STRUCTURAL TRANSFORMATION IN THE TITANIUM-ALLOY BETA-CEZ - ANALYSIS OF PRECIPITATION MECHANISMS FOLLOWING TREATMENT IN SOLUTION

The beta-CEZ, a high strength titanium alloy developed for aerospace engine applications, is a alpha/beta near beta alloy. A wide variety of phase transformations and attendant nodular, lamellar and mixed microstructures are possible according to thermomechanical treatment conditions. The aim of this present paper is to illustrate the influence of solution treatment temperature on equilibrium microstructures and continuous cooling transformations. Solution treatment temperature controls the volume fraction of primary alpha particles and composition of the beta-matrix. Therefore the transformation during continuous cooling from alpha/beta or beta field depends on beta-matrix stability and potential sites amount of alpha precipitation. After a beta solution treatment, the alpha particles are disappeared and the beta phase contains all alloying elements; the continuous cooling transformation produces a Widmanstatten structure. If the cooling rate or/and the solution treatment temperature in the alpha/beta field are sufficiently low, the microstructure consists only of nodular morphology. During alpha particles growth the alpha volume fraction increases as equilibrium and the decreasing of growth kinetic leads to supersatured beta matrix and Widmanstatten alpha precipitation. The final microstructures are mixed. The influence of solution treat-ment temperature and cooling rate on nucleation and growth mechanisms is specially developed.