Effect of cyclic DHT on microstructural evolution and mechanical properties of Ti44Al6Nb1Cr2V alloy

Abstract

To clarify effects of microstructure on tensile properties of high Nb-TiAl alloys after cyclic directional heat treatment (DHT), Ti44Al6Nb1Cr2V alloys are conducted DHT for 1, 2 and 3 cycles (DHT-1, DHT-2 and DHT-3) at 1730 K, respectively. The pulling rate is set as 4.17 µm/s. Results show the phase component mainly consists of γ, α2 and B2 phases for Ti44Al6Nb1Cr2V alloy after cyclic DHT. Directional microstructure is obtained. Lamellar clusters are columnar and B2 phases present banded morphology. They are distributed alternately. As DHT cycles increase, the radial size of lamellar clusters enlarges as well. Blocky γ phases grow up and coarsen. At room temperature, good tensile properties are obtained in Ti44Al6Nb1Cr2V alloys after cyclic DHT. Tensile strength is about 609 MPa for DHT-1 alloy. Their fractures show that cleavage microstructure is mainly formed for cracks propagation during tensile test. Both DHT-1 and DHT-3 alloys present good tensile properties stability at 900 °C. Their yield strength, ultimate tensile strength and elongation are about 355–487 MPa-3.3% and 355–421 MPa-1.5%, respectively. Combining their microstructures with fractures, it concludes for their tensile properties difference that many blocky γ phases precipitate and coarsen during cyclic DHT, which decreases deformation coordination between lamellar clusters and banded B2 phases. Some microcracks form easily near B2 phases. Consequently, tensile properties of DHT-3 alloy decrease. This study will provide basic experimental data and theoretical support for high Nb-TiAl alloys to further optimize DHT parameters and improve their mechanical properties.