Microstructure And Hydrogen Absorption/desorption Properties Of Ti-V-Mn Multicomponent Alloys

Vanadium?V?based solid solution alloys have become one of the most potential materials in the fields of mobile energy storing device and MH-Ni battery negative electrodes,due to their extremely high hydrogen storage capacity,absorption/desorption at room temperature and large electrochemical capacity.Laves phase-related BCC solid solution alloys?Ti-V-Mn?with excellent activation behavior can absorb hydrogen rapidly,but their effective hydrogen storage capacity is low,the activation and hydrogen storage mechanisms are rarely researched systematically.In this study,the hydrogen storage behavior of Ti-V-Mn alloys is systematically investigated by adjusting the lattice parameter,the ratio of two phases and thermodynamic parameter.The relationships among alloy composition,microstructure and hydrogen storage properties were established.The composition of Ti-V-Mn based alloys was optimized by using elements substitution,and Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ alloy with good activation and hydrogen absorption/desorption properties was obtained.The effect of heat treatment on hydrogen storage properties of Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ alloy has been investigated.It is attempted to further increase the hydrogen absorption capacity meanwhile keeping the fast absorption rate,and the influence mechanisms of heat treatment on microstructure and hydrogen storage properties were revealed.Different ratios of Ti/Mn?maximum/minimum atomic radius?in V₃₅Ti_xMn_65-x5-x alloys are selected,aiming to study the effect of lattice parameter on hydrogen storage performance.The lattice parameters of the main BCC phase increases with increasing Ti/Mn ratio.The hydrogen absorption capacity is strongly affected by electron-to-atom ratio?e/a?value and lattice parameters of BCC phase.The effective hydrogen capactity takes the largest proportion of hydrogen absorption capacity,which reaches at e/a value of 5.45 and a lattice parameter of 3.020?.With increasing V content from 25 at.%to50 at.%,the BCC phase increases,and the hydrogen storage capacity first increases and then decreases after 40 at.%of the V content.During hydrogenation,BCC phase works as hydrogen storage phase while C14 Laves phase acts as the“path”for hydrogen atom diffusion into the matrix.The hydrogen absorption capacity increases,which results from large amounts of BCC phase providing more interstitial sites.However,the C14 Laves phase becomes little and causes difficulties in the formation of hydrides,leading to the decreasing of hydrogen absorption capacity.V₄₀Ti₂₃Mn₃₇has the best hydrogen storage properties among Ti-V-Mn alloys:its maximum hydrogen absorption capacity is 3.5 wt.%at 293 K,and effective hydrogen storage capacity is 1.41 wt.%at 303 K.In order to improve hydrogen storage properties of Ti₂₃V₄₀Mn₃₇ alloy with the two-phase mixture of BCC and C14 Laves,the alloys with different Zr?x=0,2,4,6,8and 10,at.%?or Hf?x=0,2,4 and 6,at.%?partly substituting for Ti have been prepared.Both Zr and Hf can improve the formation of C14 Laves phase,while coarse columnar dendrite BCC phase decreases.C14 Laves phase changes the behavior of non-lateral growth to the behavior of lateral growth after substitution.When the Zr content is more than 6 at.%,the higher plateau region and the lower plateau region have appeared in desorption P-C isotherms.The hydrogen storage properties are strongly depended on the microstructure of alloys.Small equiaxed dendrite and coarse columnar dendrite BCC phases are formed in alloys with high-Zr?6,8 and 10 at.%?.Equiaxed dendric BCC phase surrounds entirely by the C14 Laves phase,where a large amount of H is easily released to form the higher platform.H is hardly diffused from columnar dendrite BCC phase,which results in the lower platform.The hydrogen storage capacity and cycle durability have been improved significantly by the Hf partial substitution for Ti,and it reaches the value of 1.82 wt.%at 293 K when the composition is Ti₁₉Hf₄V₄₀Mn₃₇.Effective hydrogen storage capacity only decreases to 93.3%of its initial values after20 cycles for 4 at.%Hf substituted alloy,which results from the less residual monohydride and the slight change of crystal lattice.The Cr substitution for Mn in Ti₁₉Hf₄V₄₀Mn₃₇ alloy has contributed to improve the hysteresis effect,due to the increasing types of Cr-rich tetrahedral interstice.It is found that the composition of Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ exhibits the highest effective hydrogen capacity,with the value of2.09 wt.%at 293 K.To explore the effect of Hf and Hf/Cr substitution on the activation and hydrogen absorption/desorption behaviors of Laves phase-related BCC solid solution alloys,Ti₂₃V₄₀Mn₃₇,Ti₁₉Hf₄V₄₀Mn₃₇ and Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ are prepared.The results show that lattice parameter of BCC phase increases and the ratio of C14 Laves phase also increases by Hf and Hf/Cr substitution.The stages in hydrogen absorption of three alloys are described by nucleation-growth and 3-D diffusion mechanisms;it means that hydrogen absorption rate is controlled from nucleation rate of hydrides to diffusion rate of H atom in this process.With the Hf substitution for Ti,the lattice interstitial increases and the attraction to H atom enhances,which leads to the fast formation of hydrides at the earlier stage of hydrogen absorption process.High density of dislocations is observed in Hf substituted alloys in favor of the diffusion of H atom in the matrix,which results in the increase of the hydrogen diffusion coefficient and the decrease of activation energy.Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ alloy exhibits the fastest hydrogen absorption rate but the lowest hydrogen absorption capacity at room temperature.The phase abundance adjustment for Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ with low-temperature heat treatment?700?,2h/8h/72h,water quenching?improves the hydrogen absorption capacity.After heat treatment,the coarse dendrite BCC phase transforms to equiaxed dendrite,the grains are refined after ball-milling and even reach nanometer size.Microstructure change on heat treatment gives rise to the increasing of dihydride,the decreasing of monohydride stability.Moreover,the formation of nanocrystalline increasing grain boundary energy to decrease the hydrogen diffusion energy.The dehydrogenation activation energies of monohydride in as-cast and heat treated alloys are calculated by using Kissinger equation,with the values of 83.30 kJ/mol and 66.26kJ/mol.The hydrogen absorption capacity and effective hydrogen storage capacity of the heat treated alloy are significantly improved comparing with that of ac-cast,with the values of 3.57 wt.%and 2.34 wt.%at 293 K,respectively.The phase transformation of fully hydrogenated Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ alloy with increasing temperature is followed by FCC?BCT,BCT?BCC,HfV₂H_3.97?HfV₂,HfH₂?Hf.