Recovery of sheet formability of cold-rolled pure titanium by cryogenic-deformation treatment

Abstract

The sheet formability of cold-rolled pure titanium (Ti) was remarkably recovered after being slightly rolled at 77 K, i.e., subjected to cryogenic-deformation treatment (CDT). This recovery occurred because CDT increased the thinning capability of the cold-rolled pure Ti, thereby suppressing cracking by restraining local strain concentration during subsequent sheet forming. The thinning capability was high because CDT formed twins, and slip in their orientations was favorable to accommodate thinning deformation. In addition, twinning during deformation contributed to high thinning capability. The twinning was possible because during deformation, the preformed twins by CDT acted as twin nuclei, enabling growth-dominant twinning without requiring a substantial activation stress for twin nucleation. CDT also increased the strength of cold-rolled pure Ti by twinning-induced grain refinement and dislocation accumulation. These abilities of CDT are noteworthy because sheet formability and strength are typically mutually exclusive. Thus, CDT achieved a superior combination of sheet formability and strength, which cannot be achieved by cold rolling alone, overcoming the trade-off between these two properties in pure Ti.