The Effects Of Multi-component Alloying And Ball-milling On The Microstructures And Hydrogen Storage Properties Of Ti-V-Fe Based Hydrogen Storage Alloys

In this thesis, the research and development of hydrogen storage alloys with BCC structure vanadium-based solid solution were exhaustively reviewed first. On this basis, Ti-V-Fe based alloys with high hydrogen absorption capacity were chosen as the objects of this study. By means of XRD, SEM, EDS analysis and hydriding/dehydriding characteristic measurements, the microstructures and hydrogen storage properties of Ti-V-Fe ternary alloys with middling and high V content have been investigated respectively. Then, the effects of multi-component alloying and mechanical ball-milling modification on the microstructures and hydrogen storage properties of the studied alloys have been investigated systemically. The purpose of such investigation is to improve the overall properties of Ti-V-Fe based hydrogen storage alloys.The study on the microstructures and hydrogen storage properties of Ti_100-x-yV_xFe_y (x= 54, 49, 44;y= 5, 7.5, 10) alloys with middling V content shows that Ti₄₁V₅₄Fe₅ alloy consists of a solid solution main phase with BCC structure and a little α-Ti secondary phase, Ti_43.5V₄₉Fe_7.5 and Ti₄₆V₄₄Fe₁₀ alloys consist of a single solid solution phase. It is found that all of these alloys have good kinetics, they can absorb hydrogen rapidly without hydrogenation gestation time at room temperature and under 4 MPa initial hydrogen pressure. All alloys can be activated after 4-5 cycles and absorb saturated hydrogen within 3 minutes. With the increase of Ti, Fe content and the decrease of V content, the maximum hydrogen absorption capacity at room temperature, the effective hydrogen desorption capacity at 300 ℃ and the efficiency of hydrogen desorption all increase. Among these alloys studied, Ti₄₆V₄₄Fe₁₀ alloy has a good overall property, such as the activation number of 4 cycles, the maximum hydrogen absorption capacity of 372.4 ml/g at room temperature and the effective hydrogen desorption capacity of 238.5 ml/g at 300℃.In order to enhance further the maximum hydrogen absorption capacity and effective desorption capacity besides reducing the temperature of hydrogen desorption, the microstructures and hydrogen storage properties of (Ti_0.1V_0.9)_100-xFe_x(x= 0, 2, 4, 6) alloys with high V content were investigated systematically. The results show that all of these alloys consist of a single vanadium-based solid solution phase with BCC structure. With the increase of Fe content, the lattice parameter descends linearly, the unit cell volume also decreases. As the Fe content increases from x=0 to x=6, the activation number decreases from 4 cycles to 2cycles, the maximum hydrogen absorption capacity at 10℃ decreases from 509.5 ml/g to 424.8 ml/g, and the effective hydrogen desorption capacity at 50℃increases first and then decreases while the maximum value of 255.6 ml/g is obtained at x=4. Among these alloys studied, Ti9.6V86.4Fe4 alloy has a good overall property, such as the activation number of 2 cycles, the maximum hydrogen absorption capacity of 494.5 ml/g at 10°C and the effective hydrogen desorption capacity of 255.6 ml/g at 50°C. The study on the change of phase structures of Ti9.6V86.4Fe4 alloy during hydrogenation/ dehydrogenation cycle shows that the main reason of low effective desorption capacity of the alloy is the very low pressure of lower plateau of P-C-T curve and the difficult dehydrogenation of VH0.8i-based hydride with high thermodynamic stability.The microstructure and hydrogen storage properties of Ti9.6Vg6.4Fe4 + 10 wt% Tio.9Zro.iMni.5 composite prepared by mechanical ball-milling for lh were investigated. The results show that the alloy has a C14 type Laves secondary phase besides the BCC solid solution main phase. Comparing with as-cast Ti9.6V86.4Fe4 alloy, the unit cell volume of BCC main phase of the ball-milled composite increases slightly, the activation behavior is improved evidently, the maximum hydrogen absorption capacity at room temperature decreases (459.8 ml/g), and the effective hydrogen desorption capacity at 50°C increases (268.5 ml/g) due to the improved plateau characteristics of P-C-T curve.On the basis of the research on the ternary alloys, the microstructure and hydrogen storage properties of Ti9.6V86.4^Cr^Fe4 (x= 11, 12, 13, 14) alloys were investigated systematically. The results show that all of these alloys consist of a single vanadium-based solid solution phase with BCC structure. With the increase of Cr content, the lattice parameter descends linearly and the unit cell volume also decreases. It is found that all of these alloys have good kinetics, they can absorb hydrogen rapidly without hydrogenation gestation time at 10°C and initial hydrogen pressure of 4 MPa, but all alloys are activated after 3-4 cycles. As the Cr content increases from x=ll to x=14, both the maximum hydrogen absorption capacity and the effective hydrogen desorption capacity decrease. However, the Cr dopping is useful to improve the plateau characteristics of P-C-T curve for these alloys. Among these alloys studied, Ti9.6V75.4CrnFe4 alloy has a good overall property, such as the activation number of 3 cycles, the maximum hydrogen absorption capacity of 465.4 ml/g at 20°C, the effective hydrogen desorption capacity of 282.6 ml/g at 50 °C and the efficiency of hydrogen desorption of 60.72% .On the basis of the above work on the quadruple alloys, the changes in microstructure and hydrogen storage properties of Ti9.6V75.4CriiFe4 alloy after modification by mechanical ball-milling for different time (t = 0,1, 2,4, 8 h) were investigated. The results show that the alloys before and after ball-milling consist of a single V-based solid solutionphase with BCC structure. The lattice parameter and the unit cell volume descend with the increase of ball-milling time. It is found that the ball-milling can improve the activation behavior of the alloy effectively, and the activation number decreases from 3 cycles of the un-milled alloy to 1-2 cycles of the milled alloys. It is found that the alloys before and after ball-milling has good kinetics, they can absorb hydrogen rapidly without hydrogenation gestation time. The hydrogen absorption capacity of the activated alloy can get to 90 percent of the maximum hydrogen absorption capacity within 5 minutes. As the ball-milling time increases, the maximum hydrogen absorption capacity at room temperature decreases gradually, while the effective hydrogen desorption capacity increases first and then decreases. The Ti9.6V75.4CrnFe4 alloy ball-milled for 2 h has a good overall property, and the effective hydrogen desorption capacity at 50°C achieves 302.9 ml/g.