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 degrees C and then cooled at different cooling rates. With preheating time of 8 minutes, the original microstructure was replaced by coarsening alpha 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 degrees C/min, the lamellar spacing was refined from 1.77 to 0.43 mu m 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 gamma 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.