Study on improving directional microstructure of Ti44Al6Nb1Cr alloy by continuous regional phase transformation

Abstract

To improve the directionally solidified (DS) microstructure and mechanical properties of high-Nb containing TiAl alloys, DS Ti44Al6Nb1Cr alloy was directionally heat treated (DHT) with pulling velocity of 4.17 µm/s at 1750 K. Microstructure and grains orientation were characterized by the OM, SEM, TEM and EBSD. Chemical composition and phase component were detected by the EDS and XRD. Results indicate that wide columnar grains are formed along the axial direction after DHT. Microstructure is still fully lamellar and composed of α2, γ and B2 phases. Crystallographic orientation relationship among phases (α2, γ and B2) is not changed. Lamellar clusters in columnar grains change from blocky to columnar, and the reticular B2 phases become banded structure after DHT. During DHT, the β phase transforms into α phase continuously and α grains grow into columnar clusters. Nb and Cr atoms can diffuse more adequately under the influence of directional heat flow and segregate at boundaries of columnar clusters. Therefore, lamellar clusters and B2 phases are distributed almost in parallel along the axial direction of columnar grains at room temperature. Tensile testing shows that the yield strength, ultimate strength and total strain of the DHT alloy are increased to 574 MPa, 636 MPa and 2.28% from 422 MPa, 523 MPa and 3.03% of as DS alloy, respectively. The tensile strength increases about 36% and this improvement is mainly caused by the directionally aligned B2 phases. In addition, a large number of dislocations and stacking fault bands are formed in lamellar γ phases of DHT alloy after testing, which also plays an important role in improving mechanical properties of DS alloy.