【作者】 李毅；

【导师】 孙建；

【作者基本信息】 合肥工业大学 ， 材料工程（专业学位）， 2021， 硕士

【摘要】 渗氮是改善金属材料表面性能的重要手段,因其工艺简单、成本低而广泛的应用在钢、钛等金属表面改性上。然而,由于表面存在氧化膜和处理温度过高等问题使得渗氮工艺很难在镁锂合金材料上获得应用。为解决这个问题,本文首先采用表面机械纳米合金化的方法,在镁锂合金表面引入一层纳米结构的Ti或Ti-Al涂层,随后利用纳米结构能够加速原子扩散的作用,在低温下对表面机械纳米合金化后的样品进行氮化,借助氮原子与涂层、涂层与基体之间的扩散与相变,期望在低温条件下使得镁锂合金表面获得性能优异、与基体结合良好的氮化层。另外,本文还通过在氮气气氛下进行表面机械纳米合金化处理,期望能够在常温条件使镁锂合金表面原位制备出氮化层,从而进一步降低氮化温度。为此,本文系统研究了LZ91镁锂合金表面高质量Ti或Ti-Al涂层的制备及其低温渗氮行为,同时还系统研究了表面氮化层的机械纳米合金化法原位制备过程及影响因素。获得的主要结论如下:（1）通过表面机械纳米合金化方法,可在LZ91镁锂合金表面制备出纯Ti涂层和Ti-Al涂层,其中Ti-Al涂层的质量优于纯Ti涂层。当使用含25%Al的混合合金粉末、处理2h后所获得的涂层具有较为优异的厚度均匀性、致密性、表面平整性以及耐蚀性,此时涂层厚度为110～160μm,表层硬度可达351.8HV。（2）Ti-Al涂层的低温渗氮效果强于纯Ti涂层的低温渗氮效果,且表面机械纳米合金化工艺时间的增加能够提高渗氮效果。当使用含25%Al的混合合金粉末、处理2h后所获得的样品经450℃、400℃渗氮后表层形成了明显的TiN、AlN等氮化物相以及Ti-Al化合物相,且表层硬度分别高达749.2HV、734.4HV,同时改性层与基体界面处形成了Mg-Al化合物层和孔洞等不利于层基间结合的组织。此样品经350℃渗氮后,表层未观察到明显的氮化物相形成,但有相当量的Ti-Al化合物相形成,使得表层硬度增至574.8HV,且由于层基界面处只形成了Mg-Al间的互扩散层而表现出较好的结合力。（3）采用氮气氛围下的表面机械纳米合金化处理可原位制备出含氮的Ti涂层。处理5h后的样品表层上形成了TiN_0.9和TiN_0.3相,且硬度达566.8HV,较氩气氛围下处理的样品表层硬度高80HV左右。同时,采用氮气氛围下的表面机械纳米合金化处理也可原位制备出含氮的Ti-Al涂层。处理1h、2h时后样品表层未发现明显的氮化物相;处理3h、4h后样品表层形成了AlN、TiN相,其表层硬度分别为651.5HV、680.6HV,较氩气处理的样品表层硬度分别高240HV和210HV左右。球磨5h得到的样品表层的EDS能谱结果表明表层中含有氮化物,但其表面的衍射峰已十分宽化,仅可观察到AlN相,而未观察到TiN相。另外,处理5h后的样品表层硬度高达743.2HV,较充氩气处理的样品表层硬度高170HV左右。更多还原

【Abstract】 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 is110～160μm,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.更多还原