Papers by Author: Xiao Yan Liu

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Authors: Xi Rong Yang, Xi Cheng Zhao, Xiao Yan Liu
Abstract: A series of experiments were conducted to evaluate the feasibility that commercially pure titanium (CP-Ti) was pressed for multiple passes by equal channel angular pressing (ECAP) at room temperature. Samples of CP-Ti were processed at room temperature using the dies with channel angles of 90° and 120°, respectively. First, each billet was processed 4 passes by ECAP using a die with an angle of 120° and a ram speed of 0.5mm s-1. And in order to eliminate residual stress, immediate annealing at 473 K for an hour was conducted between two adjacent passes. Second, CP-Ti was successfully processed by ECAP for up to 8 passes using the same die and a ram speed of 2 mm s-1 by controlling the flow of metal. Finally, CP-Ti was successfully achieved using a conventional die with an angle of 90° between the channels at room temperature. Each billet was processed for two passes with a ram speed of 26 mm s-1. These experiments show that CP-Ti may be processed by ECAP at room temperature and special attention was paid on improvements in the yield stress, ultimate strength and micro-hardness of ECAPed-Ti that are slightly higher than the improvements attained after pressing at elevated temperatures.
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Authors: Qing Juan Wang, Zhong Ze Du, Xiao Yan Liu, Ludvík Kunz
Abstract: The fatigue life, cyclic deformation behavior and microstructure of ultra-fine grained (UFG) copper produced by equal-channel angular pressing (ECAP) were tested and observed in present work. Development of surface relief and fracture surfaces were observed by means of scanning electron microscopy. The results show that the higher tensile strength UFG copper has lower ductility and its strain controlled cyclic properties are worse while comparing with those of lower strength CG counterpart. All the ECAP specimens display obvious cyclic softening behavior under the applied strain range. The cyclic softening ratio increases with increasing strain amplitude. The cyclic hysteresis loops of UFG Cu shows the tension and compression peak stresses decreased gradually with number of cycles. The formation of SBs and grain boundary sliding is the main factor causing crack nucleation as well as premature failure.
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Authors: Qing Juan Wang, Y.C. Wang, Zhong Ze Du, Xiao Yan Liu
Abstract: The corrosion behavior of ultra-fine grain (UFG) copper bulk prepared by equal channel angular pressing (ECAP) was studied in 3.5% NaCl solution. The effect of ECAP deformation on the copper corrosion is controversial in the literature, and worth to verify by means of various experimental techniques. Corrosion performances of UFG copper were investigated in comparison with that in recrystallized coarse grain (CG) copper by polarization curves, Tafel extrapolation method, electrochemical impedance spectroscopy(EIS). The shape of polarization curves and type of corrosive attack remains the same in the UFG and the coarse-grain state. UFG copper exhibited a lower corrosion current and high self-corrosion potential in comparison with CG copper. Electrochemical experimental results showed that UFG copper increased in resistance to corrosion compared with CG copper. This decrease in corrosion resistance was mainly attributed to the more compact corrosion film of UFG copper. The compact passive film led to decrease of the diffusion capability of ions within the corrosion film and corrosion rate.
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Authors: Xi Rong Yang, Xi Cheng Zhao, Xiao Yan Liu
Abstract: Equal channel angular pressing (ECAP) deformation for commercial pure titanium (CP-Ti) was successfully conducted using a conventional die with an angle of 90° between the channels at room temperature via route B, in which the billet was rotated 90° along its longitudinal axis between adjacent passes. Each billet was processed for two passes using a ram speed of 26mms-1. The microstructures and mechanical properties of these CP-Ti billets with an initial grain size of ~23μm processed by ECAP were investigated. Experiment results show thin parallel shear bands with a width of 0.3~0.4μm are generated after one ECAP pass, which are composed of large number of dislocation cell blocks. After the two ECAP passes, some various directional bands are generated and the equiaxed and smaller-than-average CBs in local areas begin to appear. In addition, the ultimate strength and microhardness are significantly enhanced to ~725MPa and ~2283MPa, respectively. Meanwhile good elongation of 18.0% to failure is still remained.
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Authors: Xiao Yan Liu, Xi Cheng Zhao, Xi Rong Yang
Abstract: Ultrafine-grained (UFG) commercially pure (CP) Ti with a grain size of about 200 nm was produced by ECAP up to 8 passes using route BC at room temperature. For ECAP processing a proper die set was designed and constructed with an internal channel angle Φ of 120° and an outer arc of curvature Ψ of 20°. Strain rate sensitivity of UFG CP-Ti and CG CP-Ti were investigated by compression tests in the temperature range of 298~673K and strain rate range of 10-4~100s-1 using Gleeble simulator machine. Evolution of the microstructure during compression testing was observed using optical microscopy (OM) and transmission electron microscopy (TEM). Strain rate sensitivity value m of the UFG CP-Ti has been measured and is found to increase with increasing temperature and decreasing strain rate, and is enhanced compared to that of CG CP-Ti. Result of the deformation activation energy determination of UFG CP-Ti indicates that the deformation mechanism in UFG CP-Ti is correlated to the grain boundaries.
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