Creep behaviour of laser powder bed fusion processed Hastelloy X

At elevated temperatures, creep becomes a critical parameter for performance under service conditions. In this work, a detailed investigation of creep and stress rupture behaviour was carried out on laser powder bed fusion processed Hastelloy X in two orthogonal directions, i.e., vertical (along build direction) and horizontal (perpendicular to build direction) directions. Stress relieving treatment (1050 °C for 1 hour) was performed on as-built specimens prior to creep tests. Higher values of creep parameters, such as apparent stress exponent, na, and apparent activation energy, Qa, for vertical (na = 8.1 and Qa = 464 kJ/mol) and horizontal (na = 6.7 and Qa = 548 kJ/mol) specimens in the stress range of 75-150 MPa and the temperature range of 750-800 °C suggested the presence of threshold stress. This threshold stress originated from the obstructed dislocation motions by M6C, M23C6, σ and µ phases, which dynamically formed during long-term thermal. By subtracting the threshold stress from the applied stress, the stress exponent values decreased to = 4.5 for vertical and horizontal specimens, respectively, indicating dislocation climb as the underlying deformation mechanism. Furthermore, the creep and stress rupture properties were notably superior in the vertically oriented samples compared to the horizontally oriented ones. This difference was primarily attributed to the columnar grain morphology observed in the vertically oriented specimen, as opposed to the equiaxed morphology in the horizontally oriented specimen.