A Study On The Microstucture And Hydrogen Storage Properties Of Ti-V-Fe Based Alloys

In this thesis, based on an overall review of the research and development of V-based solid solution hydrogen storage alloys with body-centered cubic (BCC) structure, Ti-V-Fe based hydrogen storage alloys with high and medium V content were developed and studied for the purpose of lowering cost and improving the overall hydrogen storage properties. By means of XRD, SEM, EDS, AES, TG/DSC analysis and hydriding/dehydriding characteristic measurements, the effects of multi-component alloying and heat treatment modification on the microstructures and hydrogen storage properties of the studied Ti-V-Fe-based alloys were investigated systematically. Moreover, the activation mechanism of this series alloys was also discussed and deduced.The effects of Fe content on the microstructures and hydrogen storage properties of Ti10V90-xFer(x=3-9) alloys have been investigated. The results show that all alloys have a single solid solution phase with BCC structure (Space group:Im3m), and the increase of Fe content leads to a decrease of the lattice parameter and unit cell volume. It is found that with the increase of Fe content in the alloy, the activation behavior deteriorated and hydrogen absorption capacity at room temperature decreased. In the range of x<6, the hydriding kinetics is noticeably improved with increasing x. As the Fe content increases, the P-C-T desorption plateau pressure at 333 K increases first and then decrease, reaching its maximum value of 0.65 MPa at x=7; And the hydrogen desorption capacities at 333 K against 0.1 MPa also increase first and then decrease, reaching their maximum values of 2.36 wt.%at x=6. In the studied alloys, Ti10V84Fe6 has a better overall hydrogen storage performance.In order to improve the activation behavior, by the partial substitution of Zr for V, the microstructure and hydrogen storage properties of Ti10V84Fe6Zrx (x=1-8) alloys have been investigated. The results show that the alloy with x=l still has a single solid solution phase with BCC structure, while the alloys with x=2-8 consist of a main phase with BCC structure and a secondary phase with C14 type Laves structure (Space group: P63/mmc)and the abundance ratio of the secondary phase increases with increasing Zr content. As the Zr content in the alloys increases, the activation behavior is markedly improved, but the hydrogen absorption and desorption capacities decrease gradually. The partial substitution of Zr for V can distinctly lower the stability ofβmono-hydride phase and decrease its dehydrogenation temperature. However, excessive Zr leads to a notable decrease of hydrogen absorption capacity.To explore the activation mechanism of this series Zr-contained alloys with a dual-phase, Ti10V76Fe6Zr8 alloy with excellent activation behavior was chosen as the research object. Based on the chemical components of the BCC phase and C14 phase in Ti10V76Fe6Zr8 alloy, the BCC alloy of Ti7.7V87.5Fe4.8 and C14 alloy of Ti10.2V42.4Fe11 6Zr35.8 were prepared respectively. The XRD analysis shows that above two alloys are both single phase alloys. The AES analysis shows that the thickness of the surface oxide film of BCC alloy is much bigger than that of C14 alloy. The micro-area activation model of the V-based alloy with dual-phase structure is put forward, namely:after evacuation at room temperature, due to the non-destruction of surface oxide film on the alloy, the possibility of the penetration of hydrogen through the surface oxide film of C14 phase and the occurring probability of micro-cracks and fresh surface by the hydrogenation are much higher than that of BCC phase. Hence, the less C14 phase the alloy contains, the longer incubation time the activation needs. After evacuation at high-temperature, the refresh surface can contact with hydrogen directly and the activation becomes much easier owing to the partial or entire demolishment of the surface oxide film on the alloy.Aiming at the improvement of hydrogen desorption plateau pressure and effective hydrogen capacity mainly, the partial substitution of Cr for V in Ti10V8o-xFe6Zr4Crx (x=0, 6,10 and 14) alloys is employed. It is found that all alloys consist of a main phase with BCC structure and a secondary phase with C14 type Laves structure. The Cr addition leads to a contraction of the unit cells of both phases. These alloys have good activation behavior. As the Cr content increases, the hydrogen absorption capacity decreases, especially for the alloys with x>10, while the hydrogen desorption capacity increases first and then decreases, reaching the highest value of 1.50 wt.