Study On Low Temperature Nitriding Behavior Of Mg-Li Alloy Assisted By Surface Mechanical Nano-alloying Treatment

Nitriding is an important way to improve the surface properties of metal materials.It is widely used in the surface modification of steel,titanium and other metals because of its simple process and low cost.However,nitriding technology is difficult to be applied to Mg-Li alloy materials due to the existence of oxide film on the surface and high treatment temperature.In order to solve this problem,a layer of nano-structured Ti coating or Ti-Al coating was introduced on the surface of Mg-Li alloy by surface mechanical nano-alloying treatment.Then the samples after surface mechanical nano-alloying treatment were nitrided at low temperature by using the effect of nano-structure which can accelerate atomic diffusion.By means of the diffusion and the phase transition between the nitrogen atom and the coating/the coating and the substrate,it is expected that the nitride layer with excellent properties and good bonding with the substrate can be obtained under the surface of Mg-Li alloy at low temperature.In addition,the surface mechanical nano-alloying treatment was also carried out in nitrogen atmosphere to prepare the nitride layer on the surface of Mg-Li alloy at room temperature.Therefore,the preparation of high-quality Ti coating or Ti-Al coating on LZ91 Mg-Li alloy and its nitriding behavior at low temperature were systematically studied.At the same time,the in-situ preparation process of surface nitrided layer by mechanical nano-alloying treatment and its influencing factors were also systematically discussed.The main conclusions are as follows:(1)The pure Ti coating and Ti-Al coating can be prepared on LZ91 Mg-Li alloy by surface mechanical nano-alloying treatment,and the quality of the Ti-Al coating is better than that of the pure Ti coating.When the sample was treated for 2h with the mixed-alloy-powder containing 25%Al,the coating has good thickness uniformity,density,surface flatness and corrosion resistance.The thickness of the coating is110?160?m,and the surface hardness is up to 351.8HV.(2)The low temperature nitriding effect of Ti-Al coating is stronger than that of pure titanium coating,and the increase of surface mechanical nano-alloying time can improve the nitriding effect.The samples fabricated by mixed-alloy-powder containing25%Al with milling 2h nitrided at 450?and 400?can formed intermetallic compound and nitride phases such as TiN,AlN on the surface,and the surface hardness was as high as 749.2HV and 734.4HV,respectively.At the same time,Mg-Al compound layers and pores were formed at the interface between the modified layer and the matrix.After nitriding at 350?,the surface of the SMNAT-25%Al-2h sample did not form nitride phase,but a considerable amount of Ti-Al intermetallic compound phase formed,making the surface hardness increased to 574.8HV,and the sample showed good binding force because the Mg-Al inter-diffusion layer was formed between the coating and matrix.(3)Ti coating containing nitrogen can be prepared in situ by surface mechanical nano-alloying treatment in nitrogen atmosphere.After treatment for 5 hours,TiN_0.9and TiN_0.3phases were formed on the surface of the sample,and the hardness reached566.8HV,which was about 80HV higher than that of the sample treated with argon.At the same time,Ti-Al coating containing nitrogen can be prepared in situ by surface mechanical nano-alloying treatment in nitrogen atmosphere.There were no obvious nitride-phase could find on the surface of the samples treated by 1h and 2h.While the milling time was 3h or 4h,AlN and TiN phases were formed on the surface of the samples,and the hardness could reach to 651.5HV and 680.6HV which was about240HV and 200HV higher than that of the samples treated with argon,respectively.The results of the EDS spectrum of the sample obtained by milling for 5h showed that there were nitric compounds existed in the surface,the diffraction peak on the surface was very wide,But only the AlN phase could be observed while TiN phase could not be observed through the XRD pattern,this may be caused by broadened diffraction peak.In addition,the surface hardness of the samples treated by 5h milling time is as high as 743.2HV,which is about 170HV higher than that of the samples treated with argon.