EWGlWEERlNG A Materials Science and Engineering Al 92/l 93 (1995) 950-956 ELSEVIER Phase transformations in two-phase TiAl/Ti,Al alloys during continuous heating and cooling, studied by electrical resistivity measurements D. Veeraraghavan, Vijay K. Vasudevan Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA Abstract The a + y transformation in a Ti-47.5at.%Al alloy and the a - a2 transformation in a Ti-40at.%Al alloy was studied by a novel computer-interfaced control-cum-data acquisition technique. In-situ, real-time, high-speed simultaneous measurements of resistivity and temperature were made in the system designed and constructed for this purpose. The experiments were made in the system designed and constructed for this purpose. The experiments were conducted in high vacuum. The samples were heated to the completely a region by controlled direct resistance heating, and cooled at various rates utilizing either controlled cooling algorithms or by varying the gas-jet quench. Temperature and resistivity changes were monitored during heating and cooling, and correlated with corresponding phase/microstructural changes. The results have shown that the y phase is characterized by a low resistivity (about 50 ,& cm at room temperature), whereas both a and a? phases have high resistivities (about 190 to 200 ,uQ cm at room temperature). Because of the large difference in resistivities of the a and y phases, large and measurable changes in resistivity can be observed when one of these phases transforms to the other. In the Ti-47.5A1 alloy, the lamellar y morphology was observed at slower cooling rates, the Widmanstatten-like at intermediate cooling rates, and a massive reaction at very high cooling rates. Preliminary results of the transformation start and completion temperatures for the various reactions as a function of cooling rates are presented. Keywords: Titanium; Aluminium; Alloys; Phase transformations 1. Introduction Alloys based on the intermetallic TiAl are presently receiving considerable attention as structural materials for elevated-temperature aerospace applications, owing to their unique combination of attractive properties such as light weight, high strength, high melting temperature, and superior creep and oxidation resistance [I]. The most promising of the alloys developed so far with regard to room-temperature ductility and high-temperature strength are based on the Ti-48Al composition (compositions in the text are in atomic percent) with ternary or quaternary additions [2,3]. The properties of these alloys are extremely sen- sitive to microstructure. While many aspects relating to the phase equilibria are well understood, the transfor- mation mechanisms and microstructure development during continuous cooling are just beginning to be understood. Recent studies have shown that cooling rate has a major effect on the transformation of the cc phase. Wang et al. [4,5] have reported that at low cooling rates the lamellar morphology prevails, at intermediate cool- ing rates both a Widmanstatten and a feathery micro- structure appear, whereas at very high cooling rates a massive transformation dominates. Other studies have reported similar effects of cooling rate and confirmed the occurrence of these various microstructures [6,7]. The actual temperatures at which the different reac- tions occur and their dependence on the cooling rate have been determined in the case of Ti-48at.%Al alloy [P]. In the present paper, preliminary results of electri- cal resistivity changes associated with the cz+ y trans- formation on heating and cooling are reported. 0921-5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(94)03326-9