Microstructure And Mechanical Properties Of Casting Mg-Gd Based Alloys With Low Content Of Gd

Mg-Gd based alloys have been a hotspot in recent years because of their outstanding strengthening effect and precipitation hardening character.However the commercial applications are limited due to its high cost.Hence,the research is focused on reducing the content of Gd without the loss of its strength.According to the latest literature,the precipitation hardenable Mg-Gd based alloys with the lowest content of Gd is the Mg-6Gd-1Zn-0.6Zr(wt.%)alloy.In this work,a precipitation hardenable Mg-Gd based alloy with 3 wt.% of Gd was designed.Meanwhile,The growth twins and cuboid-shaped phase were also investigated in Mg-Gd alloys with low content of Gd.Further investigation of microstructure revealed the mechanism of strengthening effect.A precipitation hardenable Mg-3Gd-3Nd-0.6Zr alloy was designed by thermodynamic calculations firstly and then investigated by experiment.(1)Massive Mg5 Gd phase formed in the as-cast alloy.Its crystal structure is fcc,with a lattice parameter of 2.26 nm.The solution of Nd atoms into equilibrium Mg5 Gd phase facilitated its formation and increased its lattice constant.The actual chemical composition of Mg5 Gd is Mg5(Gd0.2Nd0.8)(at.%).The mechanical properties of the as-cast alloy were: UTS-200 MPa,TYS-115 MPa and ET-5.6 %.The strengthening phase is Mg5(Gd0.2Nd0.8).(2)A two-step solution treatment(500 ℃ / 4 h + 535 ℃ / 4 h)was designed and performed on the as-cast alloy.Most of the Mg5(Gd0.2Nd0.8)phase dissolved into the matrix without formation of overheating defects.The mechanical properties of the solution treated alloy were: UTS-206 MPa,TYS-110 MPa and ET-11.1 %.The solution strengthening effect of rare earth elements counteracted the weakening effect for the disappearing of Mg5(Gd0.2Nd0.8)phase.The solution treated alloy had a more homogenized microstructure and a higher tensile elongation than that of the as-cast alloy.(3)Through the aging treatment,the hardness of the alloy increased from 66.2 HV(solution treated condition)to 99.2 HV(peak-aged condition),increased by 50 %.Heat treatment regime of solution treated at 500 ℃ for 4 h plus 535 ℃ for 4 h and then aging at 100 ℃ for 24 h plus 200 ℃ for 24 h was selected for peak-aged condition.The precipitate is a plate like shape with three variants,and its habit plane is parallel to the {10-10} planes of matrix.The crystal structure of the precipitate is fcc,with a lattice constant of 0.79 nm.The orientation relationship between the precipitate and the matrix is(-112)p //(10-10)m and [110]p // [0001]m.The precipitate is identified as β1(Mg3(GdxNdy))metastable phase.It is quite possible that the β1 phase is a continuous solid solution with Mg3 Gd phase and Mg3 Nd phase.The mechanical properties of the peak-aged alloy were: UTS-289 MPa,TYS-138 MPa and ET-5.3 %.The morphology and orientation of β1 precipitate can provide the most effective barriers to dislocations gliding on the basal plane of α-Mg.For Mg and Mg alloys,there are only two independent easy slip systems,both involving the slip of dislocations with 1/3<11-20> vectors within the basal plane.The β1 precipitate could provide effective strengthening effect and decrease the ductility significantly.The microstructure and mechanical properties of permanent mold casting Mg-3Gd-0.6Zr alloy with {10-12} growth twins were investigated.(1)In the as-cast alloy,the growth twins are {10-12} type.Their average thickness is 4.4 ± 2.3 μm and the volume fraction per unit volume of specimen is 25.2 ± 4.8 %.After annealed at 350 ℃ for 1 h,the growth twins are also {10-12} type.Their average thickness is 4.5 ± 2.0 μm and the volume fraction per unit volume of specimen is 55.2 ± 5.3 %.(2)The mechanical properties of as-cast and annealed alloys were tested.The strength values remain unchanged,but the ductility is increased both in the tensile and compressive test.The random texture of casting alloy,the strengthening effect of growth twinning boundaries and the weakening effect of annealing treatment are the reason for the unchanged value of strength.The increased of ductility is caused by two reasons.First,the annealing treatment relieve the thermal stress and reduce the degree of segregation of solute atoms,which will make the microstructure homogenized.Second,the increases of volume fraction of growth twins per unit volume of specimen will accommodate the plastic deformation.(3)The twinning behavior and texture of three deformed samples were tested by EBSD technology.Texture of <10-10> // Load Direction was detected in the sample of tensile to failure.No obvious preferred orientation of grains in the samples of pre-compression was detected.The maximum value of texture intensity decreased with further pre-compression from 5% to 20%.It is maybe the reason for the high compressive elongation.(4)The formation mechanism of growth twins in as-cast Mg-3Gd-0.6Zr alloy by permanent mold casting was elucidated in four factors: supersaturation,composition,cooling rate and segregation ability of the alloy elements.The identity of cuboid-shaped phase in a designed water quenching Mg-3Gd-0.6Zr alloy was identified.(1)The cuboid-shaped phase was preliminarily identified as fcc-Gd.(2)A designed annealing treatment procedure can eliminate the cuboid-shaped phase.(3)The preliminary result is that the 3s electrons of Mg filled the 5d shell of Gd and induced the formation of cuboid-shaped phase during the non-equilibrium solidification.(4)The formation of cuboid-shaped phase has an adverse effect on the strength and a beneficial effect on the ductility of the alloy.