Study Of Microstructural Evolution And Atomic Structure Of High-purity Titanium During Dynamic Plastic Deformation

In the present work,we performed low to medium plastic strains during dynamic compression in high-purity titanium with a hexagonal-closed pack?HCP?crytal structure.We systematically studied the microstructural evolution and atomic structure of the twin boundaries?TBs?by using multiple material characterization method and measurement technology.All kind of deformation twins were quantitive indentifed to investigate the singnificant effects of deformation twins on the strain accommodation and grain refinement during the dynamic plastic deformation.Meanwhile,combination of the microstructural characterization and macrotextural evolution,the two deformation mechanisms of defomation twins and dislocation slip effects on macrotexture had also been systematically analyzed.TEM experiments were performed on the characterization of TBs of {10???2} and {11???2} twins that occurred in the microstructure,to study to interfacial structure and defects belong to these two types of twins.We also characterized the morphologies,orientation relationships,twin-twin junctions and atomic structures of {11???2} and {11???4} twins that presented in this study.Moreover,to compare the results obtained from dynamic compression,quasi-static compression?QSC?tests were conducted at a constant strain rate of ¹0^-3 s^-1.The samples used for the QSC tests had the same dimensions as the samples used for the dynamic loading test,and all the tests were conducted at room temperature.The following conclusions were drawn from this work:?1?Dynamic compression was found to enhance the activity of deformation twins,{10???2}-{11???2} and {11???2}-{10???2} secondary twins were formed from a 10% thickness reduction.With the increasing of thickness reduction,the mean grain size reduced from intial size with ¹4 ?m to 1.3 ?m at a thickness reduction of 40% by twin-twin,dislocation-dislocation and dislocation-twin interacions.At the beginning of deformation,the predominant deformation twins were {11???2} compressive twins.After the thickness reduction of 20%,the volume fraction of {10???2} twins was remarkabley increased,and {10???2} twins became to the dominant deformation twinning mode.?2?During the evolution of macrotexture,twinning reoriented the orienation of the inital grains,which promoted the dislocation slip interior of the grains.The newly formed [0001] TD orientation texture component was attributed to the {11???2} primary twins.Although deformation twinning was the dominant deformation mechanism at the beginning of deformation,slip became dominant with increasing strain.Dislocations with Burgers vectors of ???c+a??? type character was predicted to accommodate strain along c-axis at higher deformation strain.The ???a???,pyramidal ???c+a??? and {11???2}-{10???2} secondary twins contributed to the formation of a basal texture at 40% thickness reduction.?3?The microhardness of the high-purity titanium was higher for the samples with greater thickness reduction under both dynamic compression and QSC.However,the microhardness of the sample that was deformed by dynamic compression was higher than that of the QSC sample for the same thickness reduction.the difference in hardness plot of dynamic compressed sample and QSC at the same strain level may be relate to the twin volume fraction increased with increasing strain rate.?4?The coexistence of {11???2} and {11???4} twins in a given grain is observed.No individual {11???4} twin is found,indicating that the formation of {11???4} twin may be always accompanied with {11???2} twin in a same grain.The crystallographic relationships between {11???2} and {11???4} variants in a given grain can be classified into four types: the former two types can be regard as {11???2} and {11???4} twin sharing the same ???10???0??? zone axis,however,the rotated direction is the opposite;the other two types belong to {11???2} and {11???4} twin variants with different zone axes.The interactions of {11???2} and {11???4} twins result in the formation of {11???2} and {11???4} junctions.The newly formed twin-twin boundaries may contribute to higher strain hardening.?5?The TEM and HRTEM analysis revealed that facets were formed along the {11???2} and {11???4} TBs,respectively.The alignment of basal and {11???2} planes across the faceting is observed on {11???2} TB.{11???4} TB exhibited facets that aligns the {11???4} pyramidal planes in the matrix with basal planes in the twin.The facets may be formed from twinning dislocations?TDs?pile-up at {11???2} and {11???4} TBs.A b₃ twinning dislocation involved shear plus shuffles for {11???2} and {11???4} twins in Ti are described.?6?The integrated TBs formed at the beginning of deformation gradually evolved into TB segments with the increasing strain.A distorted area was formed in the vicinity of the TB at 40% thickness reduction.Meanwhile,the orientation between twins and matrix deviated from the theoretical orientation and the twin boundary trace deviated from the twinning plane.TBs gradually losted the special interface structure.