Effect of Post-Annealing Process on Nb-Hf Alloy Produced by Spark Plasma Sintering

In this study, the effect of annealing treatment on microstructure and mechanical properties of the Nb- 10Hf-1Ti wt.% produced by Spark Plasma Sintering (SPS) was investigated. Scanning electron microscope (SEM), optical microscopy, X-ray diffraction analysis, hardness, and uniaxial tension test were used to characterize the samples. Annealing treatment was carried out in a vacuum of 10-3 Pa at 1150 °C for 1, 3, 5, and 7 hours and in an argon atmosphere at 1350 °C for 5 hours. Internal oxidation and subsequent hafnium oxide formation causes the hardening of the C103 alloy and drastically increases the hardness and tensile strength. However, HfO2 particles formed at the grain boundary cause brittleness and cleavage fracture of samples. Volume fraction, particle size, and mean inter-particle spacing of oxides significantly change by annealing which affects the mechanical properties. The SPSed sample at 1500 °C is softened by annealing at 1150 °C for 5 hours and its hardness and yield strength are reduced from 303 to 230 Hv and 538 to 490 MPa respectively. While annealing at 1350 °C for 5 hours increases hardness and yield strength increases to 343 Hv and 581 MPa respectively.