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Ab initio-based mean-field theory of the site occupation in the Fe-Cr σ-phase

机译:基于从头算的Fe-Crσ相占位平均场理论

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The Fe-Cr σ phase was first observed by Bain in 1923 as a hard, nonmagnetic intermediate phase, which has formed after long-term high-temperature annealing for nearly equiatomic composition. In the early 1960s, the σ phase formation has been found in some Ni-based alloys and soon after in many other high-temperature alloys. Nevertheless, the Fe-Cr σ phase is the most investigated due to relevance to different types of steels, where its formation should be avoided because of extreme brittleness. The σ phase has a low-symmetry structure with a disordered distribution of atoms in all crystallographic sites. In particular, it has a tetragonal unit cell (space group P4_2/mnm) with 30 atoms divided into five nonequivalent groups of sites or sublattices (A, B, C, D, E).1 The σ phase structure can be written in a general form as A_2~12B_4~15C_8~14D_8~12E_8~14, where the bottom and top indices denote the site multiplicities and coordination numbers for each sublattice, respectively (see Fig. 1). In Fe_(1-x)Cr_x alloys, the a phase is formed in a quite narrow concentration (0.462 < x < 0.505) and temperature range (700 K < T < 1100 K).~(1-3) It is paramagnetic at ambient temperatures. However, already at the end of 1960s it was found that the Fe-Crσ phase has a small magnetic moment at temperatures below the Curie temperature of about 50 K.~(4,5) These data were reproduced 35 years later and extended in a broad experimental study of the Fe-Cr system.~(6,7)
机译:贝恩于1923年首次观察到Fe-Crσ相是一种坚硬的非磁性中间相,它是在经过长时间高温退火后形成的,几乎是等原子的。在1960年代初期,在某些镍基合金中发现了σ相形成,随后不久在许多其他高温合金中也发现了σ相形成。尽管如此,由于与不同类型的钢有关联,对Fe-Crσ相的研究最多,由于极易碎,应避免形成。 σ相具有低对称性结构,在所有晶体学位点中的原子分布均无序。特别是,它具有一个四边形晶胞(空间群P4_2 / mnm),其中有30个原子,分为五个非等价的位点或亚晶格组(A,B,C,D,E)。1σ相结构可以写成通用格式为A_2〜12B_4〜15C_8〜14D_8〜12E_8〜14,其中底部和顶部索引分别表示每个子晶格的位点多重性和配位数(见图1)。在Fe_(1-x)Cr_x合金中,a相以非常窄的浓度(0.462

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