Optimizing microstructure and mechanical properties of directionally solidified Ti44Al6Nb1Cr2V alloy by directional heat treatment

To optimize microstructure and improve mechanical properties of directionally solidified (DS) TiAl alloys containing high alloying elements, DS Ti44Al6Nb1Cr2V alloy was conducted directional heat treatment (DHT) at 1730 K. Results indicate that columnar grains grow up straightly after DHT. It decreases from 17.55 degrees to 10.67 degrees for the angle in the axial direction between columnar grains and ingot. Morphology of B2 phases changes obviously, from reticular to banded microstructure. Lamellar clusters grow up greatly and are parallel to the growth direction of columnar grains. These changes are mainly caused by sufficient atoms diffusion during DHT. alpha grains grow up adequately and residual B2 phases reduce. Compared with that of DS Ti44Al6Nb1Cr2V alloy (505 MPa and 1.40%), ultimate tensile strength and total strain are 609 MPa and 1.86% for DHT alloy, respectively. They are increased by 21% and 33%, respectively. Fracture shows that many tearing edges and some similar dimples morphology are also found in DHT alloy, besides cleavage planes and cleavage steps. TEM image shows that many dislocations are produced in lamellar gamma phases of DS alloy. They wind and intercross each other. Dislocation interaction is lower and the stress concentration is less for DHT alloy, due to thin lamellar gamma phases. Furthermore, some stacking faults are also produced in lamellar gamma phases, which benefits to improving mechanical properties. Therefore, mechanical properties are improved greatly for DS alloy after DHT.

Automated summary

After directional heat treatment (DHT) at 1730 K, DS Ti44Al6Nb1Cr2V alloy exhibited straight columnar grain growth, significant changes in B2 phase morphology, and improved mechanical properties. The ultimate tensile strength and total strain increased by 21% and 33%, respectively, compared to the untreated alloy. The DHT process resulted in reduced residual B2 phases and improved fracture behavior with the generation of stacking faults in lamellar gamma phases.