Directional Solidification Behaviour And Properties Enhancement Of La-fe-si Magnetocaloric Alloys

With increasing concern for environment,traditional gas compression/expansion refrigeration becomes out of date due to its damage to ozone sphere,low efficiency and low density.In this case,solid refrigeration has driven lots of attention because it is the promising substitution for gas refrigeration,especially magnetic refrigeration achieved by magneotocaloric materials.Magentocaloric alloys refer to materials with magnetocaloric effect?MCE?,which undergoes magneto-structural phase transition accompanied by absorbing or releasing heat when applied magnetic field changed.The typical magnetocaloric materials include those undergoing second-order magnetic phase transition materials like Gd and first-order magnetic phase transition like Gd-Si-Ge,Ni-Mn-based Heusler alloys and La-Fe-Si.La-Fe-Si magnetocaloric alloy has driven lots of attetion because of giant magnetocaloric effect as well as its environmentally-friendliness,cheap constituted elements and small hysteresis.However,La-Fe-Si alloy is faced with several obstacles including processing complication,unclear conception about first-order magneto-structural phase transition,and comparatively low comprehensive properties?strength,thermal conductivity,et al.?.Firstly in this study,directional solidification is used to prepare LaFe_11.6Si_1.4 alloys with a dual-phase microstructure consisting of a?Fe,Si?and LaFeSi phases.It is found that the volume fraction of La?Fe,Si?₁₃ phase in directionally solidified LaFe_11.6Si_1.4 alloys can beincreased to⁶2 pct.It is also found that the volume fraction of the La?Fe,Si?₁₃ phase is dependent primarily on the temperature gradient at different growth rates,which can be the result of their wide temperature range between solidus and liquidus.A significant Si segregation is observed in directionally solidified LaFe_11.6Si_1.4 alloys.Furthermore,the solidification path is discussed,focusing on the volume fraction increase of the La?Fe,Si?₁₃phase by directional solidification.Based on large solidus-liquids range,this investigation demonstrates a novel short-time continuous descending temperature annealing?CDTA?method for La?Fe,Si?₁₃magnetocaloric alloys.Within 10 min annealing,?81 pct volume fraction of La?Fe,Si?₁₃phase and a 9.52 J/kgK?0–5 T?magnetic entropy change have been obtained.Besides,a lamella-shape microstructure consisting of two phases is observed.The relationship between chemical segregation of La?Fe,Si?₁₃ phase and cooling rate is investigated.The mechanism of CDTA method is analyzed,indicating a different interface movement behavior.Then,we introduce La-rich off-stoichiometric component.The morphology,magnetic property and mechanical property of directionally solidified off-stoichiometric La_1.7Fe_11.6Si_1.4 alloys are systematically investigated.Different from the conventional casting in which the La?Fe,Si?₁₃ phase is formed during a long time heat treatment,a large volume fraction?over⁸0 pct?of La?Fe,Si?₁₃ phase is formed directly during solidification.The Curie temperature is increased and the magnetic entropy change value under applied fields of 2T attains 9.1 J/kgK.The activation energy of the magnetic phase transition is calculated using the Johnson-Mehl-Avarmi-Kolmogorov?JMAK?equation and the Arrhenius relation for the first time,showing a higher value than that of NiMn-based alloys.Compared with the Fe-rich alloys,the directionally solidified alloys show higher bending strengthens,which is due to the existence of the preferential growth?-?Fe,Si?phase.Furthermore,the?-?Fe,Si?phase plane interface and periodic morphology are detected along crystal growth direction,and the corresponding formation mechanism is discussed.Finally,the microstructure and magnetic properties of off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x?x=0,0.1,0.4?alloys were studied over a range of annealing temperatures.The alloys were homogenized for 36 hours at the optimized annealing temperature of 1248K for LaFe_11.6Al_1.4 and LaFe_11.6Al₁S_i0.4 specimens,and 1198K for the LaFe_11.6Al_1.3Si_0.1 specimen.Compared with annealing time?30 days?of stoichiometric LaFe_11.6Al_1.4-xSi_x alloys,it saves much energy and time for off-stoichiometric La_1.7?Fe,Si,Al?₁₃ alloys.The microstructure of off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x alloys display facet-like grains.The grains of LaFe_11.6Al_1.4 and LaFe_11.6Al₁S_i0.4 specimens are separated while the grains of LaFe_11.6Al_1.3Si_0.1 are connected and continuous.Si doping caused the Curie temperature of off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x alloys to vary.The magnetic entropy change for La_1.7Fe_11.6Al_1.3Si_0.1 specimen with an applied magnetic field change from 0T to 2T was determined as 4.58 J/kgK and the corresponding relative cooling capacity was observed as173.6 J/kg.It was also shown during thermal expansion tests that when doped with Si the La_1.7Fe_11.6Al_1.4-xSi_x?x=0,0.1,0.4?alloy displayed a slight volume change when the phase transition occurred.Finally,Vickers hardness tests were performed to illustrate the mechanical performance of the specimens.