Deformation twinning in TiAl: Effects of defect clustering

Possible roles of point defect clustering in the formation of deformation twins in gamma-TiAl are critically assessed by reviewing the available models of dislocation-assisted twin nucleation and experimental data on deformation twinning in Ti-56 at.% Al single crystals and two-phase Ti-47 at.% Al alloys. According to the pole mechanism for twinning in the L1(0) structure, a reasonable combination of the stress concentration (n approximate to 27) and the vacancy supersaturation (c/c(0) approximate to 13) is needed to overcome the critical stages of twin formation. The so-called radiation-induced ductility reported in Ti-47 at.% Al alloys is attributed to the effective formation of twin embryos in the presence of interstitial-type Frank loops and the subsequent nucleation and growth of twins during plastic deformation.