【作者】 梁文萍；

【导师】 徐重；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2007， 博士

【摘要】 Ti-Al系金属间化合物比强度高，其高温强度和刚性都比Ni基和Ti基合金高，而且还具有优良的抗高温氧化性，是航空、航天飞行器理想的新型高温结构材料，在汽车工业上也有广阔的应用前景。Ti₂AlNb O相合金是在Ti₃Al基础上发展起来的：在Ti₃Al中加入β稳定元素Nb，改善Ti₃Al基合金的室温塑性和断裂韧性。国内外研究表明，Ti₂AlNb合金具有“三高一低”的优异性能，即：高的高温屈服强度、高的蠕变抗力和断裂韧性，以及低的缺口敏感性，作为新一代高温结构材料显示出了巨大潜力。然而Ti-Al系金属间化合物也存在一些性能上的缺陷，使得其应用受到限制。航空发动机有许多摩擦副零件，对材料表面的耐接触磨损、冲刷磨损或微动磨损提出了很高的要求。大量的研究与试验表明，Ti-Al系金属间化合物的耐磨性不充分而不能满足上述要求。为了尽快满足航空航天以及国防军工等部门对轻比重、高性能的高温结构材料的迫切需求，Ti-Al系金属间化合物的耐磨性能的提高与解决已成为关键的工程问题之一。本课题源于2004年国防科工委所下达的民口配套计划，是针对我国先进航空发动机领域中的关键零件—热端部件隔热套摩擦副的研制而提出的，该摩擦副由γ-TiAl制成的旋流器与Ti₂AlNb O相合金制成的喷嘴组成，零件的耐磨性急需解决。提高材料的表面耐磨性的方法很多，如离子渗氮与渗碳、离子注入、CVD、PVD以及激光表面熔覆等，但针对于Ti-Al金属间化合物尤其是Ti₂AlNb的应用研究却很少。本课题采用我国独创且具有自主知识产权的双层辉光等离子表面冶金技术，在Ti₂AlNb O相合金表面渗Mo、Cr以提高其耐磨性，并兼顾保持其优良的耐蚀性和抗高温氧化性。双层辉光等离子表面冶金技术的特点在于：表面合金层深度和成分可控，渗入元素含量随深度呈梯度分布，与基体间无明显界面，结合牢固，无裂纹。本文的研究内容有如下几个方面：①Ti₂AlNb O相合金双层辉光等离子渗Mo、Cr工艺研究，探讨工艺参数对表面合金层形成的影响规律；②对表面合金层的组织成分、相结构、硬度等进行分析研究；③表面合金化试样的摩擦磨损性能及机理研究；④表面合金化试样的电化学腐蚀性能及机理研究；⑤表面合金化试样的抗氧化性能及机理研究。经过反复试验验证，总结出Ti₂AlNb O相合金双层辉光等离子渗Mo的最佳工艺参数为：温度T=950℃～1000℃，时间t=3～5h，气压P=30～50Pa，极间距d=15mm，源极电压Vs=800～1100V，阴极电压Vc=300～600V。Ti₂AlNb O相合金双层辉光等离子渗Cr的最佳工艺参数为：温度T=980℃～1000℃，时间t=4h～5h，气压P=40～50Pa，极间距d=18mm，源极电压Vs=800～1100V，阴极电压Vc=300～600V。Ti₂AlNb O相合金双层辉光等离子渗Mo及渗Cr试样表面均形成了均匀致密的合金层。在最佳工艺条件下，Ti₂AlNb O相合金双层辉光等离子渗Mo层有效厚度达100μm，渗层组织呈板条状，表面约有6μm的Mo沉积层，其含量达95 wt％，沉积层下是Mo的扩散层，由表及里Mo含量逐渐降低，钼元素主要以纯Mo和Al₅Mo相的形式存在，表面硬度高达HV 800以上；Ti₂AlNb O相合金双层辉光等离子渗Cr层有效厚度达30μm，渗层组织呈板条状，各相间隔交织。表面的Cr含量达38 wt％，随着扩散深度的增加，Cr含量逐渐降低，呈梯度分布。渗层主要由Al₈Cr₅和AlNb₂组成，还存在少量的Cr₂Ti及Cr₂Nb相。表面硬度最高达HV 950。表面合金层中含Mo、Cr的硬质金属间化合物的形成对提高材料的耐磨性起到有益的作用。在Ti₂AlNb双辉等离子渗Mo的工艺参数中，温度、时间对其表面硬度均有较大的影响，而气压、极间距的变化对硬度影响不显著。Ti₂AlNb离子渗碳后表面硬度达HV 540，Ti₂AlNb离子渗氮后表面硬度最高达HV 575，从提高Ti₂AlNb O相合金耐磨性的目的考虑，采用双层辉光等离子渗Mo及渗Cr效果更显著。摩擦磨损性能的试验，采用了常温以及500℃高温下的干摩擦磨损性能试验两种方法。在室温下，Ti₂AlNb基材产生了较高的摩擦系数，其磨痕形貌表现为严重的粘着、擦伤、撕裂与塑性流动的特征，磨损机理为粘着磨损并伴有磨粒磨损，磨损量较大，磨损抗力较低，耐磨性能较差；渗Mo试样摩擦系数非常低，随滑动时间的延续几乎无变化，磨损失重远低于基材，磨损量很小，表现出了优良的摩擦磨损性能；渗Cr试样的摩擦系数也非常低，随滑动时间的延续几乎无变化，磨损量很小，表现出了优良的摩擦磨损性能。500℃的高温摩擦磨损试验表明：Ti₂AlNb基材表面形成的氧化物使表面变光滑，减轻了表面的粘着磨损，摩擦系数在0.32～0.75之间反复波动，其高温磨损机制主要是氧化磨损和氧化物脱落成造成的磨粒磨损，以及少量的粘着磨损；渗Mo试样表面粗化，有明显氧化剥落和蚀坑，摩擦系数较高，在0.4～1.0之间波动，磨痕既深且宽，磨损机理是以磨粒磨损和粘着磨损为主，疲劳磨损为辅；渗Cr试样表面光滑、无蚀坑、无氧化剥落，摩擦系数在0.5～1.1之间波动，磨损机理是以粘着和微犁削为主，疲劳磨损为辅。渗Cr试样的高温磨损的比磨损率较Ti₂AlNb基材和渗Mo试样低，说明渗Cr试样具有最好的耐高温磨损性能。腐蚀性能的试验采用了电化学腐蚀性能研究及中性盐雾腐蚀试验两种方法。在5％H₂SO₄溶液中，Ti₂AlNb基材及渗Cr试样均发生钝化现象，表现出良好的耐蚀性能。渗Mo试样钝化区很窄，腐蚀速度最大，耐蚀性相对较差，表面产生轻度的全面腐蚀，这是由于表面没有形成稳定钝化膜的缘故。在5％HCl溶液中，Ti₂AlNb基材、渗Mo及Cr试样均发生明显钝化现象，渗Mo试样表面产生轻微的全面腐蚀。渗Cr试样表面形成稳定钝化膜，但很小的局部发生点蚀，三者均表现出了较好的耐蚀性能。在3.5％NaCl溶液中，Ti₂AlNb基材、渗Mo及Cr试样表面均发生钝化，形成钝化膜，表现出了较好的耐蚀性能。中性盐雾腐蚀试验表明，经144小时试验的Ti₂AlNb基材和渗Cr试样表面形态和颜色没有明显的变化，耐蚀性优良；渗Mo试样表面仅发生色泽变化，无腐蚀产物，也具有较好耐蚀性。试样的抗高温氧化性能通过650℃、750℃、850℃断续氧化100h试验来评价。650℃下，Ti₂AlNb基材、渗Mo及渗Cr试样氧化增重均小于1mg／cm²，达到了完全抗氧化级别，三种试样的氧化产物均有TiO₂、Al₂O₃膜生成，氧化膜均匀致密，无孔洞裂纹，具有较好的抗高温氧化性能。750℃下，Ti₂AlNb基材及渗Mo试样增重相对较大，氧化速度服从抛物线定律，而渗Cr试样在此温度下抗氧化性能最好。850℃下，三种试样增重氧化速率均较高，呈直线上升，氧化膜明显疏松，存在孔洞与裂纹，表明超出三种试样的稳定抗氧化温度范围。最后，作者运用数理分析的方法研究了Ti₂AlNb双辉等离子渗Mo各工艺参数对硬度的影响，并对试验进行验证。分析说明各工艺参数如工作温度、保温时间对Ti₂AlNb双辉等离子表面渗Mo显微硬度均有显著影响，而工作气压、极间距对Ti₂AlNb双辉等离子表面渗Mo显微硬度无显著影响，与试验结果相吻合。采用最小二乘法求得Mo元素浓度分布曲线的回归方程，计算出在980℃双层辉光离子轰击条件下Mo在Ti₂AlNb中的扩散系数D≈5.4～6.1×10^-14m²·sec^-1，并且在30Pa～50Pa范围内，工作气压实际上对扩散系数没有影响。系统的研究结果表明，Ti₂AlNb O相合金双辉等离子渗Mo及渗Cr是提高其耐磨性能的切实有效的方法。其中，双辉等离子渗Cr在兼顾保持其优良的耐蚀性和抗高温氧化性方面更具优越性。更多还原

