Niobium carbide – Nickel-niobium alloy composites from a nickel coated powder: Microstructural development during sintering

The sintering of a 14 ​wt% nickel nano-dot coated NbC powder was investigated using dilatometry in combination with XRD, ToF-ERDA, PIXE, SEM-EDS/EBSD/TKD, and TEM-HAADF/EDS/EELS for analysis of samples heated to 1375, 1405 and 1500 ​°C, with a 30 ​min annealing. The main sintering step at 1110–1375 ​°C, was succeeded by a slower step, centred at 1405 ​°C, before the final smaller densification step to 1500 ​°C. NbC grain-growth took place on heating at 1500 ​°C for 30 ​min; from ca. 0.5–5 ​μm (average 1 ​μm) at 1375 and 1405 ​°C to ca. 5–30 ​μm (average 10 ​μm) 1500 ​°C for 30 ​min. The NbC0.91-0.94 phase showed a micro-hardness of 1700–2300 HV0.05. The binder phase consisted of the unprecedented textured c-Nb0.15Ni0.85 and h-Nb0.2Ni0.8 phases. The Nb–Ni binder phase yielded micro-hardness values of 1150–1250 (1375–1405 ​°C) to 810 HV0.05 (1500 ​°C, 30 ​min), which greatly exceeds nickel and is higher than Fe3Al