A Mixed-control Mechanism Model of Proeutectoid Ferrite Growth under Non-equilibrium Interface Condition in Fe-C Alloys

By combining the α/γ interface migration and the carbon diffusion at the interface in Fe-C alloys, a mathematical model is constructed to describe the mixed-control mechanism for proeutectoid ferrite formation from austenite. In this model, the α/γ interface is treated as non-equilibrium interface, i.e., the carbon concentration of austenite at γ/α interface is obtained through theoretical calculation, instead of that assumed as the local equilibrium concentration.For isothermal precipitation of ferrite in Fe-C alloys, the calculated results show that the rate of interface migration decreases monotonically during the whole process, while the rate of carbon diffusion from γ/α interface into austenite increases to a peak value and then decreases. The process of ferrite growth may be considered as composed of three stages: the period of rapid growth, slow growth and finishing stage. The results also show that the carbon concentration of austenite at γ/α interface could not reach the thermodynamic equilibrium value even at the last stage of ferrite growth.     

Phase diagrams calculated for Fe-rich Fe-Si-Co and Fe-Si-Al ordering alloy systems

Theoretical analysis based on the calculation of phase diagrams was employed for Fe-Si-Co and Fe-Si-Al ordering systems to clarify the necessity for the occurrence of phase separation in Fe-base ternary ordering systems. The free energy of Fe-base ternary ordering alloys where B2 and D0₃ ordered structures are formed is evaluated statistically using a pairwise interaction approximation up to second nearest neighbours, taking into account not only the atomic interaction but also the magnetic interaction, based on the Bragg-Williams-Gorsky model. The calculated phase diagrams are consistent with the experimentally obtained ones. The phase diagram calculation in this work is useful to predict the equilibrium states of the ternary ordering systems. The phase separation in ordering alloys is caused by the contribution of excess free energies due to ordering. The influences of ferromagnetism on the two-phase regions are also demonstrated.     

Electronic structure of complex Hume-Rothery phases and quasicrystals in transition metal aluminides

Aluminum based alloys which contain transition metal elements with concentrations up to 25% and additional small quantities of silicon, have a wide variety of atomic structures with unusual physical properties. Among these materials are the famous quasiperiodic alloys which were discovered in 1984 in Al-Mn by Shechtman et al. One find also numerous complex phases which atomic structures bear resemblance with medium-range structure of quasicrystalline phases. Some of these complex phases, but not all of them, are crystalline approximants of quasicrystals. This review focuses on the effect of the transition metal elements on the electronic structure of these alloys. The case of Al(Si)-Mn alloys is considered as a benchmarking one for the present study. But many of the results and concepts developed here can still be applied to other Al-rich alloys containing TM atom from the 3d, 4d, and 5d series.On the one hand, it is now well accepted that the Hume-Rothery stabilization of the valence electrons plays a crucial role in these materials. On the other hand, it has been shown that the TM atoms are also very important for their stability and their physical properties. But, until recently, there has been no model taking into account these two aspects together. We present a model that unifies the classical Hume-Rothery stabilization for sp electron metals with the virtual bound state model for TM atoms embedded in a metallic matrix. This new formalism for “spd electron phases” is applied successfully to Al(Si)-TM alloys and it gives a coherent conceptual picture of their stability and physical properties. It is compared to accurate first-principles calculations of the electronic structure for these alloys, and to the most important experimental and theoretical results in the literature.We also investigate the transition metal aluminide alloys with a non-metallic character. This can be due either to the creation of a gap or to the localization of electrons by some atomic clusters. Again in that case, the scattering of sp states by d orbitals of transition metal plays a central role.     

The migration behavior of atomic clusters in early nanocrystalline process of soft magnetic Finemet alloy

The Finemet alloys are commonly used as cores in transformers and generators, stress and field sensors in technological application for their excellent soft magnetic characteristics. To clarify the nanocrystallization mechanism of Finemet especially about the atomic migration in early stage is very essential for developing their distinctive characteristics. In this study, we investigate the migration behavior of atoms in order to clarify the mechanism of the early-stage nanocrystallization in amorphous Finemet alloys. The Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous ribbons were prepared by single-roller melt-spinning process in argon atmosphere, and then annealed at 350 degrees C-400 degrees C for 10 minutes in vacuum. The atom force microscope (AFM) and the coincidence Doppler broadening spectra (CDB) were used to characterize the migration behavior of different atoms in Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous alloy during the early-stage nanocrystallization. The X-ray diffraction (XRD) patterns show that all annealed samples are in the amorphous state. But the AFM observation shows clearly that there are many small atomic clusters (nuclei) which distribute in the amorphous matrix of the annealed samples. With increasing annealing temperature, there is a significant increase in the amount of atomic clusters and a dramatic drop in the average size of clusters with very limited Cu contention in the samples, which reflect the structural evolution into more homogeneity. The CDB spectrum indicates that the peaks of positron annihilation spectrum are gradually reduced, which means the number of grain boundary and the defects in samples are gradually increased. It can be concluded that more defects are introduced by the formation of atomic clusters through atomic migration during the early-stage nanocrystallization in Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous alloys.     

