Study On The Deformation Twinning Evolution And The Relevant Crystallography In Pure Titanium

Titanium and its alloys are wildly used in the fields of aerospace, military industry, petrochemical industry, biological medicine, precision manufacturing, construction and water desalination etc, and are known as the "universal metal" because of the excellent comprehensive mechanical properties. Researchers made efforts to explore the causes of its good mechanical properties from the point of the microscopic deformation mechanism with the purpose of finding effective methods for further improving its performance, and providing theoretical reference for improving mechanical properties of other HCP structural metals. "Twinning induced plasticity model" concludes that twinning is closely relevant to the excellent deformation and ductility of titanium. Given the above background, this paper studies the twin evolution and relevant crystallography in depth to enrich and develop the plastic deformation theory of titanium from the angle of deformation twinning.This paper adopts the "interrupted in-situ SEM/EBSD measurement" to obtain the information and real-time data of twinning activation, growth, propagation, and the interaction between twin variants by scanning specific areas discontinuously by means of SEM and EBSD in the process of deformation. This paper investigates the key issues including twinning mechanism, twin growth and propagation of pure titanium at room temperature from the angle of micro texture and micro orientation on the micrometer scale, and further discusses the variant selection of primary, secondary and double twinning in depth, which is great of importance of optimization design of titanium alloy, mechanical properties prediction, damage and destruction control, and provides technical parameters to the plastic deformation calculation and simulation.The study on the twinning evolution indicates that{1012} tension twinning, {1122} compression twinning,{1122}-{1012} and{1012}-{1122} double twinning activate in both channel-die and rolling deformation. The twinning evolution can be summarized as that twinning activates in the favorable oriented and large sized grains at the very beginning of the plastic deformation. As grains deforms, twin grows rapidly and secondary twinning activates in large amounts. As grains further deforms, the favorable oriented and large sized grains are close to saturated of twinning and twinning turns to activate in unfavorable oriented and small grains. It is also found that the grain size plays a very important role in the twinning activation and the cause could be drawn as that the dislocation sources start more difficultly in small grains in the twinning mechanism of "polar axis", in addition, twinning stain energy increases as the matrix grain diameter decrease.The calculation on the Schmid factor and NSF of activated twinning variants indicates that the primary and secondary twinning variant selection follows the Schmid law generally. When adopt Schmid law to predict variant selection, special attention should be paid on the distribution of NSF and variants with NSF higher than 0.85 would stand a good chance to activate. The calculation on the energy factor of activated variants reveals that variants with high energy factor have priority to be selected during primary and secondary twinning because the calculation model of energy factor reflects the influence of grain shape and size by Hall-Petch equation. The Schmid law and other model do not involve these influences, so the variant selection using energy factor is in better agreement with the experimental results.The calculation on the crystallography of{1122},{1012} and{1121} twin systems shows that the 216 variants in six double twin systems combined of{1122}, {1012} and{1121} twin systems could be divided into 10 groups owning to the crystallographic symmetry. However, taking account of the twinning sequence, all double twin variants should be classified into 15 groups instead of 10. The variant selection of double twinning is found to follow Schmid law as well, but comparing with the energy factor, the variant selection of double twinning using energy factor agrees with the experimental results much better.The study on the twin growth indicates that the growth of{1122} twin shows multiple variant systems and{1012} twin shows predominant variant system, which results from the difference of the activation conditions for the two twinning systems.