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

The evolution of microstructure during hot pack rolling of the orthorh ombic titanium aluminide alloy Ti-22Al-23Nb was investigated. The star ting material was a prior hot-rolled sheet with an elongated ordered b eta (B2) grain structure whose grain boundaries were decorated with or thorhombic (O) phase. Two different types of rolling schedule were uti lized, one consisting of rolling below the beta (B2) transus alone and the other comprising supertransus preheating (in order to dissolve th e second phase) followed by rolling through the transus under decreasi ng temperature conditions. For both schedules, the rolled structure co nsisted 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 ex plained by measurement of the precipitation kinetics for the second ph ase. These measurements indicated very rapid grain boundary precipitat ion, even close to the transus, an effect that leads to the pinning of the beta (B2) grain boundaries.