Microstructure Evolution And Mechanical Properties Of TiB+TiC+La₂O₃/γ-TiAl Alloys

γ-TiAl has been considered as next generation of high temperature structure materials based on its low density , high strength at elevated temperature and good creep resistant .The elastic module ofγ-TiAl and creep resistant are higher than titanium alloys, and reach the level of Ni-based alloys. Moreover ,the density of titanium aluminides is lower than half of Ni-based alloys density. The work temperature ofγ-TiAl can reach as high as 900℃,and elastic module decreasing slightly as work temperature rises is as high as 176 GPa at room temperature. Consequently, titanium aluminides can fill in the blank between Titanium alloys and Ni-based alloys. Titanium alumindes has become candidate material of key high temperature component in areo space industry, such as blade of aviation engine. However, limited ductility and hot workability hinder further applications of Titanium aluminides. At present, much effort has been devoted to microstructure refinement inγ-TiAl. The general consensus is that boron addition can refine lamellar microstructure ofγ-TiAl through formation of boride during alloy solidification. However, borides grow to long ribbon shape inevitably in as-cast boron-bearingγ-TiAl, which is detrimental to mechanical properties ofγ-TiAl, especially to room temperature ductility.In this work, LaB6,TiB2 and C were added toγ-TiAl alloys , and TiB(TiB2), rare earth oxide and TiC formed by in-situ reaction. Effects of TiB(TiB2), rare earth oxide and TiC onγ-TiAl microstructure refinement and evolution were investigated. The results showed that when two of the above ceramic phases exist inγ-TiAl, microstructure is further refined in comparison with the microstructure refined by boron addition. Moreover, it was found that ceramic phase prompted the formation lamellar microstructure and changed the morphology of primary betaβdentrite.Based on TEM observation of TiB inγ-TiAl alloys, co-effect between ceramic phases during solidification and boride morphology refinement were investigated. The result showed that presence of ceramic phase with higher melting point than TiB which supplied heterogeneous nucleation core could refine TiB morphology.To further investigate the effect of microstructure on mechanical properties ofγ-TiAl bearing ceramic phases, different annealing methods inα+γregion were used to generate different microstructures. The results showed that coarsen of lamellar microstructure happened during annealing inα+γregion, and blockyγphase formed during coarsen course. Tensile strength and ductility had been improved when blockyγphase formed.