Study On The Formation Mechanism Of Nanocrystalline Magnesium Alloys During Cold Rotary Swaging

Mg alloys,as the lightest metal structural material in practical application,have wide application prospects in the fields of aerospace,weaponry,and automotive industries.Grain refinement has been proven to be one of the most effective methods for improving the mechanical properties of Mg alloys.The most common method for grain refinement of Mg alloys is the severe plastic deformation(SPD),such as equal channel angular pressing and high-pressure torsion.However,these SPD technologies have the disadvantages of high cost,complex process,and limited sample size prepared,which limit their usage in industry now.Cold rotary swaging is a near net shape process with low cost and high efficiency,which can realize the preparation of high-strength nanocrystalline Mg alloys and have a high application value in industry.In this work,the effect of solid solution alloying elements,initial grain size,and deformation temperature on the formation of nanograins of Mg alloys during swaging was studied using Vickers microhardness test,uniaxial tensile and compressive tests,X-ray diffraction(XRD),backscattered electron diffraction(EBSD),and transmission electron microscopy(TEM).The results provide the experimental basis and theoretical guidance for further alloy optimization and process development for preparing the large-sized and extraordinary high-strength nanocrystalline Mg alloys.The main conclusions were drawn as follows:(1)The formation of dislocation arrays was indispensable for the nanocrystallization of Mg alloys during swaging.Nanograins were not obtained in pure Mg via rotary swaging,which was attributed to the low activity of non-basal slip and the difficulty of forming dislocation arrays.In contrast,nanograins were obtained in Mg alloys by adding the non-rare earth(Al,Zn)or rare earth(Y)elements due to massive non-basal dislocations activated during swaging which promoted the formation of dislocation arrays.Compared with the non-rare earth elements,the addition of rare earth elements resulted in decreasing the activity of twinning.In the early stage of swaging,the numbers of twin lamellae formed in the rare earth Mg alloy were smaller than the non-rare earth Mg alloy and many deformation bands were formed in the former.Consequently,the formation rate of nanograins during swaging in the rare earth Mg alloy was slower than the non-rare earth Mg alloy.The degree of grain refinement was also lower after five-pass swaging in the former than the latter.(2)There was little effect of the initial grain size on the size of nanograins in the swaged Mg alloys.The initial large grains of about 548μm can be refined to about 95 nm after simple three-pass cold rotary swaging.The formation process of nanograins in Mg alloys with different initial grain size all contained the refinement of twin lamellae on the initial coarse grains and the segmentation of dislocation arrays on the twin lamellae.However,as the initial grain size decreased,the activity of nonbasal slip in Mg alloys increased,which promoted the formation of dislocation arrays and accelerated the segmentation of dislocation arrays on the twin lamellae.As a result,the formation rate of nanograins increased.(3)The key factors for the nanocrystallization of the central region of Mg alloy bars via cold rotary swaging were the high strain rate and the high-frequency change of the loading direction.The formation of twin lamellae mainly depended on the high strain rate.The formation of dislocation arrays mainly depended on the high-frequency change.The effect of grain refinement gradually increased as the distance to the center of the alloy bars decreased due to the increase of applied loading directions in the grains,which promoted the formation of dislocation arrays.In comparison,the grains in the edge region of the alloy bars were subject to the single direction of loading.Consequently,only twin lamellae were formed in the edge region after five-pass swaging and no dislocation arrays were observed.After cold rotary swaging,the grain size at different positions along the radial direction on the cross-section of the Mg alloy bars showed a gradient distribution.(4)Cryogenic rotary swaging(CRS)showed a stronger effect on the grain refinement than room-temperature rotary swaging(RTRS).The region with nanograins in the center of Mg alloy bars was also larger after five-pass CRS than after five-pass RTRS.Tensile and compression yield strength of the alloys after five-pass CRS improved from 202 MPa and 103 MPa to 495 MPa and 492 MPa,respectively.The stronger effect on the grain refinement of the CRS process was attributed to the smaller size of nanograins obtained after CRS were mainly attributed to the finer twin lamellae formed in the early stage of CRS,the smaller nanoscale subgrains formed in the twin-twin intersections,and the inhibition of dynamic recovery.The whole article contains 108 figures,10 tables,254 references are cited...