Microstructural refinement of CP-Ti by cryogenic channel-die compression involving mechanical twinning

In order to refine the microstructure of commercial purity grade 2 Ti, a plane-strain cryogenic channel-die compression method was developed. During the compression, specimens and plunger were submerged inside a liquid nitrogen bath so that a temperature close to −190 °C was maintained. Depending on their orientations with respect to the compression die, specimens revealed the deformation traces belonging to different slip systems, primary as well as secondary. Various twins were identified: {101¯2}〈1¯011〉 and {112¯1}〈1¯1¯26〉 tensile twins as well as {112¯2}〈1¯1¯23〉 and {112¯4}〈224¯3〉 compressive twins. The twins and persistent slip bands contributed to the microstructural refinement significantly by increasing the stored energy and providing local sites of nucleation. Therefore, efficient microstructural refinement to the level of 100-200 nm scale was achieved with relatively small amount of deformation. Consequently, a combination of high room temperature yield strength (840 MPa) and elongation to failure (12%) was achieved with only a moderate amount of deformation strain, 1.18, by the multi-pass cryogenic compression. Recrystallization annealing of deformed specimens at 500 °C yielded a uniform fine microstructure and considerable enhancement of the ductility. In this process, coarse grains that were deformed by the primary prism 〈a〉 slip system resisted recrystallization, retaining low angle boundaries inside, which confirmed the proposition that secondary slip mechanisms should be activated for efficient grain refinement of Ti.