Study of microstructure and solute partitioning in a cast Ti-48Al-2Cr-2Nb alloy by quenching during directional solidification technique

One major hindrance to effective implementation of cast gamma TiAl-based intermetallic alloys in aircraft engines lies in the variability of their mechanical properties resulting from chemical and microstructural heterogeneities. In the present work, the buildup of microsegregation in a cast Ti-48Al-2Cr-2Nb alloy is investigated through experiments of quenching during directional solidification (QDS). The solidification process, as well as the partitioning of alloying elements, between the solid and liquid phases, is investigated. Considering experimental conditions, the α-hcp phase is found to be the primary solidifying phase. A low dendrite tip temperature of 1475 °C was estimated from thermal recordings. These observations could be explained considering the value of the thermal gradient (around 4 °C/mm). Quantitative values of partition coefficients are proposed for Al, Cr, and Nb. In addition to Al, Cr is found to segregate in interdendritic regions, whereas Nb tends to be retained in the Ti-rich inner dendrites. Considering experimental cumulative solute distributions, the buildup of microsegregation can be satisfactorily represented on the basis of Gulliver-Scheil assumptions. Due to high-temperature quenching, the QDS experiments are also found to be appropriate to the study of high-temperature phase transformations and microstructural development of TiAl-based alloys. The results of QDS experiments are discussed with regard to the range of microstructural and chemical heterogeneities determined within Ti-48Al-2Cr-2Nb investment castings. Finally, regarding solid-state phase transformations subsequent to solidification, the study attempts to explain the formation of B2 phase particles stabilized by the ternary additions. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 17–21, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.