Microstructure and mechanical behavior of the DS Ni–Al–Hf alloy

The Ni–30 at% Al–1 at% Hf alloy was prepared by a modified Bridgman method. The microstructure and mechanical properties of the alloy were studied. The results show that directional solidification (DS) of the alloy at 4 mm/min resulted in a two-phase (β+γ′) alloy which consisted of a single crystal β matrix and γ′ (Ni3Al) reinforcements. Homogenization at 1280°C/6 h followed by air cooling or oil quenching resulted in the β-NiAl phases in the alloy mostly transformed to NiAl martensite (M-NiAl). Furthermore, the ambient temperature fracture toughness of the alloy was over 20 MPa m in as-solidified state. With the solid solution strengthening of hafnium and second γ′ phase strengthening, the high temperature strength of the alloy was also improved significantly. Annealing at 1280°C for 6 h did not influence obviously the high temperature strength of the alloy, but decreased its ambient fracture toughness significantly when the cooling rate was fast enough. The microstructure/mechanical property relationships were discussed.