First-principles investigation of the Cu–Ni,Cu–Pd,and Ni–Pd binary alloy systems

Phase diagrams of copper–nickel–palladium binary alloys were determined by density functional theory cluster expansion method. The system has both magnetic and non-magnetic binaries and subtle phase coexistence areas between similar and different kind of lattice types. Furthermore, the CuPd binary has several ordered structures. Cluster expansion models were constructed by heuristic cluster selection for all of the fcc structures and for the CuPd_bcc structure. Both configurational and magnetic phase diagrams were determined. Small amount of nickel magnetize fcc palladium to 0.26 μ_B from which the magnetic moment rises almost linearly to that of pure Ni. In CuNi, 0.46 x-Ni is needed for the magnetic transition. In CuPd alloy in 0 K, configurational free energy difference between bcc and fcc lattice resulting to phase separation is only about 1.1 kJ/mol-atoms. Low temperature energetics and magnetic phase diagrams have good quantitative agreement with available experimental and theoretical results. Finite temperature properties of the alloys are in good qualitative agreement with experimental results.     

Metastable bcc Ni produced by rod milling and chemical leaching

Nanocrystalline Al₆₀Ni₄₀ and Ni have been obtained by rod milling Al and Ni powder mixtures and chemical leaching Al atoms from the rod-milled Al₆₀Ni₄₀, respectively. The rod-milled alloy powders retained their bcc structure after being treated at room temperature and at 85 °C with a 25–30 wt.% KOH solution. The leached powders are very active and easily explode when they come into contact with air. The leached powders were transformed to a ferromagnetic fcc phase at high temperature. On cooling of the specimen from 600 °C, spontaneous magnetization M sharply increased at about 350 °C, indicating that the bcc phase was transformed to an fcc phase. It has been confirmed that the leaching temperature and annealing temperature and KOH concentration have a considerable effect on structural and magnetic properties.     

Application of classical nucleation theory to phase selection and composition of nucleated nanocrystals during crystallization of Co-rich (Co,Fe)-based amorphous precursors

Classical steady-state nucleation theory is applied to Co-rich Fe,Co-based alloys to provide a rationale for experimental observations during the nanocrystallization of Co-rich (Co,Fe)₈₉Zr₇B₄ and (Co,Fe)₈₈Zr₇B₄Cu₁ amorphous precursors. The amorphous precursor free energy is estimated using density functional theory. This simple theory suggests: (i) strain or interface energy effects could explain a tendency for a body-centered cubic (bcc) phase to form during crystallization. Dissolved glass formers (Zr,B) in crystalline phases may also contribute; (ii) similar face-centered cubic (fcc) and hexagonal close-packed (hcp) free energies could explain the presence of some hcp phase after crystallization even though fcc is stable at the crystallization temperature; (iii) nanocrystal compositions vary monotonically with the Co:Fe ratio of the amorphous precursor even when multiple phases are nucleating because nucleation is not dictated by the common tangency condition governing bulk phase equilibria; and (iv) Fe-enrichment of the bcc phase can be attributed to a relatively small free energy difference between the amorphous and bcc phases for high Co-containing alloys.     

On the mechanism of nucleation of copper precipitates upon aging of Fe-Cu alloys

Structure of the Fe-5.5% Cu alloy has been studied after aging at 500°C to show that the lattice of copper precipitates changes with increasing time of holding at a temperature from bcc to 9R to fcc. The free energy of the solid solutions and the phase-separation diagrams of the bcc and fcc phases of copper have been calculated. Barriers for the nucleation of copper particles with the bcc and fcc lattices have been evaluated. Factors that favor the appearance of intermediate structural forms of copper, which precede the formation of the stable fcc phase, are discussed.     

