MICROSTRUCTURAL EVOLUTION DURING ROLLING OF TI-22AL-23NB SHEET

The evolution of microstructure during hot pack rolling of the orthorhombic titanium aluminide alloy Ti-22Al-23Nb was investigated. The starting material was a prior hot-rolled sheet with an elongated ordered beta (B2) grain structure whose grain boundaries were decorated with orthorhombic (O) phase. Two different types of rolling schedule were utilized, one consisting of rolling below the beta (B2) transus alone and the other comprising supertransus preheating (in order to dissolve the second phase) followed by rolling through the transus under decreasing temperature conditions. For both schedules, the rolled structure consisted of elongated beta (B2) grains and a fine dispersion of second-phase (O) particles. The absence of recrystallization of the B2 grains, particularly for sequences involving supertransus preheating, was explained by measurement of the precipitation kinetics for the second phase. These measurements indicated very rapid grain boundary precipitation, even close to the transus, an effect that leads to the pinning of the beta (B2) grain boundaries.