Microalloying Mechanism Of Sub-rapidly Solidified La–Fe–Si Magnetocaloric Alloys

La–Fe–Si compounds has been considered to be one of the most promising magnetic refrigeration materials.High-temperature phase transition behavior and intrinsic brittleness of NaZn₁₃-type?₁ phase in La–Fe–Si magnetocaloric materials are two key problems from the viewpoint of materials production and practical applications.In the present thesis,the classical Johnson-Mehl-Avrami-Kolmogorov equation was introduced to quantitatively characterize the formation kinetics of?₁ phase in sub-rapidly solidified LaFe_11.6Si_1.4 plates during the isothermal annealing process.In combination with X-ray diffraction results,the density-functional theory was used to reconstruct the unit-cell structure for bettering understanding the magnetic transition behavior after doping B and C element into LaFeSi alloy.The following conclusions are obtained.?1?It was suggested that centrifugal casting may become an effective approach to prepare high-performance La–Fe–Si magnetocaloric plates,which could largely accelerate the formation of?₁ phase during high-temperature heat-treatment process.The sub-rapidly solidified plate can obtain?₁ phase up to a proportion as high as 65.1%after annealing at 1373 K for 6 h,while the arc-melting bulk can obtain 59.8%?₁ phase after annealing at 1323 K for 25 h.The acceleration formation of?₁ phase was ascribed to the refined and homogeneous honeycombed microstructure.According to the JMAK equation,the Avrami index of sub-rapidly solidified plate was estimated to be0.43?⁰.5?,which implies that the formation of?₁ phase is in a diffusion-controlled one-dimensional growth mode.While the Avrami index of arc-melting bulk is 0.93?¹?,which means the formation of?₁ phase is in a diffusion-controlled two-dimensional growth mode.Meanwhile,it is revealed that the Vickers hardness as a function of annealing time for sub-rapidly solidified plates also agrees well with the JMAK equation.The Vickers hardness of?₁ phase was estimated to be about 754 Hv.Under an external magnetic field change of 3 T,the maximum magnetic entropy change was about 22.31 J/?kg·K?for plates annealed at 1323 K for 48 h,and the effective magnetic refrigeration capacity reached 191 J/kg.?2?According to the high temperature differential scanning calorimetry's results,the sub-rapidly solidified LaFe_11.6Si_1.4 plate occurred the following reactions upon heating:SLaFeSi?LLaFeSi,??Fe?+LLaFeSi??1,L?1?S?1,and S??Fe??L??Fe?;L??Fe??S??Fe?,S_?1?L_?1 and L_LaFeSi?S_LaFeSi upon cooling.Therefore,the?₁ phase which obtained from the sub-rapidly solidified LaFe_11.6Si_1.4 plate was formed through a peritectoid reaction S_LaFeSi+S_??Fe???₁ at 1373 K.All the three samples demnonstrated nearly pure?₁ phase after annealing at 1373 K for 100 h.The melting point of LaFeSi phase slightly reduced after dopping B atoms,and as a result,less LaFeSi phase can be observed.All the LaFe_11.6Si_1.4,LaFe_11.6Si_1.4B_0.06.06 and LaFe_11.6Si_1.4C_0.06.06 samples underwent a first-order magnetic transition.B doping decreased the magnetic transition temperature from 192K to 188 K,whereas C doping increased to 206 K.The annealed samples possessed the maximum magnetic entropy changes higher than 20 J/?kg·K?under a magnetic field change of 3 T,and their effective refrigeration capacities reached 200 J/kg.?3?The density-functional theory was used to reconstruct the LaFe_11.6Si_1.4 unit-cell.A co-ordination factor k was introduced to balance the X-ray diffraction experimental results and the simulated crystal structure.The results suggested that almost all B atoms?93%?occupy 96i FeII/Si substitutional site,only a small number of B atoms enter the 24d position as interstitial atoms.All C atoms prefer 24d interstitial site in the present given conditions.The relationship between the Fe-Fe bond length and the Curie temperature of La–Fe–Si alloy was well explained by directly measuring the bond length.Compared with LaFe_11.6Si_1.4,most of Fe-Fe bonds are contracted in case of B-96i substitution.Opposite tendency happens for C-24d interstitial site,where more Fe-Fe bonds stretch.