Study On Detwinning And Annealing Strengthening Phenomenon Of Mg-xGd-yY-0.3Zr Alloys

Mg-xGd-yY-0.3Zr alloys have been widely concerned for their excellent strength and corrosion resistance at room temperature and high temperature.However,for their strong anisotropy at room temperature and detwinning-prone,the application scopes have been restricted as engineering components.Recent studies show that annealing treatment can improve yield strength of the deformed Mg-Gd-Y alloys,but the factors of annealing strengthening phenomenon need further study.In this paper,the generation conditions and influencing factors of detwinning,and the effect of twin type and twin volume fraction on annealing strengthening phenomenon are investigated by means of Metallographic Microscope,Electron Backscattered Diffraction(EBSD)and Universal Testing Machine,which provide theoretical references for engineering application of Mg-xGd-yY-0.3Zr alloys.The main conclusions of the thesis are as follows:The optimal solution treatment process of Mg-0.3Zr,Mg-2Gd-0.3Zr,Mg-2Gd-2Y-0.3Zr,Mg-10Gd-3Y-0.3Zr alloy is 500?×4h,500?×4h,500? ×4h,510 ? × 6h,respectively.The Gd and Y elements have the effect of weakening texture.The addition of Gd and Y elements make the C-axis of hot rolled Mg-xGd-yY-0.3Zr alloys change from the ND direction to the RD and TD directions,often accompanied by the formation of multi-peak pole figures.With the increment of Gd and Y element,the max value decreases in pole figure,and the effect of weakening texture and eliminating anisotropy becomes more obvious.Although the C-axis orientation of most grains of Mg-2Gd-2Y-0.3Zr alloy slightly deviates from the ND direction,detwinning phenomenon still occurs when the sample is pre-compressed along TD and then re-compressed along ND direction.When the pre-compression amount along TD direction is 7%,detwinning phenomenon is the most obvious.And when the pre-compression amount exceeds 7%,detwinning phenomenon is weakened.The hot rolled Mg-2Gd-2Y-0.3Zr alloy pre-compressed along ND direction is full of {10(?)1}-{10(?)2} secondary twins.The movement of the twin boundaries becomes difficult due to the interaction between twin boundaries and dislocations when the hot rolled Mg-2Gd-2Y-0.3Zr alloy pre-compressed along ND is re-compressed along TD direction.As a result,detwinning phenomenon is suppressed.The hot rolled Mg-2Gd-2Y-0.3Zr alloy pre-compressed along TD direction by 7% is annealed at 175?for 3h,and then re-compressed along ND direction.It is found that interaction between solutes and twin boundaries during re-compression inhibits detwinning,and increases the yield strength of annealed pre-compressed samples by 97 MPa compared with samples without annealing.Moreover,a large number of dispersed second phases precipitate in Mg-10Gd-3Y-0.3Zr alloy during rolling at 500 ?.Second phases can not only produce strengthening effect to increase yield strength by 74 MPa,but also inhibit detwinning due to difficult migration of twin boundaries.{10(?)2} extension twins,{10(?)1}-{10(?)2} secondary twins and multiple types of deformed twins are generated in Mg-10Gd-3Y-0.3Zr alloy rolled at 500 ?.When the rolling reduction is less than 60%,the grain size and the twin density of Mg-10Gd-3Y-0.3Zr alloy is coarse and low,respectively.When the rolling reduction is 60%,the grain size is uniform and the density of twin is intensive.When the rolling reduction is over 60%,the dynamic recrystallization will take place and the deformed twin grains are almost disappeared.However,The hot-rolled Mg-2Gd-0.3Zr alloy has a strong basal texture.When it is pre-compressed by 7% along the TD direction,{10(?)2} extension twins are actived.The pinning effect is produced due to the Gd atoms segregate to {10(?)2} twin boundaries after annealing treatment at 175?for 3h.Thus,the yield strength of the annealed pre-compressed Mg-2Gd-0.3Zr alloy is improved by 23 MPa during re-compression along TD direction.When the hot-rolled Mg-2Gd-0.3Zr alloy is pre-compressed by 7% along the ND direction,a great number of {10(?)1}-{10(?)2} secondary twins are actived.However,the Gd atoms could not segregate to {10(?)1}-{10(?)2} twin boundaries after annealing treatment,and the yield strength is not improved followed the re-compression along ND direction.