Study On Phase Structure And Hydro-gen Absorption And Desorption Of LaMgNi_3.9Mn_0.2 Alloy After Annealing

In recent years,the Chinese government has issued a series of policies on promoting the development of new energy industry,new energy hydrogen fuel cell vehicles are more and more popular,and as the core part of new energy vehicle,Ni/MH fuel cell needs high performance and high energy density,which means that the material used as battery cathode should have the better performance in whole.At present,the commercial AB5 type hydrogen storage alloy cannot meet the current market demand because of its limited capacity due to its own structure,while REMg based AB₂ type hydrogen storage alloy has a high theoretical capacity and better activation performance,which led to a research upsurge of scholars at home and abroad.Although the theoretical hydrogen storage capacity of this kind of alloy is very high,its actual hydrogen storage capacity is very small,and its cycle stability is also poor.Therefore,how to improve the hydrogen storage capacity and cycle life of this kind of alloy has become the research focus of its commercial application.In this study,the as cast LaMgNi_3.9Mn_0.2.2 alloy was annealed at different time and temperature.The phase composition and microstructure of the alloy were detected and analyzed by XRD,SEM and EDS,and the hydrogen absorption and desorption of the alloy at different experimental temperatures were tested.In addition,in order to explore the effect of annealing on the mechanical properties of the alloy,ABAQUS 6.14-1 finite element software was used to establish the model of the as cast alloy and annealing alloy cracks.Different loads or different angle loads were applied to the as cast and annealed alloy to act as hydrogen pressure loads,so as to simulate the occurrence of cracks in the alloy.The phase composition of the alloy was detected and analyzed by X-ray diffraction?XRD?.The results show that all the alloys are composed of two main phases,namely LaNi₅phase and LaMgNi₄ phase.No matter the annealing temperature or the annealing time is different,there is no perfect improvement in the phase composition of the alloy.The results of SEM and EDS analysis show that the as cast and annealed LaMgNi_3.9Mn_0.2.2 alloy have columnar crystal structure,which shows that the annealed alloy has structural heredity,and the white area is LaNi₅ phase,and the dark area is LaMgNi₄ phase,which has the characteristics of peritectic reaction.The hydrogen absorption and desorption of as cast and annealed LaMgNi_3.9Mn_0.2.2 alloys at different time/temperature were detected,the maximum hydrogen absorption can be achieved after the first hydrogen absorption cycle,but the activation property of as cast alloy is significantly higher than that of other annealed alloys.With the increase of annealing temperature,the maximum hydrogen absorption of the alloy increases first and then decreases.When the annealing temperature is 370?,the alloy has better gas hydrogen absorption and desorption performance,and its saturated hydrogen absorption can reach 1.540%at 55?,With the increase of the experimental temperature,the amount of hydrogen absorption and desorption decreased;for the alloy treated with different annealing time,with the increase of annealing time,the maximum amount of hydrogen absorption and desorption and the saturated amount of hydrogen absorption and desorption decreased,and the saturation rate of hydrogen absorption and desorption increased firstly and then decreased.When the annealing time was 15 h,the saturation rate of hydrogen absorption and desorption was relatively high,and reached 91.72%/91.85%respectively at the test temperature of 75?.The simulation of as cast and annealed alloy with different loads of different sizes and angles was carried out by using the finite element software ABAQUS 6.14-1.It is found that the crack of as cast alloy increases with the increase of loading value,while the same load is applied to the annealed alloy,the stress of the annealed alloy is smaller than that of the as cast alloy,so the crack is obviously smaller than that of the as cast alloy,which shows that the annealed alloy is helpful to improve the internal stress of the alloy,and thus effectively hinder the progress of crack formation.When the load of 3 MPa at different angles is applied to as cast and annealed alloy,it is found that,the effect of different angle load on the crack degree of the alloy is small,which shows that the load of different angle has little effect on the crack of the alloy,but for the annealed alloy,the stress is far less than that of the as cast alloy,which further proves that the annealing treatment can effectively improve the mechanical properties of the alloy.