Microstructural And Textural Evolution Of High-purity Titanium Sheet During Rolling From Low-to-medium Strain And Annealing

In present work,a typical high-purity titanium sheet(purity of 99.999%)was rolled from low to medium strain(10-30%)and then annealed(both isochronous and isothermal annealing),with micro structural and textural characterization made by jointly using EBSD,ECC imaging and XRD techniques.After detailed characterizing the micro structural and textural evolution,the correlation of macrostructure,hardness and stored energy was analyzed quantitatively.The main conclusions are as follows,(1)During low-to-medium-strain rolling,the plastic strain in high-purity titanium is accommodated by both slip and twinning,with twinning(including {11(?)2}<11(?)(?)>compressive twins and {10(?)2}<10(?)(?)>extensive twins)acting as the main deformation mode.Massive twinning leads to significant grain refinement,greatly weakens the initial texture and produces new textural components.The primary twins force c-axes of grains to deviate from the ND while the secondary twins produce the new component of c//ND(especially in the 30%-rolled specimen).During 10-30%rolling,the hardness continuously increases from 120.8±5.7 HV to 193.3±3.6 HV.The grain refinement strengthening by twinning after 10%rolling can effectively contribute to hardening,while the contribution keeps relatively stable during subsequent rolling.As slip gradually plays a more important role at higher strains,the fraction of LABs increases continuously(2)For the 10%-rolled high-purity titanium sheet,recrystallization nucleation starts at 600℃ with prior grain boundaries as preferential sites.With increasing the annealing temperature or time,the proportion of recrystallization structures increases.After annealing at 700℃ for 1 h or at 600℃ for 20 h,the specimens still cannot complete the primary recrystallization.Before recrystallization nucleation is initiated,the texture characteristics keep almost unchanged.With the recrystallization,the main textural components of the specimens vary irregularly from ND～35° along the c-axis on the RD-TD plane.The specimen hardness always decreases with increasing annealing temperature or time.Even without recrystallization nucleation,reduced residual strain and stored energy in the specimen could also lead to hardness decrease.(3)For the 30%-rolled high-purity titanium sheet,recrystallization nucleation starts after annealing at 500℃ for 1 h or 600℃ for 0.17 h with prior grain boundaries as preferential sites.Fully recrystallized structures are obtained after annealing at 650℃ for 1 h or 600℃for 5 h.At the early stage of recrystallization,the recrystallization texture is similar to the deformation texture,and the main texture is c//ND.With increasing recrystallized proportion,the c-axis of grains will be spread along ND～35° on RD-TD plane,and a ring-like texture will be eventually formed.Hardness changes during annealing are found to be closely related to grain sizes and dislocation densities(mainly stored in low angle boundaries).Quantitative analyses reveal that the main reason for the decrease of specimen strength/hardness is that the increased grain size and the reduced LABs.When recrystallization is completed,the stored energy in LABs is significantly eliminated.The annealing temperature has stronger effect on the recrystallization behavior than the annealing time.