MICROSTRUCTURAL STUDY OF THE TITANIUM-ALLOY TI-15MO-2.7NB-3AL-0.2SI (TIMETAL-21S)

A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si- 0.150 (in wt pct)), is a potential matrix material for advanced titani um matrix composites for elevated temperature use. In order to develop a perspective on the microstructural stability of this alloy, the inf luence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied using optical and transmission electron mic roscopy (TEM). Depending on the specific thermal treatment, a number o f phases, including alpha, omega-type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized are as could be present in the microstructure of an annealed bulk alloy, b ut the microstructure of annealed sheet alloy was fully recrystallized . The mixed structure of the bulk alloy, developed as a result of inho mogeneous deformation, could not be removed by heat treatment alone at 900-degrees-C. Athermal omega-type phase formed in this alloy upon qu enching from the solution treatment temperature (900-degrees-C). Silic ide precipitates were also found in the quenched sample. Thermal analy sis was used to determine the beta transus and silicide solvus as clos e to 815-degrees-C and 1025-degrees-C, respectively. In solution-treat ed and quenched samples, a high-temperature aging at 600-degrees-C res ulted in the precipitation of alpha phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystall ized regions. During low-temperature aging (350-degrees-C), the ellips oidal omega-type phase persisted in the recrystaLlized areas even afte r 100 hours, whereas a high density of alpha precipitates developed in the nonrecrystaLlized areas within only 3 hours. The observed behavio r in precipitation may be related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging. The alpha precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-te mperature aging) always obeyed the Burgers orientation relationship. W ith respect to the microstructure, TIMETAL 21S is similar to other sol ute-lean, metastable beta titanium alloys.