| 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 sleevea 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 technologydoubleglow 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.
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