Study On The Phase Equilibrium And The Magneto-thermal Effects Of Key Compounds In The La-(Fe,Ni)-Si Ternary System

The（Fe,Si）₁₃-based compound,La（Fe,Si）₁₃,is one of the most promising magnetocaloric materials,receiving extensive research from both fundamental and practical perspectives.Current studies have found that appropriate introduction of transition metal elements,such as Co and Ni,into La（Fe Si）₁₃-based alloys not only increases the phase transition temperature but also enhances the magnetic entropy change.However,there is a lack of reported information regarding the phase diagram of the La-Fe-Ni-Si alloy system,which hinders the development of La（Fe Ni Si）₁₃alloy as a room temperature giant magnetocaloric material.Therefore,it is of great significance to establish a reliable thermodynamic database for the La-Fe-Ni-Si alloy system,which can be achieved through the construction of the phase diagram using the Calculation of Phase Diagrams（CALPHAD）method.This method allows for efficient optimization of alloy composition,preparation techniques,and performance by revealing alloy phase equilibria,phase transitions,phase fractions,and element distributions in different phases under various composition and temperature conditions.The main objectives of this work are as follows:（1）By conducting literature research,binary thermodynamic data for the La-Fe-Si system were collected.The Calculation of Phase Diagrams（CALPHAD）method was employed to extrapolate the data to the La-Fe-Si ternary system.In the La-Si binary system,intermetallic compounds dissolve a certain amount of Fe in the La-Fe-Si ternary system.Therefore,the La_x（Fe,Si）_ymodel was used to redefine the intermetallic compounds in the La-Si binary system.Additionally,a sublattice model was employed to describe the six ternary compounds in the La-Fe-Si system,resulting in the construction of a comprehensive thermodynamic database for the La-Fe-Si system.Experimental verification was carried out on the isothermal section at 1373 K,which confirmed the agreement between the calculated phase diagram and experimental data,thus obtaining a self-consistent thermodynamic database.（2）The isothermal section of the La-Ni-Si ternary system at 1073 K was experimentally determined using the alloy method with EPMA（Electron Probe Micro-Analyzer）and XRD（X-ray Diffraction）equipment.The experiment revealed the presence of 17 ternary compounds and identified 44 ternary phase regions on the isothermal section at 1073 K.Additionally,EMPA phase composition analysis was conducted to determine the solubility of certain compounds on the isothermal section at1073 K.Specifically,the maximum solubility of Si in the Ni phase was found to be 10.28at.%;the maximum solubility of Si in the La Ni₅phase was determined to be 11.80 at.%;and the maximum solubility of Ni in the b La Si₂phase was found to be 6.09 at.%.Furthermore,for the homogeneous solid solution La Ni_13-xSi_x,experimental observations indicated that when 1.4≤x≤3.25,the compound appeared in the form of La Ni10.5Si2.5with a Na Zn₁₃-type structure.When 3.56≤x≤4.6,the compound appeared as La Ni9Si4with a Ce₂Ni₁₇Si₉-type structure.（3）Based on the obtained phase diagrams of La-Fe-Si and La-Ni-Si,the influence of Ni doping on the composition,microstructure,and magnetocaloric performance of La（Fe Si）₁₃alloys was investigated.The relationship between alloy composition,microstructure,and magnetocaloric performance with varying Ni doping levels was determined.The study revealed that moderate Ni doping can enhance the Curie temperature and magnetocaloric performance of La（Fe Si）₁₃ alloys.