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镧铁基室温磁致冷材料凝固行为的研究

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

【作者】 董中奇

【导师】 涂赣峰; 高建荣;

【作者基本信息】 东北大学 , 有色金属冶金, 2009, 博士

【摘要】 NaZn13型La-Fe基磁致冷材料由于其具有良好的磁热效应,是潜在的室温磁制冷材料,愈来愈引起人们的广泛关注。通常情况下,合金铸锭在惰性(Ar)气氛中经过长时间的高温退火(1273K/15天)才能获得NaZn13型化合物;而用快速淬火加短时间高温退火(1273K/20分)的方法可以获得含量较高的NaZn13型化合物。但高温退火过程是不利于商业应用的,为此探索控制La-Fe基合金的凝固行为更具实际意义。由于La-Fe基合金凝固过程中相的形成机制尚不清楚,为此本文通过实验和理论计算系统研究了La-Fe基合金的凝固行为和相选择机理。本文首先对La-Fe基合金在电弧熔炼条件下的凝固行为进行研究。结果表明:LaFe13-xSix合金的凝固组织中主要形成α-(Fe,Si)相、LaFeSi相和La(Fe,Si)13相,当Si含量x>1.5时,出现La(Fe,Si)13包晶相,Si含量增加有利于促进La(Fe,Si)13相的形成。LaFe13-x-ySixCoy合金的凝固组织中主要形成α-(Fe,Si,Co)相、La(Fe,Co)Si相和La(Fe,Si,Co)13相,当x=1.5,y=0.2-0.6时,出现La(Fe,Si,Co)13相,Co含量增加有利于促进La(Fe,Si,Co)13相的形成。通过对La-Fe基合金在感应熔炼条件下的凝固组织和相组成的研究,探索合金在近平衡凝固条件下的凝固行为。结果表明:在冷速较低,过冷度较小的情况下,合金铸锭中很难形成1:13相。LaFe13-xSix合金凝固时析出相的顺序为:当Si含量0.5≤x≤1.5时,首先析出高温初生相α-(Fe,Si),接着发生共晶反应生成LaFeSi相;当Si含量x≥2.0时,析出α-(Fe,Si)相后析出LaFe2Si2相,剩余液相生成LaFeSi相;当Si含量x=3.0时,析出α-(Fe,Si)相后析出LaFe2Si2相加Fe3Si相;在Si含量较低(x≤1.5)和较高(x≥2.0)时,α-(Fe,Si)相和LaFe2Si2相分别是主要相;LaFe13-x-ySixCoy合金凝固时析出相的顺序:首先析出高温初生相α-(Fe,Si,Co),接着发生共晶反应生成La(Fe,Co)Si相;采用红外测温仪测定了LaFe13-xSix合金和LaFe13-x-ySixCoy合金的液相线温度,随Si含量的增加液相线温度降低;LaFe13-xSix合金中主要相的凝固温度:TN(1:2:2).采用悬浮淬火的方法和无容器电磁悬浮熔炼的方法研究了La-Fe基合金在增加冷却速率和过冷度条件下的凝固组织特征和相组成,探索快速凝固条件下合金的凝固行为。结果表明:LaFe13-xSix合金在急冷快淬时,当x=1.5时,在样品的接触面出现了La(Fe,Si)13相,当x=2.5时,在接触面La(Fe,Si)13相作为初生相析出;LaFe13-x-ySixCoy的凝固行为与x和y又很大关系,当x=0.5-1.0,y=0-0.4时,合金的显微组织由α-(Fe,Si,Co)相和La(Fe,Co)Si相组成;当x=1.0,y=0.6时,出现La(Fe,Co,Si)13相;冷却速率增加有利于La(Fe,Co,Si)13相的形成。LaFe13-xSix合金在电磁悬浮深过冷时,当x=1.5时,过冷度ΔT≥40K,在样品被直接吹气冷却的表层出现了La(Fe,Si)13相;当x≥2.5时,过冷度La(Fe,Si)13相作为初生相析出;LaFe10.9Si1.5Co0.6的凝固行为与过冷度很大关系,当过冷度△T≤10K,合金显微组织由α-(Fe,Si,Co)相和La(Fe,Co)Si相组成,当过冷度ΔT≥40K时,合金显微组织由α-(Fe,Si,Co)相、La(Fe,Si,Co)13相和La(Fe,Co)Si相组成,La(Fe,Si,Co)13相含量随过冷度的增加而增加。应用经典形核理论和瞬态形核理论模型对合金凝固过程中的形核率和晶核孕育时间进行了计算,该计算结果很好的对相选择机理进行解释,在过冷度达到临界过冷度之前,α-(Fe,Si)相的形核率大于La(Fe,Si)13相的形核率,在凝固过程中优先析出;当过冷度超过临界过冷度之后,La(Fe,Si)13相的形核率大于α-(Fe,Si)相,作为亚稳的初生相优先析出,α-(Fe,Si)相的形成将被抑制。根据瞬态形核理论:当熔体过冷度较低时,t1:13>tα-Fe,即1:13相的形核孕育时间大于α-Fe相的形核孕育时间,α-Fe相作为初生相首先从过冷熔体中析出;而当熔体过冷度较高时,tα-Fe>t1:13,此时1:13相的形核孕育时间小于α-Fe相的形核孕育时间,1:13相将作为初生相首先从过冷熔体中析出。

