Microstructure and mechanical properties of as-cast and directionally solidified large size Ti–45Al–8Nb alloys by electromagnetic confinement

Abstract Microstructure, fracture toughness and high-temperature tensile property of as-cast and directionally solidified Ti–45Al–8Nb alloys were investigated. The microstructure of as-cast alloy consists of equiaxed grains, in which there are a large number of reticular B2 phases and massive γ phases. Compared with as-cast alloy, the transverse grain boundaries of the directionally solidified alloy were eliminated, and the lamellar orientation was controlled. The volume fraction of α2 phase increased from 11.6% in as-cast alloy to 19.3% in directionally solidified alloy and the volume fraction of γ phase changed little. From as-cast condition to directional solidification, the morphology of B2 phase changed from reticular shape to elongated shape, and the volume fraction decreased from 10.2% to 1.4%. The lamellar spacing of the alloy decreased from 1500 nm to 1020 nm and it becomes more uniform. The room temperature fracture toughness increases from 21.1 MPa m1/2 in as-cast alloy to 29.5 MPa m1/2 in directionally solidified alloy. The high-temperature tensile strength and the elongation at 850 °C of the alloy increased from 376.6 ± 16 MPa and 4.18 ± 0.9% in as-cast alloy to 592.5 ± 29.5 MPa and 11.6 ± 1.5% in directionally solidified alloy. Correspondingly, the fracture mode changed from intergranular fracture in as-cast alloy to translamellar fracture in directionally solidified alloy. In addition, the toughening mechanism and strengthening mechanism were discussed.