Anisotropic creep and stress rupture behaviour of laser powder bed fusion processed Hastelloy X

Abstract

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). Stress-relieve treatment (1050°C for 1 h) was performed on as-built specimens prior to creep tests. High values of creep parameters, such as apparent stress exponent, $n_a$, and apparent activation energy, $Q_a$, for vertical ($n_a$ = 8.1 and $Q_a$ = 464 kJ/mol) and horizontal ($n_a$ = 6.7 and $Q_a$ = 548 kJ/mol) specimens in the stress range of 75–150 MPa and the temperature range of 750–800°C indicated the presence of a threshold stress. This threshold stress originated from the obstructed dislocation motion by M₆C, M₂₃C₆, σ and μ phases, which dynamically formed during long-term thermal exposure. By appropriate evaluation of the threshold stress and subtracting it from the applied stress, the stress exponent values decreased to 4.5 for both vertical and horizontal specimens, respectively, indicating dislocation climb as the underlying deformation mechanism. Still, 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 more equiaxed morphology in the horizontally oriented specimen.