First-principles Investigation Of Rare Earth Phases In Ag-4Gu-0.3Ni-La Alloy

During the past decades,development and application of rare earth elements?RE?have been always an important area of researching high-performance alloys.Usually,RE are added as a micro-alloying elements to the alloy to improve the microstructure of the alloy,to enhance properties of the alloy.Because of its special nature,RE easily combined with other alloying elements to form new phases.These rare earth phases often play an important role in improving properties of the alloy,but their contents are very low which makes it great difficulties to observe and analyze experimentally.Meanwhile,it also brings lot of troubles to investigate the functional mechanism of rare earth in the alloy.Because of its excellent electrical contact and thermal conductivity properties,silver alloy has been widely researched and applied in the field of electrical contact,especially in RE doped silver alloy.Before this project work,it is found that there are so low contents of various rare earth phase precipitations in silver-base contact material Ag4Cu0.3NiLa alloy that they couldn't be determined clearly by X-ray diffraction instrument,et al.In the dissertation,because of the lack of specific experimental observation evidence and available data about rare earth phases in the above-mentioned AgCuNiLa alloy,the relationship between structures and properties of rare earth phases in the alloy are investigated theoretically by first-principles,explored the formation rule of the microstructure and the relationship between the microstructure and performance,and then provided a theoretical basis for rare-earth element application in high-performance alloys.Firstly,the crystal structure,thermodynamical properties and electrical structure of Ag-La?LaAg,La Ag₂,La14Ag₅1 and LaAg₅ Laves phase?,Cu-La?La Cu,LaCu₂,LaCu₄,La Cu₅ and ?-La Cu₆?and Ni-La?La₃Ni,La₇Ni₃,La Ni,La₂Ni₃ and LaNi₅?system binary compounds are investigated by theoretical calculation.Geometry optimization results indicate that the calculated values of the compounds crystal structure parameters and mass densities are consistent with experimental values.The cohesive energies of Ag-La,Cu-La binary compounds are raised with increasing Lacontent,and the decreasing order of cohesive energies for La combined with Ag,Cu and Ni atoms is Ni-La,Cu-La and Ag-La,and the decreasing order of absolute value of enthalpies of formation is among those La poor phases: LaAg₂,LaNi₅,La₁₄Ag₅1,LaAg₅-f₁f₃ Laves phase,LaCu₄,La Cu₅ and LaCu₆,and their values are-27.2,-26.3,-23.2,-18.3,-11.2,-10.6 and-8.9 KJ/mol,respectively.The electronic structures show that the Ag-La system and the Cu-La system binary compounds' stability raise up with increasing La content,since the bonding characteristics of intermetallic containing covalent and ionic are enhanced with increasing La content,while the Ni-La system stability decrease with increasing La content,attributed to the decrease of covalent nature is larger than the increase of ionic nature.Secondly,the theoretical calculations of the effect of pressure on structural,elastic and electronic properties of rare earth phases with dual structures?LaAg₅,LaCu₅,and LaNi₅?confirmed in the experiment were performed.It shows that the covalent and ionic nature of those structures raise up with increasing external pressure,leading to enhance structural stability of the compounds.There are three structures for LaAg₅,f₁f₂,f₁f₃ and f₂f₃,respectively,and the f₁f₃ structure has best performance on thermodynamic,mechanical stability and elastic modulus and isotropy among them.As the pressure increased,elastic modulus and Debye temperature for La Ag₅,LaCu₅ and La Ni₅ imply nearly linear increasing,respectively,especially for LaCu₅ which Debye temperature behave same tendency with Young's modulus that maintain steady above 25 GPa.The Pugh values and passion's rates show that,at 0 GPa,those compounds are ductile materials,and LaCu₅ and LaAg₅-f₁f₂ and-f₂f₃ structures indicate high toughness but LaNi₅ suggests low toughness.For LaAg₅-f₁f₃ and La Ni₅,their toughness become better with increasing pressure until external pressure up to 20 GPa for LaAg₅-f₁f₃ and 15 GPa for La Ni₅,respectively.At 0 GPa,the decreasing order of bulk modules,shear modulus,Yong's modulus and general anisotropy index AU is,La Ni₅,LaCu₅,La Ag₅.Finally,the substitution effects on Cu replacing Ag in LaAg₅ and Ni replacing Cu in La Cu₅ were investigated.Geometry optimized results demonstrate that Ag atoms at Wyckoff 2a and 4f sites rather than 6h of LaAg₅-f₁f₃ structure can be replaced by Cu,since Cu replace Ag at 6h site,the hexagonal structure may transform into the monoclinic,and only Cu atoms at 2c site of La Cu₅ phase can be replaced by Ni,because Ni replace Cu at 3g site,the hexagonal structure may transform into the orthogonal.After 2a sites of La Ag₅-f₁f₃ structure was replaced by Cu and 2c site ofLaCu₅ phase was replaced by Ni,both the enthalpies of formation and cohesive energies of compounds were increased so that structural stability of the compounds are raised.For La?Ag,Cu?₅,Cu is prefer to replace Ag at 2a site than 4f site because of higher absolute values of enthalpies of formation and cohesive energies.It is found that Cu doped to AgLa5 lead its elastic modulus and isotropy to reduce but its toughness to increase,and when Ni doped to La Cu₅,the compound bulk modulus and toughness decrease but its shear modulus,Young's modulus and isotropic increase.What's more,the substitution elements lead the density of states to reduce at the Fermi level,and there are local effects between Ni d and Cu d of LaCu₄ Ni compound.