Copper coordination hypotheses and structural model in the glassy semiconductor Cu0.08Ge0.18Te0.74 by X-ray diffraction

The radial atomic distribution was studied in the amorphous alloy Cu_0.08Ge_0.18Te_0.74, using X-ray diffraction data of samples obtained by quenching the molten material. The short-range order proposed was determined through the interpretation of the radial distribution function, using a theoretical expression which takes into account the variation in the atomic scattering factors with s (scattering vector module) and approximating them to polynomic functions. Different coordination hypotheses for copper, quoted in the literature on glassy alloys containing this element, were taken into account. The result of the study is that tetracoordinated copper is the hypothesis which most agrees with the experimentally obtained structural information. According to this coordination for copper, a spatial atomic distribution model was generated, using the Monte-Carlo random method. A comparative analysis of the main structural parameters of this model revealed its good agreement with the values given in the literature for similar alloys.     

Basal-plane stacking-fault energies of Mg alloys: A first-principles study of metallic alloying effects

Generalized stacking-fault energies (GSFEs) of basal-plane stacking faults I₁ and I₂ in Mg alloys have been studied based on first-principles calculations, where 43 alloying elements were considered. It is found that the most contributing features of alloying elements to GSFEs are bulk modulus, equilibrium volume, binding energy, atomic radius and ionization energy. Both bulk modulus and ionization energy exhibit positive relationships with GSFEs, and the others show opposite relationships. Multiple regressions have been performed to offer a quantitative prediction for basal-plane GSFEs in Mg-X systems. GSFEs, alloying effects of elements and the prediction model established within this work may provide guidelines for new Mg alloys design with better ductility.     

Iron-cobalt-vanadium alloys: A critical study of the phase diagrams in relation to magnetic properties

A tentative explanation of some physical, magnetic, and mechanical properties in magnetically soft or semihard Fe-Co-V alloys is proposed in the light of present knowledge of their structural features. On account of important discrepancies, it is felt that metastable equilibrium diagrams, which are of greater significance for practical purposes, should be confirmed, and it would be of interest to redraw some parts of them with greater precision.     

Electrochemical migration of Sn-Pb and lead free solder alloys under distilled water

Electrochemical migration (ECM) is a potential reliability problem in electronic soldering which might become more dangerous in lead free electronic devices. In this paper, electrochemical migration tests on Sn-Pb and lead free solder alloys were conducted under distilled water by applying constant voltages with a power supply. The susceptibility of the solder alloys to ECM and the effect of the composition on ECM behavior were studied. It is found that both Sn-Pb and lead free solders investigated in present research have susceptibility on ECM. Dendrites grow from cathode to anode and show different morphologies with the different migration elements involved. In Sn-37Pb and Sn-36Pb-2Ag solders, the main migration element is Pb. While for Sn-Ag and Sn-Ag-Cu solder alloys, Sn leads the migration. For Sn-8Zn-3Bi, both Sn and Zn can migrate. Furthermore, the effect of applied voltage on the time to short and short resistance was also investigated. As could be expected, the higher the voltage is, the shorter the failure time is. The electrochemical migration mechanism of the solder alloys was also discussed. The author is now at Fraunhofer IZM, Berlin, Germany     

TiN-alloy coatings for temperature control of space vessels

The problem of combining the temperature control of a space vehicle with the mechanical and chemical stability of the surface is addressed. With the absorption of solar radiation and the emission of thermal radiation considered the static and dominant factors that determine the equilibrium temperature of a spherical object, a simple model is formulated. Realistic variations of the two material-dependent parameters, solar absorptance alpha and hemispherical emittance epsilon, permit a large change in the equilibrium temperature, from less than -50 to more than +150 degrees C. It is pointed out that for a generalized gray surface, i.e., one made from a material whose reflectance-emittance has the same value within the visible and the thermal wavelength regions, the equilibrium temperature is approximately 5 degrees C, independent of the numerical value of the reflectance. With the requirement for electrical conductivity also taken into account, TiN alloys are identified as candidate materials. Measurements and calculations of some of them indicate that this group may contain a material that fulfills all the requirements. The experimental reflectance spectrum is used in estimating the equilibrium temperature for different TiN alloys; one Ti-Al-N-alloy with a gradient content of Al is found to have the lowest equilibrium temperature, 66 degrees C.     

