J. Phys. IV France 120 (2004) 93-101 EDP Sciences, Les Ulis DOI: 10.1051/jp4:2004120010 Prediction of the kinetics of the phase transformations and the associated microstructure during continuous cooling in the Ti17 J. Da Costa Teixeira 1,2 , L. Héricher 1 , B. Appolaire 1 , E. Aeby-Gautier 1 , G. Cailletaud 3 , S. Denis 1 and N. Spath 2 1 LSG2M – UMR CNRS/INPL/UHP, École des Mines, Parc de Saurupt, 54042 Nancy Cedex, France 2 Snecma Moteurs, YKOG2, 291, avenue d’Argenteuil, BP. 48, 92234 Gennevilliers Cedex, France 3 UMR CNRS 7633, Centre Des Materiaux, École des Mines de Paris, BP. 87, 91003 Evry Cedex, France Abstract. The aim of this paper is to present recent experimental results and related simulation about the GB + WGB and WI transformations which occur in the Ti17 alloy during the thermal treatments following the heating in the phase field. These phase transformations were experimentally studied under isothermal conditions in samples with negligible thermal gradients. The IT diagram was obtained, on the basis of electrical resistivity measurements and microstructural SEM observations. The kinetics of the phase transformation was further numerically simulated for continuous cooling on the basis of a formerly developed model giving the amount of each morphology ( WGB , WI ). Experimental and calculated results are compared. 1. INTRODUCTION The mechanical properties of Ti17 titanium alloy, as well as other metastable beta titanium alloys are strongly dependent on the microstructure, namely the nature and amount of the phases, their morphology and the way they are distributed [1]. Thus the prediction of the microstructure evolution and their final distribution associated with any thermomechanical treatment would help with the process control as well as with the prediction of the mechanical properties. The relationships between the microstructure evolution and the thermomechanical treatment parameters have been widely studied for several titanium alloys [2-4]. For Ti17 (Al 5%, Mo 4%, Cr 4%, Zr 2% and Sn 2%) only few data are available [5-6]. We have thus performed experiments in order to characterise the experimental transformation kinetics, on the basis of the experimental knowledge gained from previous studies made on titanium alloys. During the cooling from the high temperature phase (BCC) field, two main precipitation mechanisms of the (HCP) phase can occur [2]. At high temperatures, the following sequence is observed GB + WGB + enriched . First, GB nucleates only on the most favourable sites which are the grain boundaries and grows by diffusion. Then, colonies constituted of parallel lamellae of WGB ( Widmanstätten morphology) with only one crystallographic orientation nucleate on GB . The growth of WGB is diffusion controlled. In isothermal transformation conditions, the ratio between the phase amount and the enriched phase, in the WGB + enriched colony, is nearly the ratio between the and phase at equilibrium for the given transformation temperature. At lower temperatures, WI + enriched occurs on intragranular nucleation sites. A more displacive character could be associated with the transformation. The nucleation can occur on less favourable sites as the dislocations, because