金属Fe/Pd成分调制膜的结构和磁性研究

用真空双源蒸镀法在玻璃衬底上制得了金属Fe/Pd成分调制膜。用X射线衍射分析了它们的调制特性和晶格结构;用振动样品磁强计进行磁性测量。结果表明:金属Fe/Pd成分调制膜中Pd层为fcc结构;随着Fe层厚度减小,Fe层结构由bcc转变为非晶态。研究了比饱和磁化强度随Fe层厚度的变化规律并探讨了它的机理。     

STRUCTURE AND MAGNETIC PROPERTIES OF Fe/Pd COMPOSITIONALLY MODULATED FILMS

Metal Fe/Pd compositionally modulated films(CMFs) were prepared by vapour depo-sition from two sources onto glass substrate under vacuum.The modulation and crystalstructures of the films were examined by X-ray diffraction.The magnetic properties weredetermined by vibrating sample magnetometer.The Pd layers in the Fe/Pd CMFs areof fcc structure,and the Fe layer structure transits from bcc into amorphous,state withdecreasing thickness of Fe layer.The dependence of specific saturation magnetizationon thickness of Fe layers has also been discussed.     

放电等离子烧结双相AlCrCoFeNi2.1 高熵合金的微观组织、耐腐蚀性能和力学性能

采用放电等离子烧结法在不同温度下制备AlCrCoFeNi_2.1高熵合金（HEA）,并对其微观组织、耐腐蚀性能和力学性能进行了研究。结果表明,烧结后的AlCrCoFeNi_2.1 HEA最大相对密度可达99.18%;该HEA主要由体心立方（bcc）相和面心立方（fcc）相组成,其比例分别为20.6%和79.4%。与fcc相相比,AlCrCoFeNi_2.1 HEA中bcc相的再结晶组织和变形组织更多,且bcc相在3.5%（质量分数）NaCl溶液中更容易被腐蚀。随着应变速率的增加,bcc相和fcc相的压力恢复速率降低,硬化效果增强。在1050 ℃下烧结的AlCrCoFeNi_2.1 HEA具有较高的极限抗拉伸强度,这主要归因于晶界强化、固溶强化和合金粒子之间良好的界面结合。该HEA的断裂形式包括bcc相的脆性断裂和fcc相的韧性断裂。     

A modified embedded-atom method interatomic potential for the Fe–H system

A modified embedded-atom method (MEAM) interatomic potential for the Fe–H binary system has been developed using previously developed MEAM potentials of Fe and H. The potential parameters were determined by fitting to experimental data on the dilute heat of solution of hydrogen in body-centered cubic (bcc) and face-centered cubic (fcc) Fe, the vacancy–hydrogen binding energy in bcc Fe, and to a first-principles calculation for the lattice parameter and bulk modulus of a hypothetical NaCl-type FeH. The potential accurately reproduces the known physical properties of hydrogen as an interstitial solute element in bcc and fcc Fe. The applicability of the potential to atomistic approaches for investigating interactions between hydrogen atoms and other defects such as vacancies, dislocations and grain boundaries, and also for investigating the effects of hydrogen on various deformation and mechanical behaviors of iron is demonstrated.     

Composition and non-equilibrium crystallization in partially devitrified co-rich soft magnetic nanocomposite alloys

The body-centered cubic (bcc) phase tends to preferentially nucleate during solidification of highly undercooled liquid droplets of binary alloy systems, including Fe–Co, Fe–Ni and Fe–Cr–Ni. We investigate a similar tendency during the partial devitrification of Co-rich amorphous precursors of composition (Co_1?xFe_x)₈₈Zr₇B₄Cu₁ by identifying the structure and composition of the nanocrystalline grains. The Co:Fe ratio of the bcc nanocrystals varies linearly with the Co:Fe ratio of the amorphous precursor, and can lie well within the single-phase face-centered cubic (fcc) region of the Fe–Co phase diagram at the crystallization temperature. Classical nucleation theory therefore suggests several potential explanations for the preferential nucleation of bcc phase from an amorphous precursor, including: (i) a reduced amorphous/bcc interface energy as compared to the close-packed phases; (ii) a lower strain of precipitation for bcc nuclei as compared to close-packed fcc and hexagonal close-packed nuclei; and (iii) stabilization of the bcc phase by dissolved glass-formers such as Zr and B.     

