Ductility degradation of vacuum-plasma-sprayed NARloy-Z at elevated temperatures

Vacuum plasma spray forming is being used in the near-net fabrication of aerospace components at the Marshall Space Flight Center (MSFC), Alabama. For example, vacuum-plasma-sprayed (VPS) NARloy-Z (a copper-based alloy with high thermal conductivity) is used to form the combustion chamber liner of liquid rocket engines. VPS NARloy-Z possesses properties comparable with the wrought alloy at temperatures ranging from −259 to 316 °C. However, VPS material exhibits lower ductility than the wrought material at chamber operating temperatures (ranging from 427 to 649 °C). Because of these findings, an investigation was initiated into the mechanisms responsible for the loss of high-temperature ductility. Fracture analysis indicated that ductility loss at elevated temperatures is associated with a transition in fracture mode from transgranular to intergranular. Low-ductility specimens featured extensive grain boundary cavitation with limited grain elongation in the stress direction. Grain size also influenced the high-temperature ductility; finer grained material is more ductile. VPS NARloy-Z had more cavities at grain boundaries and a higher bulk oxygen content than wrought NARloy-Z. The ductility loss at high temperatures appears to have been caused by compounding effects of excessive oxygen and grain boundary cavitation. High-temperature ductility was improved by altering the thermal processing (pre-hot isostatic pressing (HIP) vacuum anneal, HIP and post-HIP vacuum aging) to reduce oxygen and enhance grain boundary integrity.