Grain Refinement and Phase Transformation of Pure Titanium Foils During Asymmetrical Rolling

Abstract

Ultrafine‐grained microstructure and face center cubic (FCC) phase are produced in the pure titanium foils through asymmetrical rolling (ASR) to explore the mechanical properties of pure titanium foils with different thicknesses. Herein, the grain refinement process and phase transformation behavior are systematically investigated. The grain refinement process can be described as: 1) coarse equiaxed grains are elongated along RD due to the compressive strain and shear strain caused by the ASR process. 2) elongated grains are split into thin laths and subgrains by dislocation walls under greater strain. 3) thin laths and subgrains are further refined and ultrafine‐grained microstructures are obtained. In addition, hexagonal close‐packed (HCP)‐FCC phase transformation is also discovered during the ASR process. The orientation relationship between the HCP and FCC phases is determined as [0001]HCP//[001]FCC, (100)HCP//(20)FCC and the phase transformation is caused by Shockley partial dislocation with Burgers vector 1/6[110] gliding on each (100) plane. Uniaxial tensile tests on heat‐treated pure titanium foils of different thicknesses show a size effect on tensile properties. As the thickness decreases, tensile strength and elongation are reduced, with fewer/single‐layer equiaxed grains at the section of the specimen.