%at x=6. With increasing the Cr content, the dehydriding rate and the hydrogen desorption plateau pressure at 333K are remarkably enhanced, moreover the slope factor of the plateau is reduced. The excessive Cr addition would result in the increase of peak temperature for the dehydrogenation ofβphase.Based on the optimally prepared Ti10V77Cr6Fe6Zr, the effects of heat treatment (1373 K for 8 h,1523 K for 5 min) on the microstructures and hydrogen storage properties were investigated systematically. The results show that all alloys consist of a BCC main phase and a secondary C14 Laves phase. After heat treatments, the content of C14 Laves phase decreases and the hydrogen absorption capacity decreases, while the hydrogen desorption plateau is flattened distinctly. The sample treated at 1523 K for 5 min and followed by quenching in cold water has the best overall hydrogen storage properties, with the dehydrogenation plateau pressure of 0.75 MPa at 333 K, plateau sloping factor of 0.1, and the effective hydrogen desorption capacity of 1.82 wt.%.Mn element has been introduced into Ti10V83Fe6Zr alloy for its positive effect of flattening the dehydrogenation plateau, the microstructure and hydrogen storage properties of Ti10V83-Fe6ZrMnx (x=0-6) alloys have been investigated. It is found that the Mn-free alloy with x=0 has a single solid solution phase with BCC structure, while other alloys with x=2-6 consist of a BCC main phase and a small fraction of C14 type Laves secondary phase. The cell volume of the BCC main phase decreases with the increase of x. All of these alloys have good activation behaviors and hydriding kinetics. With increasing the Mn content in the alloys, the plateau pressure at 333 K increases first and then drops, reaching the maximum value of 0.58 MPa atx=4, and the hydrogen desorption plateau is remarkably flattened, showing slope factor of 0.09 at x=6. However, the maximum hydrogen absorption capacity at 298 K and the effective hydrogen desorption capacity at 333 K decrease gradually with the increase of the Mn content, especially for the alloys with x> 4.In order to further lowering the cost of V-based hydrogen storage alloys, the microstructure and hydrogen storage properties of Ti16Zr5Cr22V57-xFex (x=2-8) alloys with medium V content have been investigated systematically. The results show that all alloys are composed of a main phase with BCC structure and a secondary phase with C14 type Laves structure. By addition of Fe, the unit cell of the BCC main phase decreases. All of the studied alloys have good activation behavior and hydriding/dehydriding kinetics. As the Fe content increases, the hydrogen desorption plateau pressure of the alloy increases gradually, however, the hydrogen absorption and effective desorption capacities decrease. The appropriate Fe addition can also decrease the decomposition enthalpy ofβphase. The effect of heat treatment (1553 K for 5 min) on the microstructure and hydrogen storage properties of Ti16Zr5Cr22V55Fe2 alloy has been systematically studied. The results show that after heat treatment, the two-phase structure of sample has not changed, however, the lattice parameter of BCC main phase is increased and its grain grows. Compared to the as-cast alloy, the heat treated alloy has shorter activation incubation time and higher hydrogen absorption/desorption capacities. The hydrogen desorption plateau pressure is reduced and the plateau is remarkably flattened by the heat treatment.The influence of partial substitution of Mn for V on the microstructure and hydrogen storage properties of Ti16Cr22Zr5V55-xFe2Mnx(x=0-3) has been systematically investigated. The results show that all alloys are composed of a BCC main phase and a C14 Laves secondary phase. With increasing Mn content, the lattice parameter of BCC main phase decreases gradually. This series alloys have good activation behavior, however, the hydrogen absorption/desorption capacities decrease with increasing Mn content. By Mn addition, desorption plateau pressure is increased and the plateau width is decreased. Furthermore, the flatness of the plateau is noticeably improved.