RELATIONSHIP BETWEEN MICROSTRUCTURE AND COOLING RATE IN AIR-HARDENABLE TIAL-BASED ALLOY

Relationship between microstructure and cooling rate in air-hardenable Ti-46Al-8Ta (at.%) alloy using Jominy end quench test was studied. The Jominy samples were solution annealed at 1360 degrees C for 1 h in argon atmosphere and quenched by spraying a controlled flow of water. Cooling rates within the samples were calculated by finite element method (FEM) using commercial software ANSYS. Optical microscopy (OM) and quantitative image analysis are supported by transmission electron microscopy (TEM) and Vickers microhardness measurements. After quenching the microstructure of the samples depends on the local cooling rates. Microstructure in the vicinity of the water cooled surface, at which numerically calculated cooling rate is 71 K/s, consists mainly of the alpha(2)(Ti3Al) phase and equiaxed gamma(TiAl) grains with few coarse alpha(2)(Ti3Al) laths, which are surrounded by the massive gamma(M)(TiAl). Maximum volume fraction of the massive gamma(M) is achieved at cooling rates ranging from 11 to 25 K/s. Volume fraction of the massive gamma(M) decreases at a cooling rates lower than 11 K/s and reaches a minimum value at 5 K/s at the distance of l = 50 mm. Microstructure at this distance consists of gamma(M), equiaxed grains with convoluted gamma + alpha(2) type of microstructure and lamellar gamma + alpha(2) regions.