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

The Ni-30 at% Al-l at% Hf alloy was prepared by a modified Bridgman method. The microstructure and mechanical properties of the alloy were studied. Th e results show that directional solidification (DS) of the alloy at 4 mm/mi n resulted in a two-phase (beta + gamma') alloy which consisted of a single crystal beta matrix and gamma' (Ni3Al) reinforcements. Homogenization at 1 280 degrees C/6 h followed by air tooling or oil quenching resulted in the beta-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 root m in as-solidified slate. With the solid solution strengt hening of hafnium and second gamma' phase strengthening, the high temperatu re strength of the alloy was also improved significantly. Annealing at 1280 degrees C for 6 h did not influence obviously the high temperature strengt h of the alloy, but decreased its ambient fracture toughness significantly when the cooling rate was fast enough. The microstructure/mechanical proper ly relationships were discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.