【作者】 徐开东；

【导师】 黄早文； 王爱华；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 镁合金作为目前使用的最轻的金属结构材料,具有良好的减震、电磁屏蔽、机加工和易回收等优点,被誉为“二十一世纪的绿色工程材料”,是实现装备轻量化、便携化的一条重要途径,但其较差的抗蚀耐磨等性能限制了它的进一步发展,加强改善镁合金耐蚀耐磨性能的研究,对于推动镁合金更加广泛的应用并充分发挥其性能优势具有重要意义。本文通过高能撞击诱导表面自身纳米化和激光表面合金化两种工艺来达到改善镁合金材料表面性能的目的。系统研究了钢丸大小、撞击距离和处理时间等因素对高能撞击镁合金表面纳米化的影响,优化出了适用于镁合金表面自身纳米化的工艺参数:N₂和O₂压力为1.5 MPa,氮氧流量比为7∶5,煤油流量为4 L/h,撞击颗粒粒径为φ0.5 mm、撞击距离在290³20 mm范围内、且处理时间在180²40 s之间是较为合理的,利用该参数组合均能在镁合金表面成功实现纳米化。纳米化处理后横截面微观组织形貌显示,整个变形层包括严重塑性变形的表层、变形孪晶为主的亚表层及靠近基体轻微变形的过渡层,变形层呈明显的梯度变化特征。通过对不同变形层区TEM/HRTEM微观精细组织结构观察和分析,推演出了镁合金表面纳米化的内在细化机理,并抽象出了粗大晶粒在剧烈塑性形变条件下转变为纳米晶粒的晶粒细化模式。即:在镁合金表面纳米化的初始阶段以机械孪生为主,同时伴随着基面和棱柱面的位错运动;在形变中期以孪生和位错运动的协调、竞争为主,通过前期晶粒一定程度的细化和温度的升高,导致交滑移的产生,位错在后期的竞争中占据主导,进一步分割残余孪晶和微观条带状亚结构;随着畸变加剧、变形储能升高以及位错的增值、湮灭与重排,高能亚结构在足够的驱动力下发生了动态再结晶,最终形成了分布均匀、取向随机、晶界清晰的纳米晶粒。系统研究了纳米化层的行为,结果显示,纳米化层的硬度得到了显著提高,达到了基体硬度的两倍左右,且随着距表面深度的增加而逐渐减小,呈典型的梯度变化特征。摩擦磨损实验结果显示,纳米化处理后其摩擦系数和磨损失重都显著减小,磨损机制为以粘着磨损和磨粒磨损为主,同时伴随着氧化磨损。通过在不同PH值的3.5%NaCl酸、碱、盐腐蚀介质中的腐蚀行为研究,发现纳米化处理后其耐蚀性能出现了恶化现象。热稳定性实验表明,镁合金纳米化表层稳定存在的临界温度为330°C。首次利用真空微波高温实验炉对纳米化前后的镁合金进行了扩散Al、Si和Al-12Si合金化的研究,成功实现了纳米化镁合金的微波扩散合金化,结果显示,随着温度的升高,合金化层厚度逐渐增加,且纳米化处理后合金化层深达到未经纳米化处理的2³倍。实验研究了适用于镁合金激光表面合金化的最佳工艺参数组合:脉宽为0.8 ms,频率为45Hz,光斑直径约为1.0 mm,电流220 A,扫描速度350 mm/s。通过对Al-Nb/Al-TiB2/Al-（W, Ti）C系合金化层XRD衍射图谱的分析,发现在合金化层中Al与Nb以及基体中的Gd、Y等元素产生了原位反应,生成了高温硬质相Al₂Gd、Al₂Y、Al₃Nb等金属间化合物,且在合金化过程中强化相TiB₂和（W, Ti）C并未发生分解。表面宏观形貌显示,随着元素Al在合金化混合粉末中含量的减少,表面质量呈现出逐渐变差的趋势,出现了一定程度的结瘤现象。SEM和TEM对截面微观精细组织结构的观察显示,晶粒显著细化、分布均匀,整个横截面分为合金化层、过渡层和基体三部分,原位合成的新相如Al₂Gd、Al₂Y、Al₃Nb及强化相TiB2和（W, Ti）C等在合金化层中的分布弥散均匀,绝大多数形貌呈近球形,仅有小部分呈四边形块状,但与基体结合部位较为圆润,且形成的大部分金属间化合物的尺寸在100 nm左右,通过进一步观察发现,粒度稍大的形貌并不是单个的化合物析出相,而是多个粒子的团聚,与基体之间具有良好的相容性。硬度检测结果显示,合金化层的硬度提高达4⁶倍;不同合金化成分和质量配比时的干摩擦磨损结果显示,合金化层的摩擦磨损性能得到明显提高,摩擦系数由基体的0.52左右降至0.25⁰.35,分析摩擦磨损形貌得知,粘着磨损、磨粒磨损和氧化磨损为主要的磨损形式。电化学检测结果表明,在一定合金化配比范围内,激光表面合金化能改善镁合金材料在3.5%NaCl水溶液中的耐蚀性能。更多还原

【Abstract】 Magnesium alloy currently used as the lightest metallic structural materials has high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability and is easily recycled. It is an important way for equipments to achieve lightweight and portable, and acknowledged as the 21st century green engineering material. Unfortunately, magnesium has a number of undesirable properties including poor corrosion and wear resistance and high chemical reactivity that have hindered its widespread use in many applications. It has realistic significance for promoting wider application and giving full play to its unique performance advantages of magnesium alloys to strengthen the research on improving the wear and corrosion resistance. In this paper, the techniques of self-surface nanocrystallization induced by high-energy bombarding and laser alloying to achieve the purposes of improvement on surface properties of magnesium alloy. The effect of a steel shot size, impact distance and handling time on self-surface nanocrystallization induced by high-energy bombarding was investigated systematically.The process parameters were optimized to applicable to self-surface nanocrystallization of a magnesium alloy as follows: the pressure of N₂ and O₂ is 1.5 MPa, nitrogen and oxygen flow ratio of 7:5, kerosene flow rate of 4 L/h, bombing particle diameter ofφ0.5 mm, impact distance range of 290³20 mm, handling time in the range of 180²40 s. The surface nanocrystallization on magnesium alloy can be obtained successfully using the above technological parameters. Cross-sectional microstructure after nanocrystallization treatment reveals that the deformation layer presents gradient variation obviously, including the topmost surface layer of severe plastic deformation, deformation twins-based subsurface layer and a transition layer with slight deformation near the substrate. The inherent refinement mechanism of surface nanocrystallization on magnesium alloy was deduced through TEM / HRTEM microstructure observation and analysis. Meanwhile, the grain refinement mode of coarse grains converted into nano-grains was abstracted under the condition of severe plastic deformation, i.e. the deformation mode is dominated by mechanical twinning in the initial stage of the surface nanocrystallization for magnesium alloy, and accompanied with the dislocation movement of basical plane and prismatic plane. The coordination and competition between the