Continuous-Cooling-Transformation (CCT) Behaviors and Fine-Grained Nearly Lamellar (FGNL) Microstructure Formation in a Cast Ti-48Al-4Nb-2Cr Alloy

The CCT behaviors of a high-Al mid-Nb-containing cast TiAl alloy were experimentally studied in the current work. Samples with the nominal composition of Ti-48.2Al-4Nb-2Cr (at.pct) were preheated to 1420 °C and then cooled at different cooling rates. With preheating time of 8 minutes, the original microstructure was replaced by coarsening α grains. Interrupted tests demonstrated the significant decrease of microsegregation which was also predicted by theoretical calculations. The water-quenched microstructures during continuous cooling were characterized. With the increase of cooling rate from 5 to 700 °C/min, the lamellar spacing was refined from 1.77 to 0.43 μm and the microhardness was heightened from 299 to 426 HV, showing the solute strengthening effect of Nb addition compared with 4722 and Ti-48Al alloys. The CCT diagram of the current alloy was established to guide the microstructure control. Slow cooling contributed to the discontinuous coarsening of γ lamellae, suggesting a nearly lamellar (NL) microstructure. The addition of 4 at. pct Nb increased the temperatures of phase transformations, moved the CCT curves to higher temperature, and decreased the critical cooling rate for the formation of Widmanstatten and feathery structures. Finally, a fine-grained NL (FGNL) microstructure with a potential high performance was obtained.