Industrial Pure Titanium Deformation Twin And Annealing Recrystallization Behavior Research

Deformation twinning behaviour and annealing heat treatment of CP-titanium were investigated by X rays diffraction instrument (XRD), scanning electron microcopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) technique. The conclusions can be drawn as follows:(1) The major twins of CP-Ti after medium-to-low cold roll and tensile deformation were:primary compressive{1122}<1123> twin and secondary tensile{1012}<1011> twin, and the misorientation between the matrix and them were65°<1010> and85°<1120>, respectively. With the increase of deformation, LAB fraction increased when high angle boundary decreased, and the original twin-matrix orientation relationship was destroyed due to deformation via slip in both the twin and matrix in order to accommodate the deformation imposed following twin formation. Because of high valued Schmid factors of twinning that push imposed stress to induce twinning, the twinning matrix produced abundant twins easily. Both primary compressive twin and secondary tensile twin had strong dependence on matrix orientation.(2) Two stages can be depicted during the primary recrystallisation:a first stage, completed at500℃in about60min, where about80%of the material recrystallises fairly rapidly whereas about20%of the material, consisting of the so-called white grains, resists recrystallisation. A second stage, completed from60to360min, that is far more sluggish and corresponds to the recrystallisation of these white grains. During the whole annealing, about25%of the material recrystallises with an orientation change. There is much evidence that nucleation starts in the deformed grains where the twinning activity during deformation was important.In the same time a large part of the material (about75%of the material) recrystallises without any orientation change which suggests in situ recrystallisation. In the second stage the white grains, consisting after deformation in grains with a quite homogeneous dislocation distribution, undergo an in situ recrystallisation.(3) The changes in texture are more pronounced at the beginning of the grain growth process. At this stage, the size criterion and the correlation between the orientation and the size of the grains seem to be very important. Over long-term heat treatments, the texture evolves much more slowly. The predominant mechanism appears then to be a counterbalanced one with grains of some orientations growing to the detriment of others, while a reciprocal situation occurs somewhere else in the material. The disappearing orientations are widely scattered in the orientation space and characterized by a misorientation angle larger than30°. They disappear because they belong to the smallest size range at the end of primary recrystallisation and have fast moving boundaries (with misorientation higher than30°).(4) The oxidation rate of CP-titanium is small at600and700℃, and obscission don’t happen. When the temperature raise to800℃, oxidation is fast and with serious obscission. As increasing of temperature and time of oxidation, the oxide particles with polyhedron shape grow up rapidly, and the main phases of the Hardening layer are:TiO2、Ti2O、Ti3O、Ti6O and other oxide with low price. During the thermal oxidation process, oxygen pass the interface of gaseous phase and oxidation film firstly, then penetrate the the interface of oxidation film and metal matrix. Oxygen is transmitted into the oxidation film via the microcosmic or macroscopical crack and interstice, which is the main oxidation mechanism. The diffusion of oxygen and disintegration of oxidation film cause the formation of oxygen-diffusion zone. As the oxidation and disintegration of oxidation film keeping on, delamination appear gradually, which is related to the change of layer, Internal stress, compactness and lattice.