Abstract: Impact deformation behaviors of CT20 Ti-alloy with lamellar microstructure (LM), equiaxed microstruc-ture (EM) and bimodal microstructure (BM) at 77 K were investigated systematically. The results indicated the cryogenic impact toughness of the CT20 alloy with LM ( ∼30 J/cm2 ) is ∼30%, ∼11% and ∼50% higher than that of EM ( ∼23 J/cm2 ), BM ( ∼27 J/cm2 ) and Ti-6Al-4V alloy, respectively. The twinning and its syn-ergistic interactions with multiple factors are the decisive factors. The factors include fan and bifurcation twins, dislocations, α/ βinterfaces, shear bands, stacking faults (SFs) and micro-cracks ultimately inducing the highest impact toughness of LM specimens. The strongest twinning induced plasticity (TWIP) effect in LM weakened the cryogenic suppression of dislocation slip and delayed the crack initiation. The EM and BM specimens exhibited higher stress peaks as finer equivalent grains. The unique <c> type dislocations and formation of SFs were found in Ti-alloy under cryogenic impact load firstly. From atomic arrange-ment, the formation mechanisms of SFs induced by consecutive Shockley and Frank partial dislocations, the twin growth mechanisms induced by boundary steps and coarsening mechanisms induced by screw dislocations were revealed. In addition, cryogenic temperature increased the risks of crack initiation and propagation during impact deformation.

Key words: CT20 alloy, Cryogenic impact toughness, Deformation twinning, <c> dislocations , Stacking faults, Fracture mechanisms