【Abstract】 Ti-Al intermetallic compounds have high specific strength, their strength andstiff under elevated temperature are higher than that of Ni-base and Ti-base alloys.These alloys have also excellent oxidation resistance under high temperatures, sothat regarded as ideal structural materials for elevated temperature applications inaeronautics and astronautics, also exhibit great potential in automobile industry.Ti₂AlNb is an alloy developed on the base of Ti₃Al: addingβ-stabilizing elementNb to improve the fracture toughness and the plastics under room temperature.Numeral researches from domestics and over sea have revealed that the propertiesof Ti₂AlNb exhibit 4 features, which are high yield strength under elevatedtemperature, high creep resistance, high fracture toughness as well as lowsensitivity to jags. These features have supported Ti₂AlNb becoming one of themost attractive structural materials for high temperature applications.However, there are also some disadvantages in the properties of Ti-Al intermetallic compounds that have restricted their actual uses. There are manyfriction couples in air engines, which demand satisfactory performances ofresisting to contacting wears, scouring as well as micro- moving wears. Greatnumbers of researches and experiments have already indicated that the wearresistance of Ti-Al intermetallic compounds can not meet those requirements. Asthe need for high temperature structural materials is so urgent in aeronautics andastronautics especially for defence purposes, improving the wear resistance ofTi-Al intermetallic compounds has become a key engineering problem.This research was supported by State Commission of Defence Science andTechnology and Industry as a project of supporting from civil division. The task isdeveloping new friction couple of heat-separating sleeve_a key component fornext generation air engine. The friction couple is composed of a revolving fluidguider made ofγ-TiAl and a nozzle made of Ti₂AlNb. Improvements on wearresistance of both parts are needed.There are many methods to improve wear resistance of a material such ascarbonizing, nitridizing, ion implantation, CVD, PVD, laser clad, et al. But fewworks targeted on Ti-Al intermetallic compounds, especially on the appliedstudying on Ti₂AlNb alloys. In this research, an advanced technology_doubleglow plasma surface alloying had been applied to improve the wear resistance ofTi₂AlNb. The actual processes were molybdenizing and chromizing. At the sametime, it had good corrosion resistance and anti-oxidation. The advantages of DGtechnology include: the depth and composition of alloying layer can be controlled easily, the concentration of alloying elements distributed in gradient with nointerface and no cracks. The detail research contents are as blow:①Researches on the process of molybdenizing and chromizing on Ti₂AlNb,investigating the effects of major parameters on the forming of alloying layers;②Investigating the composition, phase structure, hardness of surface alloyinglayers;③Wear tests and studies on the mechanisms of wear;④Electrochemical experiments and studies on the mechanisms of corrosion;⑤Experiments of oxidation resistance under high temperatures, studying onthe mechanisms of anti-oxidation.Through systematical experiments and verification, the author had concludedthe optimum process parameters of