Effect of cooling rate on solidification microstructure and mechanical properties of TiB₂-containing TiAl alloy

Effects of cooling rate and 0.25 at.% TiB₂ addition on solidification microstructure and mechanical properties of Ti–48Al–2Cr–2Nb alloys fabricated by the investment casting with different thicknesses were studied. The results show that with the cooling rate increasing from 37 to 2×10² K/s, the solidification path of the studied alloys is unchanged. The grain size of the matrix alloy is refined from 650 to 300 μm, while the grain size of Ti–48Al– 2Cr–2Nb–TiB₂ is reduced from 550 to 80 μm. The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×10² K/s, while TiB₂ addition shows little refinement effect on the lamellar spacing. Ti–48Al–2Cr–2Nb–TiB₂ sample under medium cooling rate (69 K/s) exhibits superior microhardness (HV 550) and ultimate tensile strength (570 MPa) among the studied alloys. The refined grain size, lamellar spacing and fine TiB₂ particles could account for the favorable mechanical properties of the studied TiB₂-containing alloy. The microstructure evolution was discussed in light of cooling rate, constitutional supercooling and borides addition.