Study On Microstructure Evolution And Properties Of Pure Titanium Processed By Equal Channel Angular Drawing

Titanium and its alloys are a class of ideal biomedical materials due to their high specific strength,excellent biocompatibility and corrosion resistance and have been widely used in bone plate,orthopedics and dentistry.Among them,Ti-6Al-4V alloy with high strength is conventional applied to load bearing joints or total replacement implants.However,the alloy elements,Al and V,are toxic and might cause some illness including cancer in the long term implantation.Therefore,more and more attention has been paid to the development of high strength pure titanium containing ultrafine/nano grained structures.Sever plastic deformation(SPD)methods are efficient thchniques for obtaining submicrometer grained commercial pure titanium,such as equal channel angular pressing(ECAP)and high pressure tortion(HPT),but the size of fabricated sample is small and these methods are limited to laboratory-scale.However,equal channel angular drawing(ECAD)uses the deformation principle of ECAP to convert the deformation pressure into a tensile force.The deformation rate can be adjusted in a wide range and the production efficiency is high.The potential of grain refinement has been found in FCC metals with more slip systems.Pure titanium is close-packed hexagonal metal with few slip systems.At present,there is no research on ECAD of pure titanium.Therefore,in the present work,CP Ti(grade 1)wire with a diameter of 2 mm was used for multiple passes of drawing at 1.7 mm/s and 75 mm/s.The evolution of microstructure and mechanical properties were studied by DSC,EBSD,TEM,Cs-corrected HAADF-STEM and microhardness tests.The main results are as follows:Pure titanium(TAl)was deformed at 1.7 mm/s through multiple passes of ECAD to realize the preparation of ultrafine subgrains,and an average size of ultrafine subgrains is about 300 nm.However,the dislocation could not be recovered dynamically at room temperature and the grain could not be refined at high rate deformation.No matter at low rate or high rate,the grain gradually changes from initial equal axied grains to elongated equiaxed crystals with the increase of drawing pass,and a large number of {1122} compression twinning and {1012} tension twinning appeared in the microstructure.The difference is that a large number of secondary twins are generated at a high deformation rate of 75 mm/s.With the accumulated equivalent strain reaching 7.2,the low angle grain boundaries in pure titanium gradually increased from 11%to 28%at 1.7mm/s,and the dislocation gradually entangled to form dislocation cells,which dynamic recovery at room temperature to form ultrafine subgrains.The deformation process is mainly dislocation slip and twin deformation,which are accompanied by dynamic recovery at room temperature.when the accumulated equivalent strain reaches 1.2 at 75 mm/s,the proportion of Low angle grain boundaries sharply increases to 25%,and remains basically unchanged in the subsequent deformation pass.A large number of lamellar structures are formed under the shear force.With the increase of deformation amount,the lamellar structures are gradually segmented and become narrower in the width direction,and the effect of twin deformation becomes obvious gradually.It is worth noting that a large number of acicular lamellars ware produced during ECAD process.The orientation relationship of HCP and FCC structure are determined to be(1010)HCP//(220)FCC,[1010]HCP//[110]FCC,which is asistent with the reported results of cold rolling.It is also observed that there are multiple non-coherent atomic steps at both ends of FCC phase.It is also verified that FCC phase grows gradually along the atomic step along with the increase of ECAD pass.The starting temperature of FCC to HCP is about 592? according to the DSC results and subsequent annealing test.In addition,this is the first time to observe the transition of FCC-HCP when exposed to electron irradiation for a long time.With the extension of time,a large number of atoms were mismatch in FCC phase and phase interface,which gradually narrowed the width of FCC phase and finally completely transformed into HCP structure.The whole transformation process was about 3 min.