Effects of nano-twinning on the deformation and mechanical behaviours of TiAl alloys with distinct microstructure at elevated loading temperatures

To study the influence of the microstructure on both the deformation mechanisms and mechanical properties of TiAl alloys at elevated temperatures, Ti-44Al (at%) alloy with a fully lamellar microstructure and Ti-47Al (at%) alloy with a duplex microstructure were prepared by adjusting the Al content. Hot compression tests at temperatures ranging from 600 degrees C to 1000 degrees C were performed on both the Ti-44Al and the Ti-47Al alloys. The analysis of the deformation mechanism reveals that nano-twinning is the dominant deformation mechanism of the gamma phase. Nanoscale twins in a single gamma phase traverse the whole gamma phase grain, rather than becoming inhibited by the lamellar interfaces in the gamma/alpha(2) lamellar colonies. Thus, the length of nano-twins in a single gamma phase is much longer than that in a gamma phase lamella within gamma/alpha(2) lamellar colonies, resulting in a lower yield strength but better ductility. By increasing the loading temperature, the ductility of the gamma/alpha(2) lamellar colonies is markedly improved due to dynamic recrystallization (DRX), while the ductility of the single gamma phase is improved by dislocations that traverse the twin boundaries at higher loading temperatures. Moreover, the effects of nano twinning on the deformation and mechanical behaviours of TiAl alloys, especially at elevated loading temperatures, are discussed in detail.