Evaluation of High-Vacuum Annealing and Hot Isostatic Pressing on the Microstructure and Properties of an Additively Manufactured Niobium Alloy

Niobium alloy C-103 is a refractory alloy used in the manufacturing of spacecraft propulsion components for its mechanical properties at elevated temperatures. Fair weldability makes C-103 additively manufacturable (printable) utilizing laser powder-bed fusion (L-PBF). Printed C-103 parts are often subjected to hot isostatic pressing (HIP) to obtain a microstructure with fewer defects and more isotropic properties. Brief exposures to high-temperature environments are common for C-103 parts; experimentally, a high-vacuum annealing (HVA) process was employed to identify changes from temperature effects apart from the HIP process. This study compares the effects of HVA and HIP processes on the microstructure (i.e., grain size, morphology), formation of precipitates, microhardness, and anisotropy of C-103 fabricated by L-PBF. The results showed recrystallization and texture changes occurred during HIP but not HVA, contributing to retained, albeit altered, anisotropy in the HVA-ed material. The microhardness of HVA-ed C-103 was greater and more anisotropic than HIP-ed for annealing temperatures below 1200 degrees C. Hafnium oxide precipitates were observed in all the samples and which coarsened with increasing HVA temperature. Oxide area% determined via image analysis showed a strong negative correlation with microhardness. Overall, this paper presents novel findings on the effects of HVA and HIP on C-103 printed via L-PBF and forms a basis for future post-processing of C-103 parts.