Effects Of Cold Rotary Swaging On Microstructure And Mechanical Properties Of Mg-Y-Zn Alloy Containing LPSO Phases

The wrought magnesium alloys containing long-period ordered(LPSO)phases display excellent mechanical properties.These alloys can be further strengthened through microstructure refinement via cold rotary swaging.In this paper,Mg-8Y-2Zn(wt.%)and Mg-3Y-2Zn(wt.%)alloys were used.By adjusting the homogenization and extrusion parameters,alloys with different LPSO phase features were obtained.The alloy bars were deformed by cold rotary swaging.The ultimate tensile strength of low Y content(Mg-3Y-2Zn)alloy was increased to 509 MPa.Effects of cold swaging on the microstructure and mechanical properties of Mg-Y-Zn magnesium alloys were studied by optical microscopy,scanning electron microscopy,transmission electron microscopy and X-ray diffraction analysis,combined with vickers hardness and tensile tests at room temperature.The main conclusions are as follows:(1)Nanocrystallization was realized in Mg-Y-Zn alloy containing LPSO phase after cold rotary swaging process.The nano-scaled grains were mainly distributed in areas without LPSO phases and at the tips of block-shaped or needle-shaped LPSO phases.The formation process of nano-scaled grains in areas without LPSO phases includes the formation of twin and the division of twin by dislocation arrays.The average size of nano-scaled grains in Mg-8Y-2Zn alloy is about 50 nm and that in Mg-3Y-2Zn alloy is about 110 nm after multi-pass rotary swaging.(2)The features of LPSO phases significantly affected the grain refinement process.The high concentrated stress at the tip of the blockshaped LPSO phases and needle-shaped LPSO phases facilitated the formation of dislocation arrays and subgrains.Thus,the rise of temperature and strain promoted the subgrains to transform into nano-scaled grains.The lamellar LPSO phases and the lateral of block-shaped LPSO phases impeded the formation of dislocation arrays.Besides,the lamellar LPSO phase critically hindered the nucleation and growth of twins,leading to the obstruction of grain refinement.(3)Nano-scaled grains and nano-scaled lamellae structures were generated in the alloy within areas containing densely-distributed needleshaped LPSO phases.The nano-scaled lamellae structures were composed of nano-scaled α-Mg lamellae and needle-shaped LPSO phases.There were misorientations between α-Mg lamellae and micro-areas within α-Mg lamella.The Mg-3Y-2Zn alloy with more needle-shaped LPSO phases were significantly enhanced after cold rotary swaging with yield tensile strength,ultimate tensile strength and elongation to failure of 454 MPa,509 MPa and 5.4%,respectively.Higher strength improvement was obtained in the alloy with more nano-scaled lamellae structures.The results demonstrated that the strengthening effects of nano-scaled lamellae structures were much better than that of nano-scaled grains.