Production-scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application: Part I. Production of master alloy remelt ingots and investment casting of combustor liner model panels

Abstract

Several master alloy remelt ingots of about 100 kg each with nominal composition 45 at.% Ni, 45 at.% Al, 7.5 at.% Cr and 2.5 at.% Ta were produced by vacuum induction melting. These ingots were used for a series of investment castings with variation of casting parameters in order to develop and optimise the investment casting process for this new intermetallic NiAl-based alloy. The microstructures of the remelt ingots and of various investment castings were characterised by light optical and scanning electron microscopy (SEM). The compositions of the constituent phases were determined by energy dispersive and wavelength dispersive analysis. The 0.2% yield stress, secondary creep rates and oxidation rates were determined and compared with those of materials produced on laboratory scale. Except of a decreased toughness of the production scale material the properties were otherwise comparable to those of the laboratory scale material. Though properties of the cast alloys are found not to be sensitively affected by impurities such as oxygen, carbon, iron, hafnium and zirconium, silicon was found to promote segregation during casting which is believed to result in embrittlement of the production scale material. Silicon contaminations could be avoided by usage of a ceramic-free cold-wall induction melting aggregate, but due to the coarse resulting microstructures this did not improve toughness of the production scale material.