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

Abstract

The development of laser powder bed fusion (LPBF) has made it possible to produce complex threedimensional components for high-temperature applications. The LPBF process needs to be re ned 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 HXy specimen showed superior creep properties to the HX specimen due to the presence of carbides M 6 C and Cr 23 C 6 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.