plasma molybdenizing on Ti₂AlNb as below:temperature T=950℃～1000℃, diffusing time t=3～5h, working pressure P=30～50Pa, distance between source cathode and work-piece d=15mm, voltage ofsource cathode Vs=800～1100V, Voltage of cathode （work-piece） Vc=300～600V; the optimum process parameters of plasma chromizing on Ti₂AlNb as below:temperature T=980℃～1000℃, diffusing time t=4～5h, working pressure P=40～50Pa, distance between source cathode and work-piece d=18mm, voltage ofsource cathode Vs=800～1100V, Voltage of cathode （work-piece） Vc=300～600V.Both the molybdenizing layers and the chromizing layers are homogeneous and dense. Under optimum process condition, the effective thickness of themolybdenizing layers surpassed 100μm with a morphology of lath-shape. Therewas a 6μm deposited layer of Mo with a concentration of 95wt％. Beneath thedeposited layer formed the diffusion layer in which the content of Mo decreased ingradient. Molybdenum existed mainly in the form of pure element and Al₅Mo. Thesurface hardness of the molybdenized specimen was above HV800. Underoptimum process condition, the effective thickness of the chromizing layerssurpassed 30μm with a morphology of lath-shape intertexture. No deposited layerformed although the concentration of Cr reached 38wt％. With the increase of thediffusion depth, the content of Cr decreased in gradient. Al₈Cr₅ and AlNb₂ were themajor phases of the chromizing layer, but there were also a few Cr₂Ti and Cr₂Nb.The surface hardness of the chromized specimen reached HV950. The forming ofhard intermetallic compounds containing Mo or Cr contributed an improvement ofwear resistance of Ti₂AlNb.In the process parameters of plasma molybdenizing on Ti₂AlNb, temperatureand time had significant effects on the surface hardness of the specimen, whereasthe effects of pressure as well as distance between source cathode and work-piecewere relatively weak.Comparison processes of carbonizing and nitridizing on Ti₂AlNb were alsocarried out. The surface hardness of carbonized specimen was HV 540, and that ofnitridized specimen was HV 575. In the meaning of improving wear resistance ofTi₂AlNb, plasma molybdenizing and chromizing are more effective. The evaluation of wear behaviors were achieved through ball-on-disc dryrubbing under room temperature and 500℃.At room temperature, Ti₂AlNb matrix recorded a much high frictioncoefficient, and the surface of worn trace exhibited adherence, scratching, tearingand features of plastic flowing. The mechanism of wear was adherence as well as alittle abrading. In general, Ti₂AlNb matrix exhibited poor wear resistance as theworn volume was much larger. In contrast, both the molybdenized specimen andthe chromized specimen all recorded a very low and constant friction coefficient,and much smaller worn volumes reflected an excellent performance of wearresistance.At 500℃, the friction coefficient of Ti₂AlNb matrix waved between 0.32 and0.75, and the surface of worn trace was relative smooth due to the forming ofoxides reduced adherence. The mechanism of wear was oxidizing as well as a littleabrading resulted from the falling of oxides. The friction coefficient of themolybdenized specimen waved between 0.4 and 1.0. The worn trace of themolybdenized specimen was relative wider and deeper, It also became coarser withflakes and eroded pits were