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-Re, (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 greater than or equal to 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-Re-1.25 alloy using Bridgman technique at a low growth velocity of 30 mm h(-1). The melting temperature of the stoichiometric NiAl compound is 1674 degreesC. 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 degreesC. 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 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.