Abstract The effects of ternary alloying of Ti-48Al (with 1–3 at.% Mn, Cr and V), and micro-alloying with 0.2% W, on crack-tip deformation and shielding mechanisms are discussed in this paper, for failure under cyclic (fatigue) loading. Crack-tip deformation mechanisms are elucidated by crack-tip transmission electron microscopy examination. Twin process zone dimensions and the degree of deformation-induced twinning are also determined via optical interference and transmission electron microscopy techniques, respectively. A micromechanics-based model is proposed for the estimation of twin toughening ratios under monotonic or cyclic loading. The model, which is based on non-linear fracture mechanics concepts, assumes an average ‘smeared’ plastic stress distribution across the twin process zone. Differences in the resistance to crack growth in the ternary alloys are related to intrinsic microstructural features and crack-tip shielding phenomena.