【Abstract】 La(Fe,Si)13 pseudobinary compounds with the NaZn13 structure are enjoying more and more interest in recent years due to the large magnetocaloric effect. In common ways, the NaZn13 structure alloys are produced by arc-melting and followed by a long time (1273 K/15 days) annealing in the Ar atmosphere. Rapid quenching and subsequent short time annealing (1273 K/20 minutes) is an effective production route for La-Fe based alloys with the NaZn13 structure. From the commercial point of view, however, such a time-consuming process is unfavorable. In a word, it is of great technical interest to control the solidification behavior of La-Fe based alloys. However, the mechanism of the formation phases is not clear in La-Fe based alloys. Hence, the present work is designed to investigate the solidification behavior and the mechanism of phase election in La-Fe based compounds through experiments and theoretical calculation.First in the paper, solidification behavior of La-Fe based alloys was investigated using arc-melting. The results show that the microstructure of the LaFe13-xSix alloys is made of a-(Fe,Si) phase, the LaFeSi phase and La(Fe,Si)13 phase. With x>1.5, the La(Fe,Si)13 phase occur in the microstructure. It help to fomation La(Fe,Si)13 phase with the x value increasing. The microstructure of the LaFe13-x-ySixCOy alloys is made of a-(Fe,Si,Co) phase, the La(Fe,Co)Si phase and La(Fe,Si,Co)13 phase. With x=1.5 and y=0.2-0.6, the La(Fe,Si,Co)13 phase occur in the microstructure. It help to fomation La(Fe,Si,Co)13 phase with the y value increasing.Phase formation and structure in induction melted La-Fe based alloys has been investigated in order to research on solidification behavior of alloys under near equilibrium solidification condition. The results show that the 1:13 phase is difficult to formation under the small cooling rate or low undercooling. The phase relations of LaFei3-xSix alloys are displayed as follow. The primaryα-(Fe,Si) phase first is formed form liquid phase at high temperature, then the LaFeSi eutectic phase is crystallized at low temperature with values (x=0.5-1.5). The primary a-(Fe,Si) phase first separate out from the liquid then the LaFe2Si2 phase and the LaFeSi phase with values (x=2.0-2.5). At x=3.0, first the a-(Fe,Si) phase is formed then the lamellar eutectic phase is occurred which are LaFe2Si2 phase plus Fe3Si phase. The a-(Fe,Si) phase and the LaFe2Si2 phase is mainly phase in the microstructure of alloy with low and high Si contents, respectively. The phase relations of LaFe13-x-ySixCoy alloys are displayed as follow. The primary a-(Fe,Si,Co) phase first is formed form liquid phase at high temperature, then the La(Fe,Co)Si eutectic phase is crystallized at low temperature with values (x=0.5-1.5). The liquid temperature of LaFe13-xSix and LaFe13-x-.SixCoy alloys was measuremented by using infrared radiation thermometers. The results show that the liquid temperature of La-Fe based alloys decrease with Si content increasing; and TN(1:1:1)< TN(1:13)< TN(1:2:2) in LaFe13-xSix alloys.Solidification behavior of La-Fe based alloys was investigated under rapid solidification condition by using rapid quenching and levitating. Phase formation and structure of La-Fe based alloys has been investigated with the undercooling and the cooling rate increasing, respectively. The results show that under higher cooling rate, composition with x=1.5, the La(Fe,Si)13 phase is crystallization in localized thin area near contact surface of ribbon sample. When x=2.5, the La(Fe,Si)13 phase is directly crystallization without via peritectic reaction at contact surface of ribbon sample of LaFe13-xSix alloys. It implies that the solidification route change and La(Fe,Si)13 phase priority the primary a-(Fe,Si) phase in competition during rapid quenching. The solidification behavior of LaFe13-x-ySixCoy alloys depend on Si and Co contents x and y. The microstructure of compositions with x=0.5-1.0 and y=0-0.4 is made ofα-(Fe,Si,Co) phase and La(Fe,Co)Si. The microstructure of compositions with x=1.0 and y=0.6 occur La(Fe,Co,Si)13 phase. Under large undercooling, composition with x=1.5 and△T≥40K, the La(Fe,Si)13 phase is crystallization in localized thin area bottom of the sample LaFe13-xSix alloys. When x≥2.5 and△T≥55K, the La(Fe,Si)13 phase is directly crystallization without via peritectic reaction. It implies that the solidification route change and La(Fe,Si)13 phase priority the primaryα-(Fe,Si) phase in competition during levitating. The solidification behavior of LaFe10.9Co0.6,Sii.5 alloys depend on undercooling. When△T≤10K, the microstructure of the sample is made of a-(Fe,Si,Co) dendrites and in interdendritic La(Fe,Co)Si phase.α-(Fe,Si,Co) phase is primary phase. When△T≥40K, the microstructure of the sample is made ofα-(Fe,Si,Co) dendrites and in interdendritic La(Fe,Si,Co)B13B phase and La(Fe,Co)Si phase. The La(Fe,Si,Co)B13B phase increase with undercooling increasing. The solidification behaviors and microstructure of LaFe13-xSix alloys strong depend on Si contents, cooling rate and undercooling.The phase selection mechanisms were discussed using classical nucleation theory and transient nucleation theory. According to the calculation results of competitive nucleation, the a-(Fe,Si) phase has higher nucleation rate than La(Fe,Si)13 phase above a critical undercooling, and the primary phase isα-(Fe,Si) phase. The primary a-Fe is replaced by La(Fe,Si)13 below a critical undercooling. Based on transient nucleation theory, the calculation results of incubation time of a-(Fe,Si) phase and La(Fe,Si)13 phase in rapidly solidified alloys, it indicates that the incubation periods of a-(Fe,Si) phase are less than that of La(Fe,Si)13 phase under small undercooling. So a-(Fe,Si) phase is formed from the liquid phase at first. Under large undercooling, the incubation time of La(Fe,Si)13 phase are less than that of a-(Fe,Si) phase, and La(Fe,Si)13 phase is formed from the liquid phase at first.

  • 【网络出版投稿人】 东北大学
  • 【网络出版年期】2012年 06期
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