the Mg5 Gd phase was uniformly distributed at grain boundaries,resulting in the second phase strengthening effect.The plasticity of the alloy was improved and the strength decreased after the solution treatment.With the increase of rare earth element contents,the yield strength and compressive strength of the alloys were obviously improved,but the plasticity was decreased,which was due to the effect of refined crystalline strengthening and second phase strengthening.In the deformation process of magnesium alloy,the volume fraction of twins and the thickness of lamellae were related to grain size and pre-deformation rate.With the increase of the strain,the twin volume fraction increased linearly,and the thickness of the twins was gradually decreased.For the magnesium alloy with high content of rare earth,the grain size of the magnesium alloy was reduced,while the thickness and the volume fraction of the twins decreased.The Gd element in the alloy was more likely to increase the twinning tendency than that of the Y element.In the plastic deformation process of magnesium alloy,{10-12} twin was preferentially nucleated,but the number of {10-1 2} twins decreased with the increase of deformation rate.Due to differences in the axis ratio c/a,Gd element could effectively activate{10-11} twin,but Y element could activate the {11-21} twin.Compared with Gd,Y had a strong ability to weaken texture during the deformation process.The mechanical properties of Mg-xGd-0.3Zr(x=5,10)and Mg-x Y-0.3Zr(x=5,10)alloys without pre-deformation were almost unchanged before and after annealing.However,the alloys with the pre-deformation had a significant annealing effect.An annealing treatment at25?~250?/5h,after 5% plastic strain rate in pre-compression,resulted in a remarkable strengthening effect,rather than softening effect.With the increase of annealing temperature and pre-deformation rate,the four alloys showed the same annealing strengthening trend,namely,increased first and then decreased.The highest strengthening parameters was at150?~200? with 4%~5% pre-deformation.The results of the in-suit metallographic and scanning transmission spectrum showed that the solid solution atoms,precipitating in dislocations and twin boundaries could pin or hind the dislocation.Because the densities of the two sides of the twin boundary were different,the solute atoms were pinned on the side of the twin,which resulted in a sharp bend in the twin boundary before and after annealing.The newly generated twins were also smaller.