Microstructure and mechanical properties of rapidly solidified and annealed Ni-Al intermetallic alloys

A series of rapidly solidified Ni-Al intermetallic alloys containing a nickel concentration in the range 61.3-76 at.% have been produced by the hammer and anvil technique. Electron metallography ductility and hardness measurements have been carried out to characterize the microstructure and mechanical properties of these alloys in both quenched and 1000°C annealed conditions. Rapid solidification, which has little effect on the composition at which γ(Ni₃Al) first forms, shifts the composition at which β(NiAl) terminates towards a higher nickel content. The observed microstructures suggest the existence of a metastable β + α(Ni) eutectic and the accompanied suppression of the peritectic reaction by which γ(Ni₃Al) forms. The γ(Ni₃Al) phase is believed to form through ordering of the supersaturated α(Ni). Annealing at 1000°C produces phases the stability ranges of which are consistent with the equilibrium phase diagram. A ductility maxima of 2% observed at 65 at.% Ni, in both as quenched and annealed alloys, is neither related to strain induced martensitic transformation of β phase nor the existence of α/β interphase boundaries.