Microstructure And Hydrogen Storage Properties Of Ti-Based Multicomponent Alloys

Ti-based multicomponent hydrogen storage alloys have the advantages of high volumetric hydrogen storage density,moderate hydrogen sorption thermodynamics,and low cost.The series of TiFe hydrogen storage alloys have good hydrogen absorption and desorption properties at room temperature,showing great prospects in stationary hydrogen storage applications.However,the activation requirement of the equimolar TiFe alloy is harsh and thus limits the practical application.The activation of TiFe alloy is closely related to the oxygen in the alloy system,but its activation mechanism has not been clearly explained yet.The microstructure and phase composition of TiFe alloy can be regulated by manipulating the oxygen content of the alloy,optimizing the synthesis method,and alloying with other elements to improve the hydrogen storage properties of the alloy.In this paper,the effect of oxygen on the hydrogen storage properties of TiFe alloy was systematically studied by regulating the content of dissolved oxygen of the TiFe phase and the content of the oxide phase in powder metallurgy Ti-Fe alloy,the role of the oxide phase in the room temperature activation process of Ti-Fe alloy with high Ti/Fe ratio was explored,the evolution of microstructure,surface morphology and composition of the alloy during the initial hydrogenation at room temperature was studied,and the room temperature activation behavior of TiFe alloy with high Ti/Fe ratio was analyzed.On this basis,by further optimizing the composition and synthesis method of Ti-based multicomponent alloys,combined with the calculation of the phase diagram,the phase composition of the alloy was regulated,and the mechanisms of the effect of secondary phases on hydrogen storage properties of the alloy were analyzed.The main conclusions can be drawn as follows:（1）TiFe alloy synthesized by powder metallurgy can increase the dissolved oxygen of the TiFe phase in the alloy,increase the plateau pressure of hydrogen absorption and desorption,and decrease the enthalpy of hydrogen desorption reaction.With the addition of oxygen in powder metallurgy TiFe alloy,the content of Ti₄Fe₂O and TiFe₂ in the alloy significantly increased,while the proportion of TiFe phases in the alloy decreased correspondingly,resulting in the decrease of reversible hydrogen storage capacity at room temperature.However,the addition of high oxygen content can significantly improve the activation properties of the alloy,the equimolar TiFe alloy with 3.78 at.%of oxygen can be fully activated at room temperature.（2）When the Ti/Fe ratio in Ti-Fe alloy increases,the content of the Ti₄Fe₂O_1-x phase in the alloy increases,while the content of the TiFe₂ phase decreases.Powder metallurgy Ti_52.6Fe_47.4alloy is composed of TiFe phase and Ti₄Fe₂O_1-x phase,it has a high Ti₄Fe₂O_1-x phase content,and it can be fully activated at room temperature.Ti₄Fe₂O_1-x plays an important role in the room temperature activation process of Ti_52.6Fe_47.4alloy.In the initial hydrogenation process at room temperature,Ti₄Fe₂O_1-x embedded on the surface of the TiFe matrix absorbs hydrogen first,which leads to the cracking and crushing of alloy particles,providing a large number of non-passivated fresh surfaces for the reaction between the TiFe phase and hydrogen,thus prompted the room temperature activation of the alloy.（3）The hydrogen storage properties of powder metallurgy Ti-Fe-Mn and Ti-Fe-Zr-Mn multicomponent alloys are significantly better than that of as-cast alloys with the same composition.Compared with as-cast alloys,the TiFe phase of powder metallurgical multicomponent alloys contains higher dissolved oxygen and thus exhibits lower hydrogen absorption and desorption reaction enthalpy.Due to the more uniform composition and microstructure,powder metallurgical multicomponent alloys exhibited flatter hydrogen sorption plateaus.In addition,the powder metallurgical alloy contains a higher content of TiFe phase and less lattice strain and defects,the complete hydrogen absorption of the alloy will form aγ-phase hydride,resulting in a higher hydrogen storage capacity of the alloy.The content of the secondary phase in powder metallurgy alloy is relatively low,and the good activation performance is mainly due to the large amount of secondary phase distributed near the pores or grain boundaries.（4）Nb alloying significantly affects the phase composition and hydrogen storage properties of Ti-Fe-Zr-Mn multicomponent alloys.With the increase of Nb content in the alloy,the TiFe phase content and lattice constant increased,the reversible hydrogen storage capacity at room temperature increased,the plateau pressure of hydrogen absorption and desorption decreased,and the reaction enthalpy of hydrogen absorption and desorption increased.When the content of Nb in Ti-Fe-Zr-Mn-Nb alloy is excessively high（>2 at%）,the content of the Zr-rich phase in the alloy decreased and thus deteriorates the activation properties.The addition of a small amount of Nb enabled the combination of good activation properties and high hydrogen storage capacity of the alloy.Ti_44.5Fe_44.5Zr_2.5Mn_6.5Nb_2.0alloy obtained the maximum hydrogen absorption capacity of 2.04 wt.%on the premise of maintaining good activation properties at room temperature.Based on this study,the relationship between oxygen and hydrogen storage properties of TiFe alloy in Ti-Fe-O ternary system is established,the activation behavior of Ti-Fe alloy with high Ti/Fe ratio at room temperature is demonstrated,and the relationship between phase composition and of Ti-Fe-Mn,Ti-Fe-Zr-Mn and Ti-Fe-Zr-Mn-Nb multicomponent alloys and hydrogen storage properties,such activation kinetics,hydrogen storage capacity,are revealed,and TiFe-based multicomponent alloys with good hydrogen storage properties is obtained.