Effect of columnar to equiaxed transition on microsegregation behaviour of main alloying elements in peritectic TiAl-based alloy

Abstract

The effect of columnar to equiaxed transition (CET) on the microsegregation behaviour of main alloying elements (Ti, Al and Nb) was studied in a peritectic Ti-44Al-5Nb-0.2B-0.2C (at.%) alloy. CET microstructures formed in cylindrical samples prepared by power down technique at a constant cooling rate of 0.5 K s−1 in a Bridgman type apparatus and in a large as-cast ingot were analysed. Microstructural analysis showed that the CET is not sharp but develops gradually during solidification in the studied alloy. A mixed zone composed of columnar and equiaxed grains is formed between the columnar and equiaxed zones. The size of columnar and equiaxed grains depends on the position in the as-cast cast ingot. The severity of the microsegregation expressed by segregation deviation parameters calculated for Nb decreases from columnar to the equiaxed zone in the samples prepared by power down technique as well as in the as-cast ingot. While the calculated segregation deviation parameters for Al and Ti decrease from columnar to equiaxed zone in the samples prepared by power down technique, these parameters show no evolution with the local microstructure in the as-cast ingot. The calculated values of segregation deviation parameters for Al and Ti are significantly lower than those for Nb in all analysed columnar, mixed and equiaxed zones of the samples. K e y w o r d s: titanium aluminides, TiAl, solidification, CET, energy-dispersive spectrometry, microsegregation