Grade-4 commercially pure titanium with ultrahigh strength achieved by twinning-induced grain refinement through cryogenic deformation

The yield strength of commercially pure(CP) Ti of ASTM grade 4, the strongest among all the CP-Ti grades, is too low for structural applications that require high-strength materials. Here, we demonstrate the strengthening of grade-4 CP Ti by cryogenic-temperature rolling(CTR), which enables deformation twinning in grade-4 CP Ti to achieve twinning-induced grain refinement. CTR activated {11.22} twinning and {10.12} twinning, which are the most common twinning systems in pure Ti, whereas room-temperature rolling(RTR) did not activate any twinning system. CTR with imposing an area reduction of just 30% significantly increased the yield strength of the CP Ti to 946 MPa, which is not achievable through typical processes performed at or above room temperature and is comparable to that of commercial Ti-6 Al-4 V. The significant increase in strength was due to microstructural strengthening caused by twinning-induced grain refinement, combined with dislocation accumulation. In contrast to RTR, CTR greatly increased the stress concentration at grain boundaries(GBs), which caused the unusual activation of twinning in the grade-4 CP Ti by facilitating twin nucleation at GBs. The stress concentration increased because CTR activated the slip to a lesser extent compared to RTR, thereby reducing the strain compatibility between neighboring grains. These results will contribute to development of ultrahigh-strength CP Ti and may thereby extend its use to structural applications that require high-strength materials.