| As one of the most important research fields, high-temperature titanium alloys are widely used in aero-engine industries. In order to meet the requirements of new advanced aero-engines, more and more attention has been put on titanium alloys which can be used at600℃or even higher temperatures for a long time at domestic and abroad. Ti600, developed at Northwest Institute for Nonferrous Metal Research (NIN) in China, is a near alpha titanium alloy with the rare earth element Y, which was designed for components used in turbine engines at600～650℃. A major factor responsible for limiting the use of high temperature titanium alloys up to a temperature of600℃is their poor thermal stability. Properties for the Ti alloys will deteriorate abruptly at higher temperatures. And thermal stability has great relation with the surface oxidization. Therefore, it is necessary to explore the thermal stability and the oxidation resistance of Ti600alloy.Samples were cut from bars prepared by both normal rolling process and thermomechanical processing (TMP). Then some samples were solutioned at1008℃for2h, air cooling, plus aged at650℃for8h, air cooling (α+β treatment). Other samples were solutioned at1060℃for1h, air cooling, plus aged at650℃for8h, air cooling (β treatment). Tensile tests were made at ambient temperature and at650℃on Instron1195testing machine. Creep tests were performed at650℃with the stress of100MPa for100h, using a RD-2typed creep-rupture machine. Then the tensile plasticities were measured to obtain the thermal stability for the samples exposured at600～750℃for100h. Combined with observation of optical microscopy, scanning electron microscopy (SEM) for electronfractography, transmission electron microscopy (TEM), influence of rare earth element Y and two different processing technic on microstructures and properties has been investigated for Ti600alloy. Oxidation resistance of the Ti600alloy was explored through continious oxidating method at600～750℃. Oxidized behavior were studied by oxidization dynamics, observation of oxidization layers, phase analysis, etc. All these are expected to offer some references to the practical application of Ti600alloy.Main conclusions are summarized as follows:(1) Bimodal microstucture containing10%～15%primary a phase can be got for the alloy with α+β treatment; while lamellar microstucture can be obtained with β treatment. Thermal stability and tensile plasticity was better for the alloy with bimodal microstucture, while tensile strength and creep property was worse than lamellar microstucture. Processing also had some contribution to the microstructure of the alloy. Microstructures tended to be homogeneous after TMP process, not to be refined. CoMPared with the conventional processings, strength at ambient temperature deceased and plasticity improved after TMP processing, at the same time, its high-temperature tensile properties, thermal stability and creep properties were quite well.(2) For the Ti600alloy with small amount of rare earth element Y, Y had only negligible solubility in the alloy, most of them precipitated as rare earth oxide Y2O3in the matrix and distributed dispersely. As a result, the microstructure for the alloy was refined, the aluminum equivalent was decreased, the precipitation and growth of α2phase were restrained. The strength(UTS and YS) at ambient temperature for the alloy with rare earth element Y varied negligibly, plasticity and high-temperature tensile properties improved. In addition, the precipitation and distribution of S2-(Ti、Zr)6Si3silicides and α2phase, the conFig.uration of dislocations in the alloy can be modified, which did apparently good to the thermal stability and creep resistance.(3) For the alloy exposed at600～750℃for100h with and without rare earth element Y, one of the most important factors to decrease their thermal stability was due to the brittle oxidizing layer which can induce cracks easily. The precipitation of coarse silicides and α2phase had a rather large effect on the alloy after long time exposure. However, the precipitation varied with temperature significantly. The results indicated that the amount and sizes of the two precipitation phases matched well for properties of the alloys exposed between650℃and700℃. The plasticity for the alloy without oxidized layer was better.(4) CoMParing the oxidization daynamic curves and kinetics parameters, the two alloys with and without rare earth element Y possess favorite oxidation resistance at600～700℃. The oxidization daynamic curves basicaly obeyed to parabolic ones. On the contrary, the curves approximately agreed with parabolic-straight lines at700℃with long exposure time because of the deciduous oxidization layers and severe oxidization. Oxidization was controlled by atoms diffusion. The oxidization layers were composed of rutile typed TiO2and small amount of Al2O3. |
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