Study on improving microstructure and mechanical properties of directionally solidified Ti44Al6Nb1Cr alloy by cyclic DHT

To improve microstructure of high-Nb TiAl alloys after directional solidification (DS) further, DS Ti44Al6Nb1Cr alloy was conducted cyclic directional heat treatment (DHT) for 1 and 4 times under pulling rate of 4.17 μm/s at 1750 K, respectively. Results indicate that columnar grains grow up obviously after DHT. Compared with DS alloy (2.56 mm), the maximum width of columnar grains is up to about 9.07 mm for DHT alloy after 4 times (DHT-4). The SEM images and EDS results indicate that phase component of DS alloy has not changed after cyclic DHT for different times. It is still composed of α₂, γ and B2 phases. As cyclic DHT times increase, thickness of lamellar phases increases gradually. More massive γ phases precipitate from B2 phases and grow up. The content of B2 phases reduces due to atoms diffusing sufficiently during cyclic DHT. Tensile curves show optimal mechanical properties are obtained in DHT alloy for 1 time (DHT-1). The ultimate tensile strength is 636 MPa and total strain is 2.28%. Compared with that of DS alloy (523 MPa and 3.03%), tensile strength is improved greatly. As cyclic times increase, mechanical properties decrease for DHT alloy, due to massive γ phases coarsening and B2 phases distributing discontinuously. This microstructure could not hinder cracks propagation effectively, which may be the main reasons to result in premature fracture for DHT-4 alloy. At 973 K, the better mechanical properties are still achieved in DHT-1 alloy. The ultimate tensile strength is up to 695 MPa and total strain is 6.21%. The mechanical properties improvement is mainly resulted from directional microstructure, especially the directional lamellar clusters and B2 phases. In addition, dislocations, stacking fault bands and deformation twins are observed in lamellar γ phases of DHT-1 alloy. These microstructure changes play an important role in improving compressive mechanical properties of DHT alloy.