A statistical evaluation of the free energy of Fe-base ternary ordering alloys

The free energy of the Fe-base ternary ordering alloys whereB2 andD0₃ ordered structures are formed is evaluated. The statistical theory is employed using a pairwise interaction approximation taking into account not only the atomic interaction but also the magnetic interaction, based upon the Bragg-Williams-Gorsky model. The application of this model on Fe-Si-Co ordering alloys are demonstrated. The propriety of the calculation results are performed by comparing the experimental results. The influences of the magnetic energy to the stability of ordered structures are also demonstrated.     

B2 Fe-Al合金中与空位有关的弛豫

利用内耗方法对空冷B2 Fe-Al合金中原子缺陷的运动特征进行了研究.在210℃(称为P1峰)和410℃(称为P2峰)附近观察到两个与样品热空位浓度密切相关的弛豫型内耗峰.研究表明,P1峰产生于应力作用下双空位(V_(Fe)V_(Al))的重新取向,而P2峰起源于反位置原子与Fe空位之间的相互作用.对于富Fe的B2 Fe-Al合金,P2峰产生于应力作用下三倍体缺陷(2V_(Fe)Fe_(Al))的重新取向;而对于富Al的B2 Fe-Al合金,P2峰则产生于应力诱导下Al反位置原子在Fe空位之间的运动.     

Alloys created between immiscible elements

The development and understanding of alloys is one of the most important themes of physical metallurgy. Over the past four decades, the progress in modern processing techniques has enabled researchers to artificially create an increasing number of new alloys in systems that are immiscible in thermodynamic equilibrium. This possibility of alloying elements between which no alloys exist in nature offers exciting opportunities for many physics, chemistry, and materials science endeavors. One of the obvious questions that needs to be answered is exactly what kind of alloys have been, and can be, obtained in these systems with positive heat of mixing, in terms of the uniformity, the presence of short-to-medium range chemical and topological order/clustering, and the energy state of the new alloy phases. This issue was not adequately addressed before because, until recent years, simple diffraction measurements constituted the main method for the characterization of the alloys produced. In this article, we survey the alloys created in binary systems with positive heat of mixing. Our emphasis is on a systematic examination of the atomic-level structure, and calorimetric determination of the positive enthalpy of mixing, of several model binary alloys created between immiscible elements, covering both amorphous and crystalline solid solutions. Vapor-deposited alloys will be our primary focus, but alloys prepared via other processing routes or modeled in computer simulations will also be discussed. The experimental characterization results recently obtained using local environment probes will be reviewed, together with the insight gained through computer atomistic simulations. The local structures uncovered will be correlated directly with the thermodynamic properties. A full account of the thermodynamic and kinetic aspects of the phase selection and the details of the transformation mechanisms involved, on the other hand, is a much broader subject to be dealt with in a separate review.     

Simulation of processes of high-temperature gas corrosion of titanium alloys in vacuum

A physicomathematical model of the process of high-temperature interaction of titanium alloys of the system Ti-Al-Mn in a rarefied medium has been considered that takes into account gas saturation of the metal and sublimation of the alloying element. The example of the alloy OT4-1 illustrates the fact that the model representations and the experimental data are in satisfactory agreement. The model representations can be apparently extended also to other types of alloys. The model can be used for expert estimation of the effect of a vacuum in the heat treatment or the utilization of titanium alloys.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 6, pp. 29–34, November–December, 1990.     

A model for two-step ageing

In commercial practice, two-step ageing is commonly used in Al-Zn-Mg alloys to produce a fine dispersion of η′ precipitates to accentuate the mechanical properties and resistance to stress corrosion cracking. While this is true in Al-Zn-Mg alloys, two-step ageing leads to inferior properties in Al-Mg-Si alloys. This controversial behaviour in different alloys can be explained by Pashley’s Kinetic model. Pashley’s model addresses the stability of clusters after two-step ageing. In the development of the model, the surface energy term between cluster and matrix is taken into account while the coherency strains between the cluster and matrix are not considered. In the present work, a model is developed which takes into account the coherency strains between cluster and matrix and defines a new stability criterion, inclusive of strain energy term. Experiments were done on AA 7010 aluminium alloy by carrying out a two-step ageing treatment and the results fit the new stability criterion. Thus it is found that the new model for two-step ageing is verified in the case of Al-Zn-Mg alloy.     

