INTERACTION OF HfB₂ WITH NICKEL AND NICKEL ALLOY Ni–20% Cr (NICHROME)

Abstract

The interaction in the Ni–HfB₂ and NiCr–HfB₂ systems is studied. No new reaction products are found to form in the system with pure nickel in solid-phase interaction up to the contact melting temperature. Solid-state interaction in the NiCr–HfB₂ system resulting from the counter diffusion of boron from hafnium boride and chromium from nichrome leads to a layer of Cr₂B in the contact area. The contact melting begins at 1370 ± 10°C for the Ni–HfB₂ system and at 1450 ± 10°C for the NiCr–HfB₂ system. The main products of the liquid-phase interaction in the hypoeutectic part of the phase diagrams of both systems are the initial metals and the Hf₃Ni₂₀B₆ and Ni₃Hf phases, to which the Cr₂Ni₃B₆ phase in the NiCr–HfB₂ system is added. In the hypereutectic part of the phase diagrams, the metal component disappears and unreacted HfB₂ appears additionally in the NiCr–HfB₂ system. Both systems are eutectic and have eutectic melt-ing points of 1370 ± 10°C and 1450 ± 10°C for the Ni–HfB₂ and NiCr–HfB₂ systems in the range 20 ± 2% HfB₂ of their quasibinary phase diagrams. Therefore, nickel and nichrome can be used as sintering activators acting through the mechanism of the vanishing liquid phase. On the other hand, heat-resistant hafnium boride products can successfully perform in contact with nickel and nichrome products in the temperature range up to 1200°C.