Cooperative Control Of Mechanical And Corrosion Properties Of Precipitation-strengthened Magnesium Alloy

In this paper,precipitation-strengthened magnesium Alloy as research objects,are investigated systematically for the cooperative control of corrosion and mechanical behavior by microstructure regulation.First of all,the influence of ultra-fine twin(UFT)structure prepared by multi-pass pre-compression on the mechanical and corrosion behavior of hot rolled AZ80 Alloy was studied;then,the composite twin structure was produced by the pre-compression deformation in ZK60 alloy,and the mechanism for reducing the mechanical and corrosion anisotropy was systematically studied.After,the effects of ultrasonic surface rolling process(USRP)on the microstructure and corrosion behavior were studied in hot-rolled AZ80 alloy.At last,the microstructure and mechanical properties of Mg-20Al alloy with high aluminum content after equal channel angular process(ECAP)were systematically studied.The research results of this paper are as follows:(1)For AZ80 Alloy,with increasing the three-dimensional pre-compression passes,twin lamellae has high density and small size,and the distribution of precipitates is more uniform after aging treatment.The ultra-fine twin structure with an average twin thickness of 204 nm can be prepared,thus the ultra-fine structure with an average grain size of 303 nm can be formed through 12 passes for pre-compression deformation(UFT-4).UFT structure can significantly improve strength(ultimate tensile strength of aged UFT-4 is 463 MPa),and reduce tension compression yield asymmetry.Furthermore,UFT structure can effectively change the morphology and distribution of the precipitates,which can reduce the micro galvanic corrosion between?-Mg₁₇Al₁₂ and?-Mg matrix and completely inhibit the local corrosion.Particularly,the average corrosion rate of aged UFT-4(0.0755 mg/cm2/day)after immersion for 300 h is approximately one order of magnitude lower than the AZ80-T6.(2)For ZK60 alloy,the composite twin structure can not only improve the strength of sample without any compromise of ductility,but completely removes the tension-compression yield asymmetry along the RD,TD and ND(Orientation of“ND”,“RD”and“TD”in the sample represent normal direction,rolling direction and transverse direction of the rolled plate,respectively.).Namely,the CYS/TYS or TYS/CYS(CYS and TYS refer to compression yield strength and tension yield strength,respectively)ratio for pre-compression sample is approximately 0.98 along the RD,0.99 along the ND and 0.99 along the TD,in contrast to 0.62 along the RD,0.81 along the ND and 0.83 along the TD for the initial sample.And the twins resign the crystallographic planes along the RD,TD and ND,and generate a more uniform distribution of crystallographic orientations in three dimensions,which greatly reduces the effect of corrosion anisotropy for different crystallographic planes on the corrosion anisotropy of strong textured samples.(3)For AZ80 Alloy,the depth of deformation zone and nano layer through USRP can reach 900?m and 350?m respectively,and the grain size increases gradiently from surface to interior.Severe plastic deformation will lead to serious anodic dissolution and reduce the surface corrosion resistance.After aging treatment,the dislocation density decreases,and the morphology of?-Mg₁₇Al₁₂ precipitates can be effectively changed.This change can effectively reduce the micro galvanic corrosion reaction,and can completely inhibit the severe local corrosion and improve the corrosion resistance.(4)For the first time,the texture of?-Mg₁₇Al₁₂ phase and its evolution with RD-ECAP conditions were disclosed in Mg-20Al alloy.The large blocks of?-Mg₁₇Al₁₂phases consist of many fine grains with different crystallographic orientations.For?-Mg₁₇Al₁₂ phase,a random texture is observed at 623 K.A higher processing temperature and more passes of RD-ECAP will generate more uniform distribution of?-Mg₁₇Al₁₂ phase and finer?-Mg₁₇Al₁₂phases.In addition,due to the influence of a large number of?-Mg₁₇Al₁₂ phase,the texture of?-Mg matrix for the ECAPed Mg-20Al alloy is much different from that of the ECAPed Mg alloys with a relative low Al content(e.g.AZ31 or AZ80).