Hualei Zhang

Theoretical elastic moduli of ferromagnetic bcc Fe alloys

The polycrystalline elastic parameters of ferromagnetic Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 ≤ x ≤ 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poissons ratio (ν) of bcc Fe. The complex composition dependence of the C(44) single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Youngs modulus (E), and Debye temperature (Θ). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects.

Alloying effects on the elastic parameters of ferromagnetic and paramagnetic Fe from first-principles theory

The elastic properties of paramagnetic face-centered-cubic (fcc) Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys are investigated using the exact muffin-tin o ...

Ab initio determination of the elastic properties of ferromagnetic body-centered cubic Fe-Mn-Al alloys

The elastic properties of ferromagnetic Fe1−x−yMnyAlx (0 ≤ x ≤ 0.5, y = 0, 0.1, and 0.2) random solid solutions in the body-centered cubic (bcc) crystallographic phase have been investigated using the ab initio exact muffin-tin orbitals method in combination with the coherent-potential approximation. Comparison with the experimental data demonstrates that the employed theoretical approach accurately captures the observed composition dependence of the lattice parameter. The predicted elastic parameters follow complex composition dependence. The C11, C12, and C′ = (C11 − C12)/2 single-crystal elastic constants, the bulk (B), shear (G), and Youngs (E) moduli, and the Cauchy pressure (C12 − C44) mainly decrease with increasing Al content, whereas the Zener anisotropy ratio (C44/C′) strongly increases with x. C44 exhibits a non-linear x dependence. The Poisson (v) and Pugh (B/G) ratios first decrease with x but show non-monotonous behavior in high-Al alloys. In terms of the Pugh criterion, these trends sugges...

Elastic anomalies in Fe-Cr alloys

Using ab initio alloy theory, we determine the elastic parameters of ferromagnetic and paramagnetic Fe(1-c)Cr(c) (0 ≤ c ≤ 1) alloys in the body centered cubic crystallographic phase. Comparison with the experimental data demonstrates that the employed theoretical approach accurately describes the observed composition dependence of the polycrystalline elastic moduli. The predicted single-crystal elastic constants follow complex anomalous trends, which are shown to originate from the interplay between magnetic and chemical effects. The nonmonotonic composition dependence of the elastic parameters has marked implications on the micro-mechanical properties of ferrite stainless steels.

Tensile strain-induced softening of iron at high temperature

In weakly ferromagnetic materials, already small changes in the atomic configuration triggered by temperature or chemistry can alter the magnetic interactions responsible for the non-random atomic-spin orientation. Different magnetic states, in turn, can give rise to substantially different macroscopic properties. A classical example is iron, which exhibits a great variety of properties as one gradually removes the magnetic long-range order by raising the temperature towards its Curie point of  = 1043 K. Using first-principles theory, here we demonstrate that uniaxial tensile strain can also destabilise the magnetic order in iron and eventually lead to a ferromagnetic to paramagnetic transition at temperatures far below . In consequence, the intrinsic strength of the ideal single-crystal body-centred cubic iron dramatically weakens above a critical temperature of ~500 K. The discovered strain-induced magneto-mechanical softening provides a plausible atomic-level mechanism behind the observed drop of the measured strength of Fe whiskers around 300–500 K. Alloying additions which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated here in the case of Fe-Co alloy.

Adhesion of the iron-chromium oxide interface from first-principles theory

We determine the interface energy and the work of separation of the Fe/Cr2O3 interface using first-principles density functional theory. Starting from different structures, we put forward a realistic interface model that is suitable to study the complex metal-oxide interaction. This model has the lowest formation energy and corresponds to an interface between Fe and oxygen terminated Cr2O3. The work of separation is calculated to be smaller than the intrinsic adhesion energy of pure Fe or Cr2O3, suggesting that stainless steel surfaces should preferentially break along the metal-oxide interface. The relative stabilities and magnetic interactions of the different interfaces are discussed. Next we introduce Cr atoms into the Fe matrix at different positions relative to the interface. We find that metallic Cr segregates very strongly to the (FeCr)/Cr2O3 interface, and increases the separation energy of the interface, making the adhesion of the oxide scale mechanically more stable. The Cr segregation is explained by the enthalpy of formation.

