Creep anisotropy reduction and improvement via post-heat treatment in yttrium-added Hastelloy-X fabricated by laser powder bed fusion

The development of laser powder bed fusion (LPBF) has made it possible to produce complex three-dimensional components for high-temperature applications. The LPBF process needs to be refined to address several key factors, such as high-temperature elongation, microstructure heterogeneity, and mechanical anisotropy. Hastelloy-X Ni-based superalloy was used to illustrate these issues in this study. First, yttrium (Y) was added to Hastelloy-X alloy to prevent grain boundary embrittlement. The second step involves post-heat treatment at a higher temperature to eliminate the microstructural heterogeneity and creeps anisotropy. We used Hastelloy-X (HX) as reference material, along with 0.046 wt.% Y (HX-y). The aging was done at 850°C for 2 h (DA) for carbides and solution heat-treatment at 1240°C for 8 h upon aging at 850°C for 2 h (HTA) to eliminate microstructural heterogeneity. The creep properties were studied at 900°C/ 80 MPa. The HX-y specimen was strengthened by solid solution and Y-rich oxides and stabilizing oxygen-based contamination at the grain boundary. The DA and HTA HX-y specimens had better creep properties than HX specimens. The HX-y specimen showed superior creep properties to the HX specimen due to the presence of carbides M6C and Cr23C6 inside grains and at grain boundaries. However, carbides remained stable even at high temperatures within grains and at grain boundaries. Nevertheless, the HTA HX-y specimen exhibited superior isotropic creep properties. As a result of grain boundary pinning, serrated grain boundaries prevented grain boundary sliding. In contrast, HX specimens exhibited poor creep properties.