observed clearly. The wear mechanism of themolybdenized specimen was the combination of adherence, abrasion andfatigue-wear. The friction coefficient of the chromized specimen waved between0.5 and 1.1, and the wear mechanism was the combination of adherence,micro-plowing and fatigue-wear. The specific wear ratio of the chromizedspecimen was the smallest one that indicated an excellent wear resistance at 500℃. The corrosion properties were investigated through electrochemicalexperiments and neutral salt spray test.In 5％H₂SO₄ solution, both Ti₂AlNb matrix and the chromized specimenexhibited favorable corrosion resistance as constant passivation took place. Themolybdenized specimen behaved a narrow passivation and had a relative biggercorrosion rate. Its surface had been eroded slightly due to failed to form a constantpassivation film. In 5％HCl solution, 3 specimens all exhibited favorable corrosionresistance as constant passivation took place. The surface of the molybdenizedspecimen eroded slightly homogeneously. A few etch pits were observed on thesurface of the chromized specimen. In 3.5％NaCl solution, all the specimensbehaved constant passivation so that exhibited favorable corrosion resistance.Neutral salt spray test continued 144 hours. The configurations of Ti₂AlNbmatrix and the chromized specimen had no visible change that indicated anexcellent anti-salt-spray performance. The color of the molybdenized specimenturned to blue, but no corrosion product had been spotted.The behaviors of oxidation resistance were evaluated through non-continuesoxidizing in air under 3 temperatures. At 650℃, the weight gains of all thespecimens did not exceed lmg/cm² that indicated a complete grade of oxidationresistance was achieved according to national standard. The oxidizing films werecomposed of TiO₂ and Al₂O₃. They were also dense without any cracks. At 750℃,the weight gains of Ti₂AlNb matrix and the molybdenized specimen were relativelarger. Their oxidizing rates obeyed parabola rule. The chromized specimen behaved a better oxidization resistance. At 850℃, the weight gains of 3 specimensincreased sharply. The oxidizing films became loose as many holes and cracksexisted. It is confirmed that 850℃exceed the durability of stable oxidizationresistance.Finally, using the method of mathematical analyses, the author discussed theeffects of key process parameters on the surface harness and verified theexperimental results. Analyses revealed that temperature and time playedremarkable roles, but pressure and the distance between source cathode andwork-piece were ignorance. Another work of the author was deducing out theregression equation of the concentration distribution curve of Mo with the help ofthe least squares method. Then calculated out the diffusion coefficient of Mo inDG process at 980℃, that is D≈5.4～6.1×10^-14m²·sec^-1. Actually, in the range of30Pa～50Pa, the effects of pressure on the diffusion coefficient of Mo wasignorance.Through systemic research, we can give out the conclusions of this study asbelow: Double glow plasma molybdenizing or chromizing is an effective andpractical process to improve the wear resistance of Ti₂AlNb Orthorhombic alloy.Chromizing had a better performance in respect of sustaining excellent corrosionresistance and oxidation resistance under elevated temperature.更多还原