| Titanium and its alloys are ideal structural materials and widely used in aviation, aerospace, marine and ocean industries because of their specific strength, good hot workability and good corrosion resistance. However, their plasticity is low at room temperature and they are easy to crack during their cold deformation, which restrict their applications. It need study further about the fundamental research on cold deformation of titanium alloys and its application, because cold deformation is one of the most economic methods to form titanium alloys, and the parts deformed at room temperature have good mechanical properties, high accuracy, good surface quality and high efficient. Thermohydrogen processing (THP) can enhance the mechanical properties of titanium alloys. However, the influence of hydrogen on the deformation behavior of titanium alloys at room temperature and its mechanism still lack systematic study. This paper systematically studied the problem.The effects of hydrogen content on microstructural evolution of Ti–6Al–4V alloy were investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Microstructural evolution of hydrogenated Ti–6Al–4V alloy during compressive deformation was also studied. Results show that martensites and hydride appear after hydrogenation. The amounts ofβphase,α'' martensite and hydride phase increase with an increase in the hydrogen content, and hydride prefers to precipitate along grain/phase boundaries. Hydrogen can promote the increment of dislocation.Tensile tests were carried out at room temperature through INSTRON-5569 matrials tester to study the influence of hydrogen content on the tensile properties of Ti–6Al–4V alloy at room temperature. Results show that hydrogen deteriorates the tensile properties, indicating hydrogen has disadvantage on the tensile deformation of titanium alloys at room temperature. In order to investigate the influence of hydrogen on fracture behavior of Ti–6Al–4V alloy and its mechanism, the whole process of crack initiation, propagation and failure during tensile deformation was observed and recorded in real time by in-situ tensile tests, and the distributions of stress and strain during tensile deformation were analysed through finite element method (FEM).In order to study the influence of hydrogen content and strain rate on the compressive properties of Ti–6Al–4V alloy at room temperature, compressive tests were carried out at room temperature through ZWICK-Z100 matrials tester and electromagnetic forming (EMF). Results show that hydrogen has favorable effets on the compressive proerties of Ti–6Al–4V alloy, can enhance the ultimate compression (the maximum increases 56.3%) under quasi-static compression and EMF, and can reduce the demand for equipment and die.The modification mechanisms about the effects of hydrogen content on the tensile and compressive properties were disclosured, and its model was built. The dissimilar effects of hydrogen content on the tensile and compressive properties are caused by the hydrogen content and stress state. The effects of hydrogen content include hydrogen in solid solution and hydrogen-induced phase transformation. As hydrogen content increases, the tensile properties decrease gradually. Intergranular deformation dominates at the tensile state, which is caused by the increased hydrogen atoms in solid solution and hydrides at grain/phase boundaries. While at the compressive state, intragranular deformation dominates at lower hydrogen content. The increased amount of plasticβphase improves the ultimate compression with the increasing hydrogen content. The intergranular deformation plays an increasing role during compressive deformation with the increasing hydrogen content because of the increased amounts of hydrogen atoms in solid solution and hydrides and leads to the degradation of compressive properties.The dehydrogenation procedure was determined according to the results of TG test of hydrogenated Ti–6Al–4V alloys. The hydrogenated Ti–6Al–4V alloys were dehydrogenated, and their microstructure and mechanical properties at room temperature were studied. Results show that the metastable phases decompose to stableαandβphases during the procedure of dehydrogenation, hydrogen in solid solution and hydride are removed, leading to the refinement of microstructures, but the grain can not be refined because of the heredity of titanium alloys. The mechanical properties can be restored, but can not be fully restored after dehydrogenation.The dry sliding wear properties of non-hydrogenated, hydrogenated and dehydrogenated Ti–6Al–4V alloys sliding against GCr15 steel were investigated using an M-200 pin-on-disk tribometer in air at room temperature. The wear mechanisms were investigated by studying the morphology and chemical element of pins and steel using SEM and EDS. Results show that wear rate increases after hydrogenation. Wear rates of dehydrogenated Ti–6Al–4V alloys are higher than those of non-hydrogenated Ti–6Al–4V alloys, although they are lower than those of hydrogenated Ti–6Al–4V alloys. The wear rates are attributed to their hardness and thermoconductivity. The non-hydrogenated Ti–6Al–4V alloy is controlled by oxidative mechanism, hydrogenated Ti–6Al–4V alloys by abrasive mechanism, and dehydrogenated Ti–6Al–4V alloys by oxidative and abrasive mechanisms. Results indicate that the dehydrogenated Ti–6Al–4V alloys should be treated to increase abrasion resistance before they are used.The optimal hydrogen content was determined for the cold deformation of hydrogenated Ti–6Al–4V alloys according to the experimental results. The alloy should be formed under compressive stress when hydrogen is applied on its cold deformation, while not under tensile sress. The optimum hydrogen content is the range of 0.6 wt.%~0.8 wt.% when the alloy is deformed under quasi-static compression. While the optimal hydrogen content is 0.1 wt.% when deformed under EMF, and the discharging voltage is 1.1 kV.
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