Tensile behavior of cast γ-TiAl alloys with varied boride morphologies

Abstract

Boride particles of various sizes and morphologies were obtained by adjusting the boron content and solidification rate of castings in Ti–45Al–2Mn–2Nb alloys. The initial and deformation microstructures, as well as the tensile properties at 25 °C and 650 °C, were examined. Furthermore, the fracture behavior of the alloys and the influence of boride particle with different morphologies on the tensile properties were discussed. Boride particles are curved flaky in 0.5B alloys, and varied from straight rod/needle-like to curved ribbon-like with increasing cooling rates in 1B alloys. Abundant phase interfaces, along with the presence of growth stacking faults and lattice distortions, are prominently observed in curved flaky and ribbon-like boride particles. Conversely, straight boride particles exhibit better regularity, with fewer lattice distortions and a lower density of interfaces and stacking faults. At both 25 °C and 650 °C, samples containing flaky boride particles exhibit the highest yield strength but the lowest elongation, while samples containing rod/needle-like boride particles show the lowest yield strength but the highest elongation. Curved boride particles exhibited no deformation, while their interfaces with the matrix accumulated a significant volume fraction of dislocations. This accumulation of dislocations causes stress concentration and created sites for the initiation of cracks. Additionally, the larger surface area of flaky boride particles promotes the growth of larger cracks, thus reducing ductility. On the other hand, straight rod/needle-like particles underwent deformation, resulting in the formation of deformation stacking faults on (0001) of C32–TiB2 and (10 1‾) of B27–TiB, showing better deformation compatibility between the borides and matrix.