Microstructural variation of a cast Ti48Al2W0.5Si alloy during quenching and tempering

By tempering the quenched samples in the single alpha region, a very fine fully-lamellar microstructure (170–250 μm) was obtained in the homogenized Ti48Al2W0.5Si alloys with an initial grain size of about 460 and 860 μm, respectively. The refining effect was found to be greatly dependent on the type of quenched microstructures which is controlled by prior cooling rate, but little on the initial grain size which is determined by homogenization condition. With the cooling rate increasing, the following α-decomposed products were observed: the lamellar structure, the Widmanstatten laths, the feathery γ plates, the massive γ and very fine indiscernible lamellar laths. The massive γ structure consists of fine γ domains containing abundant dislocations, stacking faults and antiphase boundaries. While the feathery γ structure comprises of slightly misoriented γ plates with relatively low density of dislocations, but no stacking faults and antiphase boundaries. During tempering, formation of new α laths in the feathery γ structure is mainly along the γ plate boundaries with a length comparable to the γ plates. In contrast, recrystallization occurs in the massive γ structure by nucleation of very fine α laths on stacking faults, responsible for the noticeable grain refinement. Long-time homogenization slightly retards the massive transformation but favours the formation of feathery γ and indiscernible lamellar structures.