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For SLMed Ti–6Al–4V- x O (x = 0.11, 0.16, 0.21, and 0.25 wt%) alloys, both microstructure and mechanical properties are sensitive to oxygen content. The α′ martensites in as-built state transform to a 'bimodal' microstructure containing α lamellae interspersed with β transformed regions after 2 h annealing at 800 °C, 850 °C, 900 °C, and 950 °C followed by air cooling. The α lamellar width decreases and the β phase volume fraction increases with increasing oxygen content, due to reduced diffusivity of Al and V. The solid solution hardening effect of oxygen was quantified as: σ 0 = 472 MPa + 2159 c 0.5 , where σ 0 is the lattice frictional stress and c is oxygen atomic fraction of the alloy. Increased oxygen content causes reduced fracture toughness and higher fatigue crack growth rate in the unstable growth regime, and the appearance of bilinear growth rate of the Paris regime at high annealing temperatures. The absence of the bilinear behaviour in the low oxygen content alloy and for annealing at low temperatures was interpreted in terms of changes in microstructural characteristics caused by oxygen impeding the diffusion of Al and V. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted in the recent past to assess different nickel-based superalloys fabricated using laser-based powder bed fusion (PBF-LB). Most of these works focused on the processability, microstructural evolution, post-fabrication treatments and the mechanical behaviour of components manufactured by PBF-LB under static loading. However, limited efforts have been made to investigate the fatigue crack growth (FCG) characteristics of PBF-LB built alloys, thereby imposing a barrier to their use as critical components. As such, the aim of the present study was to evaluate and understand the effect of build orientations (0° and 90°) and post-fabrication treatments (solution heat treatment and hot isostatic pressing) on the FCG behaviour of Hastelloy X (HX) fabricated by PBF-LB process. The results from the FCG tests showed that the build orientations had no discernible influence on the fatigue threshold and FCG rate within near-threshold and Paris regions. Nevertheless, the post-fabrication treatments led to an increase in the fatigue threshold and crack growth resistance within the near-threshold region because of the alleviation of detrimental tensile residual stress alongside the crack growth direction. In conjunction with FCG tests, scanning electron microscopy and electron backscattering diffraction were used to investigate the crack path and fracture surfaces within near-threshold and Paris regions. • FCG tests were performed on as-built, SHT and HIP compact tension specimens with crack growth parallel and perpendicular to build direction. • The effect of anisotropic microstructure and residual stress on fatigue crack threshold and fatigue crack growth rate is investigated. • Transgranular mode of crack propagation was observed in near-threshold and Paris regions of crack growth in as-built and post-treated PBF-LB HX. [ABSTRACT FROM AUTHOR]