Determination of the constitution of the quasi–binary eutectic NiAl–Re system by DTA and microstructural investigations

The constitution of the quasi–binary eutectic NiAl–Re section of the ternary Al–Ni–Re phase diagram was reinvestigated by using differential thermal analysis, metallography and scanning electron microscopy. The alloys studied were stoichiometric NiAl and various NiAl–Rex (0.75<x<2.5 at.%) alloys. The microstructure of the as-cast NiAl–Re eutectic is of irregular fibrous morphology. Ingots with Re concentrations ⩾1.25 at.% reveal non-equilibrium microstructures which consist of primary Re crystals, NiAl crystals, and eutectic. An ideal eutectic microstructure was achieved by unidirectional solidification of a NiAl–Re1.25 alloy using Bridgman technique at a low growth velocity of 30 mm h−1. The melting temperature of the stoichiometric NiAl compound is 1674 °C. The eutectic point of the quasi–binary section of the NiAl–Re system is determined to be at 1.25 at.% Re and a temperature of 1668 °C. The solidus temperatures of the NiAl–Re ingots exhibit a slight dependence on the Re concentration. The solid solubility of Re in NiAl is 0.6 at.% at the eutectic temperature and 0.2 at.% at 1300 °C.