Study On Effect Of Boron And Carbon On Microstructure And Properties Of TiAl Alloys

The Ti-48 Al and Ti-48Al-2Cr-2Nb with boron and carbon addition were casted by vacuum induction melting in the water-cooled copper crucible were used in this paper. The microstructure of Ti Al was observed and analyzed by Scanning Electron Microscope(SEM) and X-ray Diffraction(XRD), while the mechanical properties of the alloys had been analyzed through fracture toughness, tensile test, compression test, air bending test. The effect of boron and carbon on the microstructure and mechanical properties of Ti Al were researched in this paper.The result of Ti-48 Al containing boron and carbon which boron and carbon, Ti B2 and Ti C, B4 C were added into respectively came into such conclusions as follow. The boron and carbon could refine the grain, and there were some rod-like, banding, nubbly borides and rhabditiform carbides in the microstructure. From the results of the fracture toughness, tensile test, compression test, air bending test, it was found that when the content of boron and carbon was same, the Ti-48 Al containing B4 C could acquired the best mechanical properties among the alloys while the mechanical properties of Ti-48 Al containing Ti B2 and Ti C was worst. It was observed that the size of grains, borides and carbides which distributed unforomly in the microstructure of Ti-48 Al containing B4 C was smallest. And there were a lot of long banding Ti B2 which result the bad properties around the grain boundaries in other two teams of alloys.The result of Ti-48Al-2Cr-2Nb containing different proportion of boron and carbon came into such conclusion as follow. When the proportion of boron and carbon was 2:1, the borides and carbides distributed uniformly and did not gather around the boundaries obviously. From the results of fracture, compression test and air bending test, it was found that when the proportion of boron and carbon reduce from 4:1, 2:1 to 1:1, mechanical properties of alloys became well first and then became bad. It was observed by SEM that the size of carbides became bigger and bigger and the content of carbides became more and more with the decrease of the proportion of boron and carbon. When the content and size excess a number, the properties would be bad.The heat treatment process towards the Ti-48Al-2Cr-2Nb containing boron and carbon was prepared to improve the properties according to the two element alloy phase diagram of Ti-Al. It was found that borides and carbides could pin the boundaries to prevent the size of grain increasing, but the layer of(α2+γ) became much thicker. According to the compression test and air bending test, heat treatment could improve the strength and toughness, except the alloy containing the proportion of boron and carbon was 4:1. It was observed that the borides and carbides distributed more uniformly in the microstructure of alloys had been heat treatment than which did not be heat treatment. It could show the casting properties resulting from the precision-investment casting by Zr O2 shell.