Anti-correlation between solution strengthening and precipitation strengthening in lamellar O microstructures of a Ti₂AlNb based alloy

Anti-correlated change between yielding strength (YS) and ultimate tensile strength (TS) was found in heat-treated lamella O microstructures of Ti–22Al–25Nb (at. %) alloy. The B2-phase-region-forged microstructure contained lamella O precipitates and residual B2 matrix. The initial forged lamella O microstructure was solution-treated in B2+O phase region and fine O precipitates transformed into B2 phase. The YS of the lamella O microstructure increased significantly while the TS decreased dramatically after the solution treatment. On the one hand, electron probe microanalyses indicate that the composition of the strong strengthener Al in the B2 matrix got enriched in the solution-treated microstructure and the composition of the weak strengthener Nb in the B2 matrix got diluted. Higher Al composition resulted in stronger solution strengthening effect (SSE) in the B2 matrix of the solution-treated microstructure. Since the B2 phase in such lamella O microstructures dominated the yielding behaviors, higher Al composition further resulted in the higher YS. On the other hand, transformation from O precipitates to B2 matrix during the solution treatment caused fewer O/B2 boundaries, which resulted in weaker precipitation strengthening effect (PSE) and led to the lower TS of the solution-treated microstructure. Inversely, the YS got decreased while the TS got increased when the solution-treated microstructure was subsequently aged at a lower temperature, where abundant acicular O re-precipitated from the B2 matrix. It is concluded that the anti-correlated change between YS and TS is attributed to the anti-correlation between SSE and PSE due to the appropriate composition of strong strengthener Al and weak strengthener Nb.