| With the demand for hydrogen as a clean energy carrier is expected toincrease rapidly in recent years, membrane materials for hydrogen purificationwere widely concerned. Pd and its alloy membranes used in commerce are veryexpensive, thus limiting their applications in hydrogen production. MultiphaseV55Ti30Ni15alloy with low cost, high hydrogen permeability and mechanicalstability become a new alloy material for potential replacement of Pd-Ag alloy.Currently, the formability and mechanical properties, anti-poisoning andhydrogen permeability before and after processing of V55Ti30Ni15alloy were notrelated to study, so it is necessary to study them to promote the application of V-Ti-Ni multiphase alloy membranes.According to the characteristics of the argon arc-melting, the effect ofcooling rate on the as cast microstructure and hardness of V55Ti30Ni15alloy wereinvestigated, and the relationship between volume fraction of V-based solidsolution (Vss), temperature, pressure and hydrogen permeability of V55Ti30Ni15alloy. Studies show that the volume fraction and size of Vss, secondary dendritearm width, and secondary dendrite arm spacing of the as cast alloys decreasedwith increasing cooling rate but the microhardness. At400℃, hydrogenpermeability of V55Ti30Ni15alloy is linearly related to the volume fraction of Vssand increased with temperature, and hydrogen flux increased with osmoticpressure.In order to examine the effect of impurity gases on hydrogen permeability ofthe V55Ti30Ni15alloy under various concentrations and temperature, thepoisoning effect of H2S, CO and CO2for the V55Ti30Ni15alloy was evaluated.The study found the effect of impurity gases on the hydrogen permeability in order of H2S, CO, and CO2. At low concentration and temperature (250-300℃),gas competitive adsorption lead to hydrogen permeability decreasing, at highertemperature (350-400℃), molecular thermal motion in the system gets fasterand stronger, weakening the effect of gas adsorption, the effect of impurity gason the permeability was not obvious. When the impurity gas concentrationincreases, the blocking effect of impurity gas to hydrogen transferring plays thedominant role, the higher the temperature is, the greater the impediment isbecause the mutual diffusion coefficient of the gases increase, hydrogenpermeation decreased more serious.Homogenization heat treatment process of V55Ti30Ni15alloy was studied,and the effect of heat treatment on microstructure evolution, hardness, and phaseprecipitation of the alloy were analyzed by XRD, SEM, TEM and EDS. Theresults show that the changes in hardness of V55Ti30Ni15alloy occurring whensubjected to isochronic heat treatment at different temperatures are due to theinternal structural changes that occur in the V-matrix during heat treatment.Nanometer NiTi particle precipitated and dissolved into the Vss, leading to thedecrease and increase in the hardness of the alloy.800℃/18h is appropriatehomogenized heat treatment parameters for V55Ti30Ni15alloy.PFZ formed at Vss boundary of V55Ti30Ni15alloy during heat treatment wasstudied, analyzing its forming mechanism and the effect on rolling. At the earlystage of heat treatment or lower temperature, the PFZs formation is caused byvacancies depletion. When holding time extends or temperature increases, thedepletion of vacancies and solute atoms maybe result together in the increase ofthe PFZs width at this stage. The PFZs played an important role in restrictingcracks extension and coordinating deformation during rolling process.The rollability of V55Ti30Ni15alloy were tested and discussed, and thedeformation mechanism of the alloy was explored using TEM. The study foundthe alloy rollability increases and the microstructures become uniform withincreasing temperature. Heat treatment can improve the rollability more than30%. The interface between circular nano NiTi particles and Vss belongs to thecoherent interface. Slipping is the main deformation mechanism when the alloywas rolled at room temperature; at the middle temperature rolling, deformationmechanism is slipping and dynamic reply is soften mechanism. With the increase of rolling reduction, obvious (211) plane orientation of the alloy has graduallychanged into the (200) crystal plane orientation; low temperature rolling has noobvious effect on the changes of the alloy texture, but grain orientation decreasedat700℃rolling. Annealing can eliminate the rolling deformation texture of thealloy.In order to study the influence of forming process on the mechanicalproperties and hydrogen permeability of the V55Ti30Ni15alloy, the tensile andhydrogen permeation testing were performed. The study found the strength of therolled and annealed alloy is about20%lower than that of the as-cast alloy, butthe elongation increases about25%compared that of the as-cast alloy andhydrogen permeability also recovered to the level of the as-cast alloy. The rollingand subsequent high temperature annealing treatment is effective for thefabrication of V-Ti-Ni alloy membranes of hydrogen permeation. Defects (suchas dislocations) and microstructure anisotropy in the alloys have a great impacton hydrogen permeability. High-density dislocations are hydrogen trap positions,resulting in the decrease of hydrogen permeability. When the alignment of alloyphases is parallel to the hydrogen permeation direction, hydrogen permeabilitywill be higher, namely hydrogen permeation direction is perpendicular to therolling direction, and hydrogen permeability will be low. |
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