Magnetic effect on the interfacial energy of the Ni(1 1 1)/Cr(1 1 0) interface

The work of separation and interfacial energy of the Ni(1 1 1)/Cr(1 1 0) interface are calculated via first-principles methods. Both coherent and semicoherent interfaces are considered. We find that magnetism has a significant effect on the interfacial energy, i.e. removing magnetism decreases the interfacial energy of the semicoherent interface by around 50% . Electronic, magnetic and atomic structures at the interface are discussed. An averaging scheme is used to estimate the work of separation and interfacial energy of semicoherent interfaces based on the results of coherent interfaces. The limitations of the scheme are discussed.

Alloy Steel: Properties and Use First-Principles Quantum Mechanical Approach to Stainless Steel Alloys

Accurate description of materials requires the most advanced atomic-scale techniques from both experimental and theoretical areas. In spite of the vast number of available techniques, however, the experimental study of the atomic-scale properties and phenomena even in simple solids is rather difficult. In steels the challenges become more complex due to the interplay between the structural, chemical and magnetic effects. On the other hand, advanced computational methods based on density functional theory ensure a proper platform for studying the fundamental properties of steel materials from first-principles. In 1980’s the first-principles description of the thermodynamic properties of elemental iron was still on the borderline of atomistic simulations. Today the numerous applicationoriented activities at the industrial and academic sectors are paired by a rapidly increasing scientific interest. This is reflected by the number of publications on ab initio steel research, which has increased from null to about one thousand within the last two decades. Our research group has a well established position in developing and applying computational codes for steel related applications. Using our ab initio tools, we have presented an insight to the electronic and magnetic structure, and micromechanical properties of austenite and ferrite stainless steel alloys. In the present contribution, we review the most important developments within the ab initio quantum mechanics aided steel design with special emphasis on the role of magnetism on the fundamental properties of alloy steels.

Thermo-Physical Properties of Iron-Magnesium Alloys

According to the common phase diagrams, iron and magnesium are almost immiscible at ambient pressure Massalski (1986). In the liquid phase, the solubility of Mg in Fe is of the order of 0.025 atomic percent (at.%). The maximum solid solubility of Fe in Mg is 0.00041 at.% and the Fe content in Mg at the eutectic point is less than 0.008 at.% (Haitani et al., 2003). Below 1273 K the solubility of Mg in α-Fe is below the detection limit and about 0.25 at.% Mg can be solved in δ−Fe at the monotectic temperature. The immiscibility of Fe and Mg at ambient conditions is in line with the well-known Hume-Rothery rules, according to which more than 15% atomic size difference between alloy constituents hinders solid solution formation (Massalski, 1996). In spite of the negligible solubility of Mg in Fe, several Fe-rich metastable Fe-Mg solid solution have been synthesized. According to the pioneering work by Hightower et al. (Hightower et al., 1997), mechanical alloying produced Fe-Mg substitutional solid solutions with up to 20 at.% Mg and having the body centered cubic (bcc) crystallographic phase of α-Fe. Later, using the similar alloying procedure, Dorofeev et al. (Dorofeev et al., 2004; Yelsukov et al., 2005) found that about 5 − 7 at.% Mg in α-Fe forms supersaturated solid solution. These authors suggested that the driving force for the formation of Fe-Mg solid solutions is associated with the excess energy of coherent interfaces in the Fe-Mg nanocomposite, which facilitates incorporation of Mg into α-Fe. Indeed, based on semiempirical thermodynamic calculations, Yelsukov et al. (Yelsukov et al., 2005) obtained 6 kJ/mol for the enthalpy of formation for Fe-Mg solid solutions, compared to 20 kJ/mol calculated for the corresponding Fe-Mg nanocomposites. In addition to the mechanical alloying techniques, pressure was also found to facilitate the solid solution formation between Fe and Mg. Dubrovinskaia et al. (Dubrovinskaia et al., 2004) reported that at pressures around 20 GPa and temperatures up to 2273 K, the solubility of Mg in bcc Fe was increased to 4 at.%. They found that the lattice parameter of the bcc Fe-Mg alloy increased approximately by 0.6 % per at.%Mg. Furthermore, recent experimental measurements in combination with theoretical simulations demonstrated that at the megabar pressure range more than 10 at.% Mg could be dissolved in liquid Fe, which then could be quenched to ambient conditions (Dubrovinskaia et al., 2005). The mechanism behind the 4