Evaluation of processability, composition, and residual strain of electron beam powder bed fusion produced Mo–7Re–1HfC alloy

Due to their excellent strength, refractory metal alloys have historically been a preferred material for applications at elevated temperatures. However, overcoming the challenges posed by their high ductile–to-brittle transition temperature (DBTT), which increases crack susceptibility, along with the poor processability, is crucial for their practical applications. Electron beam powder bed fusion (EB-PBF) has emerged as a promising additive manufacturing (AM) technology suitable for processing refractory alloys with limited ductility, in part because of the elevated build temperatures and tightly controlled vacuum environment. From the composition perspective, additions of Re in refractory alloys have proven to be an effective route to enhance the ductility of the matrix. Furthermore, the incorporation of high-melting-point dispersion strengtheners has been recognized for increasing the high-temperature strength of otherwise ductile materials. In this context, a pre-alloyed and spheroidized Mo–7Re–1HfC (wt.%) powder serves as the raw material for EB-PBF fabrication. This study explores the EB-PBF processing of this material, scrutinizing the corresponding microstructure and process-induced defects through metallurgical and computed tomography analyses. High-density solid material with only internal gas porosities was achieved. The original HfC alloying component was discovered to transform into HfO₂ within the material, with some Hf–O-rich segregation regions appearing alongside the lack-of-fusion defects. Moreover, the assessment of process-induced strains of the as-fabricated sample was conducted using neutron diffraction (ND), a non-destructive method offering insights into residual strain build-up during additive manufacturing. The results indicate that the individual parameter setting has minimal impact on the overall compression strain generated along the build direction, likely due to the elevated powder bed temperature characteristic of the EB-PBF process. The hardness property of as-fabricated material was also evaluated via micro- and nanoindentation, and the result corroborates the observed compression gradient along the build direction detected by ND.