DENSIFICATION MECHANISM OF TiAl PRE-ALLOY POWDERS CONSOLIDATED BY HOT ISOSTATIC PRESSING AND EFFECTS OF HEAT TREATMENT ON THE MICROSTRUCTURE OF TiAl POWDER COMPACTS

Owing to the low density, high strength, good creep properties at elevated temperatures, TiAl alloy is considered for high temperature applications in aerospace industries. However, a major issue to the industrial applications is the alloy's intrinsic brittleness at room temperature. Therefore, extensive efforts have been made to overcome this defect by near net shape fabrication techniques. An alternative for fabricating TiAl alloy is the powder metallurgy processing of pre-alloyed powders, and by this technique TiAl alloy with fine and homogenous microstructure can be obtained. In this work, TiAl pre-alloyed powders with nominal composition Ti-47A1-2Cr-2Nb-0.2W-0.15B (atomic fraction, %) and Ti-45Al-8Nb-0.2Si-0.3B are produced by electrode induction melting gas atomization (EIGA). The pre-alloyed powders are consolidated by hot isostatic pressing (HIP). The effects of heat treatment on the microstructure of TiAl compacts and the influence of cooling rate on the solid-state transformations which occurs during continuous cooling of the TiAl compacts have been studied. It is found that the cooling rate of the pre-alloyed powders is between 10(5)similar to 10(6) K/s. As the cooling rate increases, the martensitic transformation, i.e., beta ->alpha' occurs in some fine pre-alloyed powders. The heating DSC curves indicate that the transformation from alpha(2) phase to gamma phase takes place between 700 similar to 800 degrees C. During the HIP processing, the pre-alloyed powders particles randomly accumulate, and the relative density of HIP compact is increased by the particles moving, rotating and rearranging at the initial stage. As the temperature increases, alpha(2) phase transforms into gamma phase. With further temperature increasing, significant plastic deformation and the following formation of sintering necks occur in the powder particles. With the annealing time increasing, the pores between the particles are closed by means of surface diffusion, volume diffusion and diffusion creep. The microstructure of Ti-47A1-2Cr-2Nb-0.2W-0.15B powder compacts consists of fine gamma and a small number of alpha(2) and beta; and the microstructure of Ti-45A1-8Nb-0.2Si-0.3B powder compacts consists of fine gamma phase, a small number of alpha(2) phase and dispersed xi-Nb5Si3phase. The area fractions of gamma phase, a, phase and alpha(2)/gamma lamellar structures vary with the annealing temperatures, depending on the Gibbs free energies of the phases. The cooling rate has a significant effect on the continuous cooling transformation of both TiAl powder compacts. For Ti- 47A1-2Cr-2Nb-0.2W-0.15B alloy, the microstructure is composed of predominant equiaxed alpha(2) phase after water cooling, but of lamellar structures after air, oil or furnace cooling. For Ti-45A1-8Nb-0.2Si-0.3B alloy, the microstructure is composed of gamma(m) phase and large alpha(2) phase after water cooling; after oil and air cooling the alloy consists of a mix of feathery like structures, Widmansatten laths and lamellar structures; while furnace cooling leads to fully lamellar structures. Comparing the continuous cooling transformation curves, the increase of Nb can effectively extend the continuous cooling transformation to the diffusionless area.