Papers by Author: Chao Ying Xie

Abstract: Effects of equal-channel angular extrusion (ECAE) process at 773K and heat treatment on phase transformations and superelasticity of a Ni-rich TiNi shape memory alloy were investigated by differential scanning calorimeter (DSC) measurement and cyclic tensile test. The R phase transformation of Ni-rich TiNi alloy was stimulated after ECAE processes and within a larger temperature range. The martensitic transformation start temperature Ms of Ni-rich TiNi alloy decreased sharply after ECAE processes at 773 K, then rapidly rose back after the specimen aging at 773K for 30min. Reasons for the changes of phase transformation behaviors have been discussed. ECAE processed TiNi samples exhibit better super-elasticity than solution treated one. With the increase passes of ECAE, the superelasticity becomes more stable, and a completely recoverable strain of 6% is obtained for TiNi sample after 8 passes ECAE.

Abstract: The effect of ECAE pass number on the electrochemical properties of CP-Ti was investigated by electrochemical techniques (potentiodynamic polarization test, potentiodynamic polarization test and surface analyses (OM, scanning electron microscopy (SEM) in Ringer’s solution at 37°C. The results show that the corrosion resistance of muli-pass ECAE CP-Ti samples is superior to the coarse grain CP-Ti. The effect of texture of ECAEed samples parallel to the surface on the corrosion properties has been discussed.

Abstract: Microstructure and mechanical behavior of Ti-25 at. % Nb shape memory alloy processed by equal-channel angular pressing (ECAP) at 673 K have been investigated. The effect of multi-passes ECAP on the martensitic transformation temperature and superelastic recovery strain of Ti-26 at. %Nb alloy have been analyzed. It is found that the Ti-25 at. % Nb alloy exhibits superelasticity with a recovery strain of 1.6 % after one pass ECAP process. As the ECAP pass numbers increases to four passes, the superelasticity increases a little. The yielding stress (σ0.2) of Ti-25 at. % Nb alloy increases to 430 MPa and the ultimate tensile strength (UTS) is near 678 MPa after one pass ECAP process at 673 K, but the yielding stress and ultimate tensile strength increases little after four pass ECAP. The grain size decreases sharply from 100 μm to no more than 500 nm after four passes ECAP process. Effect of ECAP on the microstructure evolution and Young's modulus has been analyzed.

Abstract: α-phase forming on the surface layer and precipitation of α-phase inside a Ti-Mo alloy during annealing in air has been investigated thermodynamically. α-phase layer is increased rapidly when annealing temperature exceeds 700°C. When the 850°C solution-treated specimen annealed at 400°C for 1 hour, there is no α-phase precipitates. As annealing temperature increased to 800°C, the amount of α-phase precipitates first increased to a maximum at 600°C, then decreased.

Abstract: The initial coarse grains of Ti-50.9at%Ni alloy were refined into submicron grains, small than 0.5 um in size, after eight passes Equal Channel Angular Extrusion (ECAE) at 500°C. Optical microscopy and high temperature DSC tests were applied to investigate recrystallization behavior. It is found that the recrystallization start (Rs) and recrystallization peak (Rp) temperatures of Ti-50.9at%Ni specimens processed by eight passes ECAE are lower than that of the samples processed by one pass ECAE. Ti-50.9at%Ni specimens processed by eight passes ECAE with submicron grains are characterized by higher stability and need less energy to finish recrystallization process.

Abstract: Ultrafine-grained (UFG) CP Ti were successfully prepared by Equal Channel Angular Extrusion (ECAE) at 390°C~400°C, small than 0.5 um in size. The compressive tests for coarse grain (CG) and UFG Ti were carried out at room temperature (RT) and 77K. UFG Ti showed excellent ductility and higher strength than CG Ti at RT and 77 K. The strain hardening of UFG Ti was improved at 77 K. The compressive ultimate strengths of CG Ti and UFG Ti were both enhanced as the strain rate increased, but CG Ti showed more obvious temperature and strain rate dependence of flow stress, comparing with UFG Ti. When the strain rate increased to 1×10-1/s, the compressive ultimate strength of UFG Ti was kept almost constant, while the ultimate strength of CG Ti increased to the strength level of UFG Ti.

Abstract: The initial coarse grains of Ti-50.9at%Ni were obviously refined after eight ECAE processes. Effects of pressing number on the transformation behavior have been investigated in detail. Comparing with the initial solution-treated Ti-50.9at%Ni, the R-phase transformation was stimulated and the martensitic temperature Ms decreased with ECAE processes at 773K. The shape memory properties of Ti-50.9at%Ni alloy before and after ECAE processes at 773K have been measured. Good one way shape memory and superelasticity were obtained for the submicron-grained Ti-50.9at%Ni alloy.

Abstract: Submicron-grained Ti-50.9at%Ni alloy was prepared by Equal Channel Angular Extrusion (ECAE) at medium temperature(500°C). Microstructure evolution and phase transformations of Ti-50.9at%Ni alloy after ECAE processes at medium temperature have been investigated by optical microscope, TEM and DSC tests. It is found that the initial coarse grains of Ti-50.9at%Ni alloy were refined into submicron grains, smaller than 0.5 um in size, after eight passes ECAE at 500°C. Martensitic transformation temperatures decreased remarkably after one pass ECAE and more gently with increasing the pass number of ECAE. R phase transformation was stimulated after ECAE processes and underwent within a larger temperature range with an ambiguous thermal peak. The reverse R phase and reverse martensitic transformations, during heating, were separated eventually after eight passes ECAE.

Abstract: NiMnGa alloys are attractive for the magnetic induced shape memory effect, especially as promising functional elements in smart composite materials and structures. Recently, more attentions are put on NiMnGa composite materials. In this paper, NiMnGa particles have been dispersed and oriented in a polymer matrix with high content under magnetic field. The damping behavior of NiMnGa/polymer composites was investigated in by DMA, contrasting to the pure polymer.

Abstract: In this paper, 3-dimension finite element model was applied to analyze the commercial pure Ti billet subjected to four-pass equal channel angular extrusion process at 400°C with Bc route. The effective strain distribution and the damage within up to four-pass were described with friction conditions in 3D. The propensity for cracks was predicted, which did correlate approximately to the experiment.