Effects of crystallographic structure and Cr on the rate of void nucleation in BCC Fe: An atomistic simulation study

Atomistic Monte Carlo simulations based on modified embedded-atom method (MEAM) interatomic potentials have been carried out to clarify the differences in swelling rates between bcc and fcc Fe and between pure bcc Fe and bcc Fe−Cr alloys. Assuming that the transient regimes prior to the onset of steady-state swelling correspond to the void nucleation stage, the effect of crystallographic structure (bcc vs. fcc) or Cr alloying on the void nucleation rate under a given amount of supersaturated vacancies was examined. It was found that the void nucleation rate is much higher in fcc Fe than in bcc Fe. Randomly distributed Cr atoms slightly increase the void nucleation rate in bcc Fe, but microstructural evolutions such as the precipitation of Cr-rich phase have more decisive effects, serving as a vacancy sink. The reasons for the individual results are rationalized in terms of the binding energy of vacancy clusters and the size difference between Fe and Cr atoms.     

Co基纳米晶软磁合金研究进展

本文对钴基非晶合金的晶化及其磁性研究的进展进行了综述,内容包括：（1）零磁致伸缩Co基非晶合金的晶化行为;（2）Vitrovac 6025型合金的纳米晶化和磁性;（3）bcc结构的Co-Fe-Nb-Si-B纳米晶合金的研究。对有关晶化产物及磁性的问题进行了讨论。     

Understanding Solid-Solid (fcc→ω+bcc) Transition at Atomic Scale

An atomic transition model of a face-centered cubic (fcc) crystal to a primitive hexagonal ω and body-centered cubic (bcc) structures has been crystallographically built. The fcc structure can transform into the ω structure through a local shuffling or displacement of atoms about 0.4014 Å in iron for a _{fcc iron} = 3.59 Å. The bcc structure can form either after the ω formation or concurrently by the similar mechanism, or the ω structure can be treated as an intermediate stage during the transition of fcc → bcc. Such a transition (fcc → ω + bcc transition) can be confirmed by Widmanstätten pattern formed in an iron meteorite, pearlitic structure and martensite composed of bcc-ferrite and ultra-fine ω particles in iron-carbon steels. The present fcc-bcc orientation relationship matches with Pitsch’s one.     

Energy of mixing and magnetic state of components of Fe-Mn alloys: A first-principles calculation for the ground state

Methods of first-principles computer simulation have been used to calculate the magnetic moments at the component atoms and the energies of mixing in substitutional fcc and bcc solid solutions of manganese in iron. It has been established that the energies of mixing in α and γ solid solutions have not only different magnitudes but even different signs. It is shown that in α solid solutions there is a thermodynamic anomaly at a manganese concentration of approximately 1.5 at. %, namely, a change in the concentration dependence of the energy of mixing caused by a reorientation of magnetic moments at manganese atoms. With increasing Mn content in dilute solutions, there occurs a continuous variation of the orientation of magnetic moments of manganese atoms from strictly antiparallel with respect to the ferromagnetic iron matrix to predominantly parallel one.     

Solid-state reactions upon mechanical alloying of an Fe32Al68 binary mixture

The sequence of solid-state reactions that occur upon mechanical alloying of powder mixtures of Al and Fe taken in an atomic ratio of 68: 32 has been studied by the methods of X-ray diffraction analysis, M?ssbauer spectrometry, and Auger spectrometry. Upon the formation of a nanocrystalline state (<10 nm), there takes place a mutual penetration of Al atoms into Fe and Fe atoms into Al particles. The rate of consumption of the fcc Al is substantially higher than that of the bcc Fe. The process of the mechanical alloying (MA) was found to be two-stage. At the first stage, up to 2 at % Fe is dissolved in the fcc Al, and an amorphous Fe₂₅Al₇₅ phase is formed in the interfaces, whose amount reaches 70 at % at the finish of the initial stage. In the interfaces of the ??-Fe phase, a disordered bcc phase of composition Fe₆₆Al₃₄ is formed, which contains up to 12 at % Al segregates. At the second stage, the amorphous phase crystallizes into an orthorhombic intermetallic compound Fe₂Al₅. The residual ??-Fe, bcc Fe₆₆Al₃₄, and segregated Al form a bcc phase of composition Fe₃₅Al₆₅.     

