Study On Oxidation And Wear Resistance Of TiAl Based Alloy Treated By Plasma Surface Chromium Alloying

TiAl-based alloy has comparable mechanical properties to the currently widedly used nickel-based super alloy, whilst its density is roughly half of the later. Therefore, the application of TiAl-based alloys in aero-industries would be very beneficial to weight reducing of machine parts and the enhancing of thrust loading of aero-engine. However, there still exist major obstacles which restrain the introducing of TiAl-based alloys to aero-engine, including insufficient oxidation resistance at high temperature and wear resistance. To upgrade oxidation and wear resistance of TiAl-based alloys with surface engineering processes has long been a focus of intensive researches. Among the available surface techniques, coatings or hard films are hard to be qualified for this purpose because of the interface problem and compatibility of the modification layer with substrate, since the aero-engine parts are usually working within complex circumstances where coexistence and even synergism of vibration, heat cycle, oxidation and wear are inevitable.In this study, to improve oxidation and wear resistance of TiAl-based alloy, plasma surface metallurgy technique was used for preparing TiAl-Cr alloy on its surface through chromizing process. The surface TiAl-Cr alloy is a diffusion layer metallurgically combined with the substrate material, which could effectively overcome the interface problem of coatings. The process provides a new choice to the surface modification of TiAl-based alloy. First-principle calculation was employed to evaluate the feasibility of the alloying process theoretically. The preparation process was optimized by investigating the effects of various processing parameters on the formation of surface alloy. The oxidation and wear resistance of TiAl-Cr surface alloy were examined compared with the bare original material. The mechanisms responsible for the improvements brought about by the addition of chromium were revealed. Besides, the characteristics of ion bombardment in the glow discharge were also discussed. The main results of the study are as follows:The calculation through first-principle method showed that the addition of chromium to TiAl lattice would increase the stability of the system, and the result proved the feasibility of the surface alloying process. With the optimized processing parameters, dense and continuous TiAl-Cr alloy with thickness of about 60μm was formed on surface of TiAl-based alloy. The surface chromium concentration was over 90% and the hardness reached 650 HV0.01, almost twice to that of the substrate material.At 650℃, the TiAl-based alloy showed superb oxidation resistance, even superior to the TiAl-Cr surface alloy. At 900℃, the TiAl-Cr surface alloy presented a remarkable improvement, the weight gain was only about a quarter to that of the untreated material. The scale of the untreated TiAl was TiO2-riched mixture of Al2O3 and TiO2 was loose and porous, which caused repeated spalling. For the TiAl-Cr surface alloy, the scale was mainly composed of Cr2O3, then evolved into combination of Cr2O3+Al2O3 +TiO2, and finally turned to an Al2O3 riched mixture of Al2O3 and TiO2. The oxide scale was dense and integrated during the whole oxidation process, with no peeling and breaking. The main reason for the improvement of oxidation resistance was owe to the enhancing of scale adhesion and the preferential oxidation of aluminum brought by the alloying effects of chromium.During the room temperature ball-on-dic sliding test, the TiAl-Cr surface alloy showed reduced friction and improvement of wear resistance. For the 500℃sliding and room temperature fretting tests, the coefficient of friction of TiAl-Cr surface alloy was a little higher than that of TiAl-based alloy, but the wear volume showed a significant reducing, and the wear resistance was improved obviously. The addition of chromium increased the strength and hardness of the TiAl-based alloy, and the load bearing and anti-adhesion capacity of the surface was also enhanced, these were the main mechanisms for the improvement of wear resistance.The ionization ratio of the glow discharge was calculated as 10-3 order of magnitude, proving that glow discharge was a weak ionization process. Majority of the power dissipation of discharge was allocated to the bombardments of ions and neutral particles on the surface of cathode. The hollow cathode effect could be used as an effective way to intensify the ionization and ion bombardment.