Evolution Of Microstructure And Texture Of High Purity Titanium Under Different Strain Rate

In this paper, the study object was polycrystalline Titanium sheet, with high purityof99.995%. Using electron backscatter diffraction and secondary electron imagingtechnology with a scanning electron microscope （SEM）, and combining with X-raydiffraction （XRD）, micro-structure and texture of the pure Ti with different strain werecharacterized under the conditions of dynamic plastic deformation （DPD） andquasi-static compression （QSC）. Zerilli-Armstrong model theory was used to calculatethe adiabatic temperature rise and theoretical thermal diffusion distance. Theconstitutive equation of dynamic plastic deformation （DPD） of pure Ti at roomtemperature was put forward by Zerilli-Armstrong model parameters fitting. Theconclusions were as follows:（1） Pure Ti deformed under compression, at low strain levels, deformationtwinning mechanism and slip deformation mechanism affected together, while twinningplayed a leading role. However, at middle strain-high strain phase, dislocation slipdeformation turned into dominant one. This change of dominant deformationmechanism demanded different strain （εDPD=0.2；εQSC=0.3） in the conditions of differentstrain rate. Deformation twinning refined the initial equiaxed grains and saturated（εDPD=0.2；εQSC=0.3） because twinning was restrained when lamellar spacing in themicro-structure reduced to a critical value.（2） In DPD pure Ti, four types of deformation twins were found, including {1012},{1121},{1122} and {1124} twins. While in QSC pure Ti, there were three types ofdeformation twins, including {1012}、{1121}、{1122} twins. The overall content ofdeformation twins was related to the strain rate, and the higher the strain rate, the moredeformation twins, which was in the same strain conditions. Among them, thepercentage content of {1121} and {1124} twins was quite rare. The formation of circledistribution was due to {1012} tensile twins formed a circle distribution in the （0002）pole figures and {1122} compression twins weakened the initial bimodal texture.（3） The evolution of texture of pure Ti at different strain rate was divided into threestages:①bimodal texture changed to circle distribution;②circle distribution changedto base surface texture;③base surface texture was strengthened. The main differencebetween texture evolution of DPD and QSC was that the start strain was inconsistent（start point εDPD=0.2, start point εQSC=0.3） in the second stage, when circle distribution changed to base surface texture. DPD sample analysis according toorientation distribution function （ODF） showed that at low strain phase, the maximumof pole density f（g）maxkept in the position （φ1=0°,=35°, φ2=30°）; at middle strainphase, f（g）maxmoved to the center pole; at high strain phase, f（g）maxwas stable in thecenter pole （=0°）．（4） Adiabatic shear bands （ASBs） had been found in pure Ti when ε≥0.6in theconditions of dynamic compression plastic deformation （about5×102s-1）, no ASBsappeared in the Quasi-static compression conditions. It showed that the appearance ofASB was related to high strain rate and high strain. The shear width slightly increasedwith the increase of strain, the forming equiaxed grains became larger and the width oftransition zone on both sides of the shear zone became significantly longer. The microhardness analysis results showed that hardness first decreased and then increased fromthe matrix to the shear zone center, and hardness had improved obviously in the shearzone center relative to its both sides. When Strain was0.6calculated by usingZerilli-Armstrong model, the adiabatic temperature rise was678K and the theoreticalthermal diffusion distance was15.5m. Zerilli-Armstrong model constitutive equationwas fitted to get the constitutive equation：σ=154.8+365.2, in the conditions ofstrain rate5×10²s^-1of pure Ti at room temperature.