Microstructure and Properties of Al0.4FeCrNiCo1.5Ti0.3 High Entropy Alloy Prepared by MA-HP Technique

采用机械合金化-真空热压烧结(MA-HP)法制备了Al0.4FeCrNi Co1.5Ti0.3高熵合金。利用XRD、SEM和力学压缩试验机分析Al0.4FeCrNiCo1.5Ti0.3合金的微观组织、相转变以及力学性能。结果表明:经高能球磨10 h,合金中形成了简单固溶体fcc和bcc相,而经过热压烧结的Al0.4Fe Cr Ni Co1.5Ti0.3合金以单一fcc相及2种bcc相(bcc1、bcc2)组成。热压烧结Al0.4Fe Cr Ni Co1.5Ti0.3合金致密度达99.48%,其微观硬度(HV),屈服强度、断裂强度、压缩率分别达到725 MPa,2.13 GPa,2.54 GPa,20.1%,合金优异的力学性能主要是因为合金的固溶强化;断裂模式为解理断裂及塑性断裂的混合机制。     

Mechanisms of martensitic phase transformations in body-centered cubic structural metals and alloys: Molecular dynamics simulations

Two different mechanisms of the stress-induced martensitic phase transformation at the crack tip in body-centered cubic (bcc) structural metals and alloys have been studied by molecular dynamics simulations. For cracks with 〈1 0 0〉 crack fronts, the bcc (B2) to face-centered cubic (fcc) (L1₀) phase transformation along the Bain stretch occurs. Whereas for cracks with 〈1 1 0〉 crack fronts, either the bcc (B2) to fcc (L1₀) or the bcc (B2) to hexagonal close-packed (hcp) transformation is the candidate. We have found that the combination of local stress and crystal orientation plays an important role in the mechanism of the martensitic transformation. Thus a simple way to determine the mechanism of the martensitic transformation is developed. The complicated deformation behaviors at the crack tip in bcc iron and B2 NiAl are discussed in terms of this method.     

Theoretical study of phase stability in Ni-Al and Ni-Ti alloys

The thermodynamic stability of the bcc and fcc based ordered phases in Ni-Ti and Ni-AI has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in Ni-AlB2 ordered intermetallic alloys is discussed with relation to the existence of intermediate structures between bcc and fcc based phases. It is shown that closely related ordered structures can exist on fcc and bcc lattices in the composition range where the transformation occurs. The Ni-rich side of the Ni-Al phase diagram has been computed, and a comparison with a recent assessment is made. In addition, the rather unusual appearance of the NiTi B2 ordered structure in the phase diagram is discussed. This paper was presented at the International Phase Diagram Prediction Symposium sponsored by the ASM/MSD Thermodynamics and Phase Equilibria Committee at Materials Week, October 21–23,1991, in Cincinnati, Ohio. This symposium was organized by John Morral, University of Connecticut, and Philip Nash, Illinois Institute of Technology.     

Direct evidence of magnetically induced phase separation in the fcc phase and thermodynamic calculations of phase equilibria of the Co–Cr system

Two-phase equilibria between the ferromagnetic fcc and the paramagnetic fcc phase from 800 to 900 °C in the Co-rich region was determined using the diffusion couple technique. It was confirmed that a magnetically-induced miscibility gap of the fcc phase is formed along the Curie temperature.Thermodynamic calculation of the phase equilibria of the Co–Cr system was performed by optimizing the present results and the thermodynamic data in the literature. A set of thermodynamic values for describing the Gibbs energy of liquid, fcc hcp, bcc and sigma phases yielded good agreement between the calculated phase diagram and the experimental data. Moreover, the magnetically-induced miscibility gap between the ferromagnetic and paramagnetic hcp phases was also predicted. This kind of thermodynamic calculation of Co–Cr base alloys is quite useful for the alloy design of the magnetic recording media.