| With shape memory effect and pseudo-elasticity, Ti-Ni based alloys are widely used in the fields of spaceflight, aviation, machine, electron, chemical industry, biomedicine, etc. With the spread of the application fields, friction and wear become more serious, and that may significantly influence the performance of Ti-Ni based alloys. Therefore, the tribological behavior of Ti-Ni based alloys becomes a hot spot for research. Grain refinement has been an effective way to improve the mechanical and tribological properties of Ti-Ni based alloys.In this study, in view of the microstructure is a key factor which influences the performance of the Ti-Ni based alloys, and machining characteristic of Ti-Ni based alloys is poor at a low temperature, an advanced equal channel angular extrusion technology has been used to process Ti-Ni based alloys in order to refine the microstructure, and then improve the mechanical and tribological properties of the alloys. Some significant original results have been obtained.Firstly, high temperature ECAE processing was used for the microstructural refinement of Ti-Ni based alloys, the effects of preheating temperature, extrusion numbers and annealing treatment on the microstructure of Ti-Ni based alloys were analyzed by optical microstructure photographs, and the microstructural refinement mechanism produced by high temperature ECAE processing was discussed. The results showed that the elongated fibriform microstructure of Ti-Ni based alloys was obtained when the first preheating temperature was 850℃and the second was 750℃. After annealing at 400~600℃, the fibriform microstructure of the alloys was restored and refined microstructure was obtained. Secondly, the effects of high temperature ECAE processing on the phase transformation temperature and the pseudo-elastic strain recovery of Ti-Ni based alloys were analyzed from the thermodynamic point of view. In addition, different scanning calorimeter (DSC) and pseudo-elasticity were used to analyze the mechanism of high temperature ECAE processing for the phase transformation and the pseudo-elasticity of Ti-Ni based alloy. The results showed that high temperature ECAE processing reduced the martensitic phase transformation temperature and extended the range of pseudo-elasticity of Ti-Ni based alloy. Moreover, the second phase was precipitated during annealing at 500℃after ECAE, which strengthened the matrix, increased the parent strength, and then improved the friction and wear behaviors of Ti-Ni based alloys.Thirdly, the friction and wear behaviors of the Ti-Ni based alloys under both dry sliding and oil-lubricated conditions were investigated in detail. The results showed that the tribological properties of the alloys were greatly improved by high temperature ECAE processing. The Ti-Ni based alloys treated by high temperature ECAE processing registered the better tribological properties than untreated alloys, and the alloys treated by high temperature ECAE and annealing at 500℃showed the best tribological behaviors.Fourthly, the wear mechanisms of the Ti-Ni based alloys under dry sliding and oil-lubricated conditions were investigated. Under dry sliding condition, the main wear modes of untreated Ti-Ni based alloys are abrasive wear, oxidization and surface fatigue wear. When high temperature ECAE processing was used, the microstructure of Ti-Ni based alloys was refined, and the pseudo-elasticity and the hardness were increased, therefore, the main wear modes of the alloys were slightly abrasive wear and oxidization. Under oil-lubricated condition, the direct contact of the friction pair was reduced by the oil film. The friction heat was carried away by oil, resulting in a great decrease in plowing. The surface oxidization of the alloy was prevented, and the main wear mode was only abrasive wear.Fifthly, the mechanism of the influence of high temperature ECAE processing on the tribological properties of the Ti-Ni based alloys was discussed. High temperature ECAE processing refined the microstructure and increased the pseudo-elasticity and the hardness of Ti-Ni based alloys. It also reduced the plastic areas and strengthened the elastic deformation ability of the alloy, and then decreased the wear of the alloys. The superior tribological properties of Ti-Ni based alloy were obtained due to proper balance between the pseudo-elasticity and the hardness when the annealing temperature was 500℃. It is the first efforts to use high temperature ECAE processing for Ti-Ni based alloys, and investigate the influence of high temperature ECAE processing on the microstructure and tribological properties of the alloys. DSC and tensile test were used to characterize the martensitic phase transformation behaviors and the pseudo-elasticity. The friction and wear behaviors of the Ti-Ni based alloys under both dry sliding and oil-lubricated conditions were investigated in detail. The interrelationship between the friction and wear behaviors of the alloys and experimental conditions was obtained. The wear mechanism the alloys was analyzed. This study is promising to be improved in the properties of Ti-Ni based alloys and the application of the alloys.
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