Adiabatic Shearing In TC4Alloy With Lamellar Microstructure And The Ultrafine Grained Titanium

Abstract:Titanium and titanium alloy have excellent properties, such as high strength, good corrosion resistance, etc. Grain refinement can significantly enhance the properties of materials. The study of adiabatic shearing in ultrafine grained titanium can explore the dynamic property and provide scientific evidence for preparing high strength and high toughness titanium alloy.The article took TC4alloy with lamellar microstructure and ultrafine grained TA2as study object. Split Hopkinson Pressure Bar (SHPB) technique was carried out to obtain adiabatic shear band (ASB). The ultrafine grained titanium with uniform grain size was prepared by multi-axial compression (MAC). The deformation characteristic (the microstructure characteristics of ASB, the microstructure evolution and phase transformation) of adiabatic shearing was investigated by means of Optical Microscope (OM), Electron Back-Scattered Diffraction (EBSD), and Transmission Electron Microscopy (TEM). The grain refinement evolution and mechanism was also explained. The result shows that:The shear deformation time in TC4alloy was60μs. The temperature rise in ASB was about1500K, which is high enough to meet the need of recrystallization and phase transformation. The core of ASB in TC4alloy was consisted of very fine equiaxed grains with low density of dislocations, a-phase grain and α"-phase grain with plate structure were coexisted in the shear band. Phase transformation was occurred in ASB. The white band observed in ASB was the result of phase transformation. Recrystallization microtextures were formed within ASB and the grain boundaries within ASB were geometrically necessary boundaries (GNBs) with high-angles. It was suggested that the dynamic recrystallization was responsible for the formation of fine equiaxed grains in ASB.The coarse initial grains with the size about25μm in TA2were rapidly reduced to about0.2μm after six passes. The mechanical twinning played a significant role in grain refinement. It was suggested that the mechanical twinning inducing continuous dynamic recrystallization was responsible for the formation of ultrafine grained microstructure.The shear deformation time in ultrafine grained TA2was50μs. The temperature rise in ASB was about867K, which is high enough to meet the need of recrystallization. The central region of ASB in ultrafine grained TA2was consisted of a number of equiaxed grains0.05-0.08μm in diameters with low dislocations density and scattered grains0.1-0.15μm in diameters. Some little equiaxed grains in ASB have grown up. The formation of the nanosized new grains and their growth up in the ASB of ultrafine grained TA2processed by MAC during the deformation process is the result of the grain boundaries rotation supported by the mechanical force and assisted by heat effect.