deformation twinning and dislocation motion dominated the deformationprocess in the mid-stage of the severe plastic deformation. Subsequently, cross slip can be activated owing to a certain degree of grain refinement and temperature rising derived from high-energy bombarding, Dislocation movement dominates the competition in the later stage, and resulting in further fragmentation of the residual twins and micro-banded substructure. Then, dynamic recrystallization occurred under the sufficient driving force that coming from high-energy substructure with the increasing distortion, deformation storage elevated, dislocation multiplication, annihilation and rearrangement. Eventually, the nano-grains formed with clear grain boundary, homogeneous distribution and random orientation.The behaviors of nano-layer were studied systematically. The microhardness of the most top surface layer is about twice comparison with the substrate, and it is gradually decreasing with the increase of the depth from the surface. The results of friction and wear experiment shows that the friction coefficient and wear weight loss were reduced significantly, and adhesive wear and abrasive wear is the main wear mechanism, meanwhile, coupled with oxidation wear. It is found that the deterioration of corrosion resistance occurred in acid, alkali and salt 3.5% NaCl solution with different PH values after surface nanocrystallization treatment through the investigation on corrosion behavior. Thermal stability experiments show that the critical temperature of stabilized existence for nanocrystalline of magnesium alloy is 330°C. The vacuum microwave oven was used for experiments of diffusion alloying Al, Si and Al-12Si alloy in magnesium alloy before and after the surface nanocrystallization treatment. It’s worth noting that the diffusion alloying of magnesium alloy utilizing microwave has not been reported. The results showed that the thickness of alloyed layer increasing with the processing temperature increase. Moreover, the thickness of nano-treated alloying layer is more 2 to 3 times than that without nano-treatment.The optimized process parameters of laser surface alloying for Mg alloys are obtained through experimental study as follows: pulse duration of 0.8 ms, frequency of 45 Hz, spot diameter of about 1.0 mm, current of 220 A and the scanning speed is 350 mm/s.It is found that in situ reaction produced in the alloyed layer between the element Al and Nb, and the matrix elements Gd, Y, and generated a high-temperature hard-phase, such as Al₂Gd, Al₂Y and Al₃Nb intermetallic compounds. Furthermore, strengthening phase TiB2 and （W, Ti）C did not decompose by XRD diffraction analysis of the Al-Nb / Al-TiB2 / Al-（W, Ti）C system alloyed layer. Surface macro-morphology shows that there has been some degree of nodulation phenomenon on the quality of laser surface alloying with the content of alloying element Al tapering off in the mixed powders. The entire cross-section is divided into three parts: alloyed layer, transition layer and matrix. The SEM and TEM observations show that the grain refinement is significant and uniform distribution, furthermore, the new phases such as Al₂Gd, Al₂Y, Al₃Nb, etc. of in situ synthesis and strengthening phase TiB2 and （W, Ti）C are uniformly dispersed in the alloyed layer, and most present nearly spherical shape, only a small part of that shows the quadrangle blocks like, and the size of most of intermetallic compounds is about 100 nm. It is worth noting that the morphology of larger size is not a single precipitates, but agglomeration of a number of fine particles by further observation.Hardness test results show that the hardness of alloyed layer is to improve up to 4⁶ times compared with the substrate. The results of dry friction and wear reveal that friction and wear performance of the alloyed layer with different alloying elements and the mass ratio can be improved significantly, and the friction coefficient changed from about 0.52 to 0.25⁰.35. Analysis of wear morphology indicated that the main form of wear for the alloyed layer is the adhesive wear, abrasive wear and oxidation wear. Electrochemical test results show that corrosion resistance of magnesium alloy in 3.5% NaCl solution can be improved by laser surface alloying.更多还原