Low temperature transformation in the b-metastable Ti-5553 alloy A. Settefrati 1,2 , E. Aeby-Gautier 1a , B. Appolaire 3 , M. Dehmas 1 , G. Geandier 1 , G. Khelifati 2 1 IJL, CNRS/Université de Lorraine, Parc de Saurupt 54042 Nancy, F. 2 AIRBUS Operations, 316 route de Bayonne, 31060 Toulouse, F 3 LEM, CNRS/ONERA, 29 Avenue Division Leclerc 92322 Chatillon, F a elisabeth.gautier@univ-lorraine.fr Keywords: titanium alloys, a'' and w precipitation, kinetics, high-energy X-Ray diffraction. Abstract. Phase transformation of the b metastable phase was characterized at low temperature in the Ti-5553 alloy. Electrical resistivity and High Energy X-Ray Diffraction were utilized to characterize transformation kinetics along two thermal paths: i) direct quenching from stable b- domain to the isothermal transformation temperature of 325°C, and ii) quenching from stable b- domain to room temperature and further ageing at 325°C. Both treatments led to different transformation sequences with formation of w and/or a''. Mean cell parameters of b and a'' were obtained all along the transformation. Results suggest a transformation with poor solute partitioning and large stress building of in each phase. Introduction Mechanical resistance of b-metastable titanium alloys can largely be improved by the control of the amount of precipitates and their morphologies. These features are closely dependent on the transformation mechanisms involved during the thermal treatments. Indeed, different transformations may occur in succession, depending on the cooling and heating rates. During slow cooling from the stable b temperature range, an a HCP structure forms and its growth is controlled by solute diffusion. After rapid quenching from the stable b temperature range, an a'' metastable orthorhombic structure [1-6] may be formed or, for increasing amounts in b stabilizer species, metastable b with athermal w. Further isothermal ageing treatments of the metastable mixture (b + athermal w at temperatures ranging between 350°C and 400°C, lead to isothermal w precipitation. Formation of a and a'' is also mentioned in the literature: a precipitation is generally reported for studies involving TEM characterizations [12,13], while orthorhombic a'' is mentioned for studies using X-Ray diffraction [9-11]. Moreover, time-resolved high-energy X-ray diffraction experiments revealed that the occurrence of these transformations is dependent on the heating rates [10, 11]. Formation of w and a'' is generally associated with a martensitic transformation mechanism. Nag et al. characterized the chemical composition of precipitates for specimens aged at 350°C and 400°C in the Ti-5553 alloy using 3D atomic tomography [13]. Partitioning of solutes was observed, but in very small proportions. In order to increase our knowledge on these transformations, we studied the isothermal transformation of the Ti-5553 alloy. As compared to different b-metastable alloys, Ti-5553 presents a good quenchability, avoiding phase transformation during cooling even at low cooling rates and allowing a better control of the microstructure in large parts during thermal treatments. The transformation sequences and their influence of final microstructure have to be determined. The transformation was characterized thanks to electrical resistivity and time-resolved high- energy X-Ray diffraction. Two transformation conditions were studied: i) after direct quenching from the b temperature range to the transformation temperature, ii) after cooling to room temperature and further ageing. Materials Science Forum Vols. 738-739 (2013) pp 97-102 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.738-739.97 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 193.48.49.245-16/01/13,09:33:50)