Microstructure and defects in γ-TiAl based vacuum arc remelted ingot materials

The intention of this paper is to describe the cast microstructure as well as to identify the nature of different defects in a Ti-46.5at%Al-4at-%(Cr,Nb,Ta,B) ingot produced by vacuum arc remelting (VAR) on an industrial scale. The ingots were triple remelted in order to obtain a good chemical homogeneity. Several defects were detected in the alloy, which can be assigned to at least 4 different categories: The first category includes any defects caused by the incorporation of interstitial impurities such as boron, nitrogen oxygen and carbon, whereas fine dispersed borides and carbides may be used for strengthening gamma alloys. he second category includes high melting/high density inclusions, which result from an insufficient dissolution of refractory alloying elements, such as niobium and tantalum. The third category of defects is the local depletion of aluminum accompanied by an enrichment of titanium. This results in an increased amount of the beta/B2-phase, and in the case of extreme depletion, titanium-rich inclusions are formed. The last category is correlated to the deviation of Al as the main reason for microstructural inhomogeneities. It is demonstrated that additional remelting steps result in improved Al deviation of less than +/- 0.5 at-% over the entire ingot. Based on our investigations, the processing technology of TiAl ingots by vacuum arc remelting has been optimized which is reflected in improved ingot quality and cost-saving aspects.