【摘要】：相變是指物質(zhì)系統(tǒng)從一種狀態(tài)轉(zhuǎn)變?yōu)榱硪环N狀態(tài)的過程。鐵磁體在低溫下的內(nèi)部自旋,相互作用表現(xiàn)出有序的排列。當(dāng)溫度超過臨界值時(shí),有序旋轉(zhuǎn)布置被破壞,這個(gè)時(shí)候鐵磁體被轉(zhuǎn)換成順磁性。計(jì)算機(jī)技術(shù)的快速發(fā)展,鐵磁相變模型不斷完善。磁團(tuán)簇?cái)U(kuò)展模型考慮到原子的磁矩和原子在合金中的位置,可應(yīng)用于所有大小的團(tuán)簇,是一種研究鐵磁相變的有效方法。本文以二元Fe-Cr合金為研究對(duì)象,通過磁簇?cái)U(kuò)展模型對(duì)Fe-Cr二元合金磁性相變進(jìn)行系統(tǒng)研究。從微觀角度理解二元合金鐵磁相變的本質(zhì)原理,本文的主要內(nèi)容和研究結(jié)果如下:(1)利用基于密度泛函數(shù)理論的平面波贗勢(shì)方法的VASP程序擬合得出磁團(tuán)簇模型的各個(gè)參數(shù)值,將模擬結(jié)果與實(shí)驗(yàn)數(shù)據(jù)進(jìn)行比較,驗(yàn)證了參數(shù)的準(zhǔn)確性。(2)用磁團(tuán)簇模型研究無序狀態(tài)下Cr含量與居里點(diǎn)的關(guān)系:在Cr含量較低時(shí)(0-10%),FeCr的居里溫度先上升,在Cr含量為6%左右達(dá)到最大值,后隨著Cr含量的增加,而逐漸降低。(3)用磁團(tuán)簇模型研究Fe-Cr合金中Cr含量與體系焓的關(guān)系:在低成分時(shí),混合焓是一個(gè)負(fù)值,此時(shí)Fe-Cr合金結(jié)構(gòu)穩(wěn)定;混合焓隨著Cr的增加,先降低后上升,在Cr6%時(shí)到達(dá)一個(gè)最小值,此時(shí)結(jié)構(gòu)最為穩(wěn)定;當(dāng)成分到達(dá)大概12%時(shí),混合焓符號(hào)為正,從而出現(xiàn)相分離的趨勢(shì)。(4)用磁團(tuán)簇模型研究有序狀態(tài)下Cr含量與居里點(diǎn)的關(guān)系,并將有序和無序?qū)Ρ?得到在同一組分的樣品中,有序結(jié)構(gòu)的居里溫度高于無序樣品,特別是在6%樣品中的差別最大。與有序結(jié)構(gòu)結(jié)構(gòu)相比,無序態(tài)下Cr原子形成團(tuán)簇的幾率增加,Cr-Cr之間反鐵磁相互作用能夠部分抵消磁矩,從而降低了相變溫度。(5)采用基于磁團(tuán)簇模型的相圖計(jì)算方法,計(jì)算α-γ和γ-δ的結(jié)構(gòu)相變,從而得到相圖。
[Abstract]:Phase transition refers to the transition of material system from one state to another. The internal spin of ferromagnet at low temperature shows an orderly arrangement of the interaction. When the temperature exceeds the critical value, the ordered rotation arrangement is destroyed, and the ferromagnet is converted to paramagnetism. With the rapid development of computer technology, the ferromagnetic phase transition model has been improved. Considering the magnetic moment and the position of atoms in the alloy, the magnetic cluster expansion model can be applied to all kinds of clusters. It is an effective method to study ferromagnetic phase transition. In this paper, the magnetic transformation of binary Fe-Cr alloy is systematically studied by means of cluster expansion model. To understand the essential principle of ferromagnetic transformation in binary alloys from a microscopic point of view. The main contents and results of this paper are as follows: (1) using the VASP program of plane wave pseudopotential method based on the density function theory, the parameters of the magnetic cluster model are obtained, and the simulation results are compared with the experimental data. The accuracy of the parameters was verified. (2) the relationship between Cr content and Curie point in disordered state was studied by using magnetic cluster model: when the content of Cr was low (Curie temperature of 0 / 10%), FeCr), the Curie temperature increased first and reached the maximum value at about 6% of Cr content. (3) the relationship between the content of Cr and the enthalpy of the system in Fe-Cr alloy was studied by using magnetic cluster model: the enthalpy of mixing was a negative value when the composition of Cr was low, and the structure of Fe-Cr alloy was stable; The mixing enthalpy decreases first and then increases with the increase of Cr, and reaches a minimum value at Cr 6%, and the structure is most stable at this time. When the composition reaches about 12%, the symbol of enthalpy of mixing is positive, which leads to the trend of phase separation. (4) the relationship between Cr content and Curie point in ordered state is studied by using magnetic cluster model, and the order and disorder are compared. It was found that the Curie temperature of the ordered structure was higher than that of the disordered sample in the same component, especially in the 6% sample. Compared with the ordered structure, the probability of forming clusters of Cr atoms in out-of-order states is increased, and the antiferromagnetic interaction between Cr-Cr can partially counteract the magnetic moments. Thus the phase transition temperature is reduced. (5) the phase diagram based on magnetic cluster model is used to calculate the phase transitions of 偽-緯 and 緯-未, thus the phase diagram is obtained.
【學(xué)位授予單位】：南京郵電大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG132.27

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