Preparation and properties of master alloys Nb-Al and Ta-Al for melting and casting of g-TiAl intermetallics

Abstract

The advantages of gamma TiAl-based alloys including their specific modulus, specific high-temperature strength and oxidation resistance make them attractive candidates as high-temperature structural materials in the automotive, aerospace and power industries. Currently most attention is paid to the alloys of the third and fourth generations. However, this type of alloys contains relatively high amounts of refractory metals such as Nb and Ta. The high melting points of these metals (2477 and 3017) °C are problematic for the preparation of these products with the conventional casting, because it is necessary to use higher temperatures and thus, generally, longer total melting times. This may result in increased oxygen amounts in the products and in decreased mechanical properties. The use of Nb-Al and Ta-Al master alloys for the preparation of the resulting Ti-Al-Nb and Ti-Al-Ta alloys is highly suitable because of the reduction in the temperature during melting. This article describes the preparation of selected master alloys Nb-60Al and Ta-80Al (x/%) with the melting points of about 1600–1650 °C using plasma melting. The optimum conditions for the preparation of these master alloys (current density, feed speed, distribution and size of charge) were characterised in order to maximise the purity and homogeneity. The prepared alloys were studied with light microscopy (LM), backscattered scanning electron microscopy (BSE), energy-dispersive spectrometry (EDS), and the melting temperature was evaluated with a differential thermal analysis (DTA).