Study On Preparation Of Twins In High Strength Mg Alloy Containing Rare Earth

Magnesium alloy has received increasing attention as the lowest density metal material,which find man potential applications in aerospace,military industry,automobile lightweight and other industries.However,magnesium and magnesium alloys have a low absolute strength,which limits their industrial applications.Twinning is one of the main plastic deformation modes at room temperature,and can refine the grains,improving the strength.In this paper,Mg-15 Gd and EW75 rare-earth magnesium alloys are selected as the research materials.Multidirectional compression is used to prepare high-density twins,and the effect of multidirectional compression on the microstructure and mechanical properties is studied.The main finding are as follows:(1)Three-directional compression can accumulate a large amount of deformation,causing strong work hardening;and generate {10(?)2} twins in magnesium alloys,which could improve the strength of the alloy.Compared with the original sample,the yield strength of the compressed Mg-15 Gd and EW75 samples after aging can be increased by 41 MPa and 111 MPa,respectively..(2)There is a certain limit to the strengthening effect of three-directional compression to harden Mg alloys.For the Mg-15 Gd alloy,the optimum process is compression for 6 passes,and for EW75 alloy,the optimum process is compression for12 passes.A further compression passes hardly increase the strength.(3)The aging treatment at 200 ℃ will not remove the twin structure,but it will recover some dislocations and residual stress,weakening the work hardening.The dislocations,twin boundaries and other defects generated by the three-directional compression can promote precipitation of aging,reduce the peak aging time,and improve the aging strengthening effect and the strength performance.The segregation of the precipitated phase at the twin boundaries can be observed by TEM.However,the aging treatment will not completely balance the work hardening effect and the compressed samples have a poor plasticity after aging.(4)The texture of the rare earth magnesium alloy is very weak,and the rare earth element will promote the activity of non-basal slip at room temperature.Therefore,it is difficult to prepare a high-density twin structure in Mg alloys containing high content of rare earth.An effective method to generate twins is to increase the compression pass and cumulative deformation within a certain range.For EW75,there are already some twins in the sample after 12 passes of compression.The factors that cause the increase of twin density are a lower deformation temperature and a higher deformation rate.For weakly textured rare-earth magnesium alloys,the preparation of the fine high-density twins common in strong-textured magnesium alloys such as AZ/ZK series may require very harsh conditions,such as forging after cooling in liquid nitrogen.