Refining Ti₂AlC in the liquid region and improving the microstructure and mechanical properties of Ti-46Al-8 Nb-2.6 C alloy by adding Y

To reveal the refinement mechanism of Ti₂AlC particle by in-situ Y₂O₃ in the liquid region and the improvement mechanisms of refined Ti₂AlC in solid-state phase transformation process, Ti-46Al-8 Nb-2.6 C-xY alloys were prepared. Microstructures were observed by SEM, XRD and TEM, and the mechanical properties were examined. Calculated results indicated that the phase boundary energy of in-situ Y₂O₃ combined with TiC was low in the liquid region. TiC particle was formed initially, followed by Y₂O₃ particle. The Y₂O₃ particle moved to the surface of TiC particle to inhibit its growth, which left small-sized TiC nucleation substrate for the formation of Ti₂AlC. The orientation relationship was [2̅200]_Ti2AlC//[06̅6]_Y2O3 . A stepped atomic diffusion zone was formed in the interface of Y₂O₃ and Ti₂AlC to inhibit lateral and longitudinal growth of Ti₂AlC. As Y increased from 0.02 to 0.1 at%, the average aspect ratio of Ti₂AlC particles decreased from 7.7 to 3.4, the Ti₂AlC content with the aspect ratio of 1–5 increased from 13.7 to 91.3%. The lamellar colony size and lamellar spacing reduced from 47.5 to 26.0 µm and 0.74 to 0.58 µm, respectively. Both Y₂O₃ and refined Ti₂AlC served as heterogeneous nucleation sites and inhibited the lateral thickening rate of γ lath. The Y-type YAl₂ phase was formed as Y content exceeded 0.1 at%. The compressive strength and Vickers hardness were improved from 1801 to 2201 MPa and 443.2 to 464.5 HV, respectively. However, the subsequent decrease in strength was caused by the embrittlement of grain boundary after the precipitation of YAl₂ phase. The significant refinement of Ti₂AlC and lamellar colony by in-situ Y₂O₃ and the precipitation strengthening of Ti₂AlC contributed to the mechanical properties.