The Room-temperature Deformation Mechanism And Strengthening Of Ultra High-plasticity VK21 Alloy

Due to inadequate independent slip systems can be activated during room-temperature deformation,the magnesium alloy has low room temperature plasticity and poor formability,and the traditional processing technology cannot avoid strength-ductility trade-off,which severely limits its wide application.Ultra high-plasticity VK21 wrought magnesium alloy was fabricated by one step conventional fast extrusion process in this paper(A>45%).The controllable microstructure and mechanical properties of ultra high-plasticity VK21 alloys can be achieved by adjusting the extrusion parameters and heat treatment routes.The texture evolution,twinning behavior,activation of slip system and dislocation configuration of investigated alloy during tensile deformation at room-temperature were analyzed by EBSD assisted quasi in-situ slip trace analysis and TEM two beam diffraction.Then the room-temperature deformation mechanism of VK21 alloy was revealed by multi-technology and multi-methods.The tensile strength of ultra high-plasticity VK21 alloy was further enhanced by alloying with a trace amount of Zn.The effect of Zn addition on the microstructure and mechanical properties of VK21 alloy was studied,and the strengthening mechanism was also elucidated.The extruded VK21 alloy bar prepared by conventional fast extrusion process contains a large number of fibroid fine-grain bands distributed along the extrusion direction,and exhibits the favorable characteristics of RE-texture.By adjusting and optimizing the extrusion parameters and heat treatment process,the microstructure and mechanical properties of ultra high-plasticity VK21 alloys can be controlled.The higher tensile yield strength can obtain by increasing the number of fibroid fine-grain bands,and the higher elongation can be achieved via coursing regular grains.In the process of tensile deformation at room temperature,tensile twinning occurs in the regular grains of VK21 alloy,but no twinning observed in the fibroid fine-grain bands.A large number of basal slip and tension twinning were activated during tension along ED,their interaction accommodates large deformation.With the increase of deformation strain,pyramidal<c+a>slips were activated.VK21 alloy show higher grain boundary bond,it is difficult to produce grain boundary cracks during tensile deformation,which ensures that VK21 alloy has higher elongation at room temperature.The addition of Zn can significantly improve the tensile yield strength and tensile ultimate strength of as-cast and as-extruded alloys.This is mainly attributed to the fact that the addition of Zn can effectively refine the grain size of VK21+xZn alloy,introduce abundant micron and nanometer scale__Mg-Gd-Zn t__ernary phases,give rise to texture sharpening,the pole changes from<1011>to<0110>.Mg-2Gd-0.4Zr-3.0Zn exhibits an excellent balance between strength and ductility with tensile yield strength(YS)and ultimate tensile strength(UTS)of 285 and 314 MPa,accompanied with elongation of 24%,they are superior to those of commercial AZ31.