Study On Local Structure Of Liquid And Amorphous FeB-based Binay And Tinary Alloys By First-principle Methods

Owing to Fe-based amorphous alloys have particular soft magnetical performances,they have been widely investigated during these years,Bulk metallic glasses are usually obtained by quenching their liquid mother alloys.Element B exits in most Fe-baesd amorphous alloys,so it is significant to investigate the molten structure of glass-forming regions of FeB-based binary alloys for seeking new Fe-based amorphous alloys.In this dissertation,the relationship between the liquid and amorphous FeB-based binary and tinary alloys have been studied by ab initio molecular dynamic (AIMD) method and reverse monte carlo(RMC) method.For Fe-B binary system,we get the partial pair correlation fuctions by AIMD method.The results show that there are splittings in the second peaks of g_FeFe(r),g_FeB(r),g_BB(r) at different compostions and with the increasing B contents,the splittings become more obvious.These phenomene show that the second peaks start to exhibit splitting evidence at the superheated stage.We also introduce Voronoi analysis and the investigation finds that in the region 10%-30%of B contents,the local environment of Fe and B atoms exhibits certain regularity-one B atom is nearly surrouded by prismatic Fe atoms,and around Fe atoms there are small quantitative changes for(0,2,8,2),(0,3,6,3) polyhedra while there are few(0,0,12,0) polyhedra representing perfect icosahedron.This can be explained that in this area,there are certain leading clusters in the molten alloys,but beyond this region,the molten structure shows strongly disordered state,so it is difficult to produce bulk metallic glasses by rapid quenching.We also investigate the dynamic performances and find that the diffusion coefficients of the two elements exhibit similar changing trend, which results from the high temperatures.In the region 10%-30%of B contents, values of the diffusion coefficients of the two elements are small and close to each other.But out of this zone,the values of B atoms are many times larger than those of Fe atoms.On the one hand the size of B atom is small,on the other hand,the possibilities of forming clusters are lower.The Fe₅₀B₅₀ composition shows the maximal chemical order,which explains that the glass-forming ablity of FeB binary is inclined to be related with geometrical short order rather than chemical short order.Based on above discussion,we employ the combined AIMD and RMC methods to investigate the local structure between liquid and amorphous Fe₇₈Si₉B₁₃ alloys and get the partial pair correlation fuctions in the two states.Comparing with the structural information of the liquid and amorphous states,we gained that the local environment of Fe and B atoms only takes small changes,but in the amorphous Fe₇₈Si₉B₁₃ alloys nearly all the Si atoms are coordinated by only Fe atoms to form cluster units resembling that in the DO₃ typeα-Fe(Si) solid solution.We also have obtained the space distribution of compositive elements by RMC methods,and the results show that in the amorphous Fe₇₈Si₉B₁₃ alloy,B and Si atoms are nearly surrounded by Fe atoms,but it is hardly for a B atom and a Si atom to bond with each other directly,and there are few Si-Si clusters in the amorphous alloy.In orde to provide more local structural information at different temperatures during quenching,we investigate the structural evolution of Fe₇₈Si₉B₁₃ alloys from 1473K to 300K.The second peak splitting has been perceived even at 1473 K in the partial pair correlation functions though not in the total pair correlation function.With decreasing temperature,full icosahedra are rich around Si atoms.The(0,3,6,0) polyhedron are abundant in the liquid state while the distorted(0,3,6,0) polyhedron are the featured local structure around B atoms in the amorphous state.The diffusion coefficients of Fe and Si are nearly the same at the entire temperature range.B atoms have much larger diffusion coefficient than Fe and Si atoms above 1173 K,but it decreases more rapidly than that of Fe and Si atoms.And from 1173 K to 873 K the three coefficients are coherent.We think this temperature range corresponds to supercooled liquid region,and 873 K serves as the glass transition temperature.