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Study on Solidification Behavior of La-Fe Based Room Temperature Magntic Refrigerant Materials

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Tutor: TuGanFeng GaoJianRong
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: Lanthanum iron-based alloys,Peritectic alloys,Rapid solidification,Undercooling,
CLC: TB64
Type: PhD thesis
Year:  2009
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Abstract:
NaZn13 type La-Fe-based magnetic refrigeration materials due to its good magnetocaloric effect, is the potential temperature magnetic refrigeration materials, more and more aroused widespread concern. Typically, the alloy ingot in an inert (Ar) atmosphere at a high temperature for a long time annealing (1273K/15 days) in order to obtain NaZn13 type compound; but with short rapid quenching temperature annealing (1273K/20 points) method to obtain higher levels of NaZn13 type compound. But the high temperature annealing process is not conducive to commercial applications, this exploration controlled La-Fe-base alloy solidification behavior is more practical significance. Since La-Fe-based alloys during solidification phase formation mechanism is unclear, this article through experiments and theoretical calculations systematically studied La-Fe-base alloy solidification behavior and phase selection mechanism. This article first La-Fe-based alloys in the arc melting solidification behavior under the conditions studied. The results showed that: LaFe13-xSix alloy microstructure mainly formed of ¦Á-(Fe, Si) phase, LaFeSi phase and La (Fe, Si) 13 phase, when the Si content x gt; 1.5 occurs when La (Fe, Si) 13 peritectic phase, Si content increases and promoting La (Fe, Si) 13 phase formation. LaFe13-x-ySixCoy alloy microstructure mainly formed of ¦Á-(Fe, Si, Co) phase, La (Fe, Co) Si phase and the La (Fe, Si, Co) 13 phase, when x = 1.5, y = 0.2-0.6 occurs when La (Fe, Si, Co) 13 phase, Co content increases and promoting La (Fe, Si, Co) 13 phase formation. By La-Fe-based alloys under conditions in induction melting and solidification phase composition research, exploration alloy solidification conditions near equilibrium solidification behavior. The results showed that: the lower the cooling rate, the undercooling is smaller, the alloy ingot is difficult to form 1:13 phase. LaFe13-xSix precipitated during solidification phase sequence: When the Si content is 0.5 ¡Ü x ¡Ü 1.5, the first temperature of the primary phase precipitated ¦Á-(Fe, Si), followed by reaction LaFeSi eutectic phase; when the Si content is x ¡Ý 2.0 , the precipitated ¦Á-(Fe, Si) phase was precipitated LaFe2Si2 phase, the remaining liquid phase generating LaFeSi; when the Si content of x = 3.0, the precipitated ¦Á-(Fe, Si) phase and the precipitated phase LaFe2Si2 addition Fe3Si; on Si content is low (x ¡Ü 1.5) and a higher (x ¡Ý 2.0) is, ¦Á-(Fe, Si) phase and the main phase, respectively LaFe2Si2 phase; LaFe13-x-ySixCoy precipitated during solidification phase in order: first the precipitation temperature primary phase ¦Á-(Fe, Si, Co), followed by the eutectic reaction of La (Fe, Co) Si phase; measured by infrared thermometer LaFe13-xSix alloys and LaFe13-x-ySixCoy alloy liquidus temperature , with the Si content increases liquidus temperature decreases; LaFe13-xSix main phase alloy solidification temperature: TN (1:2:2). quenching method of suspension and non-container electromagnetic levitation melting method to study the La-Fe based alloys increase the cooling rate and undercooling conditions, microstructural features and phase composition, exploring rapid solidification solidification behavior. The results showed that: LaFe13-xSix quenched alloy quenched, when x = 1.5, the contact surface of the sample appeared La (Fe, Si) 13 phase, when x = 2.5 when the contact surface La (Fe, Si) 13 phase as a primary phase precipitates; LaFe13-x-ySixCoy solidification behavior of x and y has a great relationship, when x = 0.5-1.0, y = 0-0.4, the alloy's microstructure consists of ¦Á-(Fe, Si , Co) phase and the La (Fe, Co) Si phase composition; when x = 1.0, y = 0.6 occurs when La (Fe, Co, Si) 13 phase; cooling rate is increased in favor La (Fe, Co, Si) 13 phase formation. LaFe13-xSix alloy electromagnetic levitation undercooling, when x = 1.5, the undercooling ¦¤T ¡Ý 40K, blowing in the sample is directly cooled surface appeared La (Fe, Si) 13 phase; when x ¡Ý 2.5 when undercooling La (Fe, Si) 13 phase as a primary phase precipitates; LaFe10.9Si1.5Co0.6 solidification behavior and undercooling great relationship, when undercooling ¡÷ T ¡Ü 10K, alloy microstructure consists of ¦Á - (Fe, Si, Co) phase and La (Fe, Co) Si phase, when the undercooling ¦¤T ¡Ý 40K, the alloy microstructure consists of ¦Á-(Fe, Si, Co) phase, La (Fe, Si , Co) 13 phase and the La (Fe, Co) Si phase, La (Fe, Si, Co) 13 with undercooling phase content increases. Application of classical nucleation theory and transient nucleation theory model of the alloy during solidification nuclei nucleation rate and incubation time were calculated, the calculated results well on the phase selection mechanism explained that in reaching critical undercooling over undercooling before, ¦Á-(Fe, Si) phase nucleation rate greater than La (Fe, Si) 13 phase nucleation rate, preferentially precipitated during solidification; when undercooling exceeds the critical undercooling after, La ( Fe, Si) 13 is greater than the nucleation phase ¦Á-(Fe, Si) phase as a primary phase metastable precipitate first, ¦Á-(Fe, Si) phase formation will be suppressed. According to the transient nucleation theory: When undercooling is low, t1: 13 gt; t¦Á-Fe, ie 1:13 phase nucleation incubation time is greater than ¦Á-Fe phase nucleation incubation time, ¦Á-Fe The first phase as a primary phase precipitation from supercooled melt; And when undercooling is high, t¦Á-Fe gt; t1: 13, time 1:13 phase nucleation incubation time is less than ¦Á-Fe phase nucleation incubation time, 1:13 first phase will serve as the primary phase precipitation from supercooled melt.
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