A Two-Patch Predator-Prey Metapopulation Model

A minimal model for predator-prey interaction in a composite environment is presented and analysed. We first consider free migrations between two patches for both interacting populations, and then the particular cases where only one-directional migration is allowed and where only one of the two populations can migrate. Our findings indicate that in all cases the ecosystem can never disappear entirely, under the model assumptions. The predator-free equilibrium and the coexistence of all populations are found to be the only feasible stable equilibria. When there are only one-directional migrations, the abandoned patch cannot be repopulated. Other equilibria then arise, with only prey in the second patch, coexistence in the second patch, or prey in both patches but predators only in the second one. For the case of sedentary prey, with predator migration, the prey cannot thrive alone in either of the two environments. However, predators can survive in a prey-free patch due to their ability to migrate into the other patch, provided prey is present there. If only the prey can migrate, the predators may be eliminated from one patch or from both. In the first case, the patch where there are no predators acts as a refuge for the survival of the prey.     

On the glass forming ability of liquid alloys

By using the concepts of the short-range order (SRO) and middle-range order (MRO) characterizing structures, an attempt has been made to describe the glass forming ability (GFA) of liquid alloys. This includes the effect of more than two kinds of SRO in the liquid caused by the addition of second and third elements to a metallic solvent. The minimum solute concentration is related to the atomic volume mismatch estimated from the cube of the atomic radius. The optimum solute concentration for good glass formability in several binary and some ternary alloys is discussed on the basis of the empirical guideline. A new approach to obtaining good GFA of liquid alloys is based on four main factors: (i) formation of new SRO and coexistence of two or more kinds of SRO, (ii) stabilization of dense random packing structure by restraining the atomic redistribution for initiating the nucleation and growth process, (iii) formation of a stable cluster (SC) or the MRO by the harmonious coupling of SROs, and (iv) difference between SRO characterizing the liquid structure and the near-neighbor environment in the corresponding equilibrium crystalline phases. The use of interaction parameters, widely used in the thermodynamics of multicomponent metallic solutions, is proposed for effectively selecting the third solute element (X₃) for enhancing the GFA of a metallic liquid (M) containing the second solute (X₂). Fe₇₀-B₂₀-(X₃)₁₀ alloys (X₃=Cr, W, Nb, Zr and Hf) are used for illustration. Two typical model structures denoted by the Bernal and chemical-order types are used in describing the new glass structure as a function of solute concentration.     

Local order hypotheses and atomic arrangement in the glassy alloy Cu0.20As0.35Te454545by X-ray diffraction

An analysis of the atomic radial distribution function of Cu_0.20As0.35Te_{0 45} amorphous alloy obtained from quenching of the molten mixture of the elements, has been performed. The short-range order was determined by interpreting the radial distribution function (RDF), using a theoretical expression which takes into account the variation in the atomic scattering factors with the scattering vector module, s, and approximates them to polynomial functions. The tetraand di-coordinated copper hypotheses, quoted in the literature for glassy alloys containing this element, have been considered. It was found that only the tetra-coordinated copper hypothesis is compatible with the structural information that was experimentally obtained. A spatial atomic distribution model was generated in accordance with this copper coordination, using a conveniently modified MonteCarlo random method. A comparative analysis of the structural parameters of this model revealed good agreement with the values given in the literature for similar materials.     

Interfacial dislocations dominated lateral growth of long-period stacking ordered phase in Mg alloys

Understanding the interface between strengthening precipitates and matrix in alloys, especially at the atomic level, is a critical issue for tailoring the precipitate strengthening to achieve desired mechanical properties. Using high-resolution scanning transmission electron microscopy, we here clarify the semicoherent interfaces between the matrix and long-period stacking ordered(LPSO) phases, including 18 R and 14 H, in Mg–Zn–Y alloys. The LPSO/Mg interface features the unique configuration of the Shockley partial dislocations, which produces a near zero macroscopic strain because the net Burgers vectors equal zero. The 18 R/Mg interface characterizes a dissociated structure that can be described as a narrow slab of 54 R. There are two dislocation arrays accompanied to the 18 R/54 R and 54 R/Mg interface, resulting a slight deviation(about 2.3°). The 14 R/Mg interface exhibits the dislocation pairs associated with solute atoms. We further evaluate the stability and morphology of the corresponding interfaces based on elastic interaction, via calculating the mutual strong interactions between dislocation arrays, as well as that between the dislocations and solute atoms. The synchronized migration of interfacial dislocations and solute atoms, like move-drag behavior, dominates the lateral growth of LPSO phases in Mg alloys.     

Sluggish,chemical bias and percolation phenomena in atomic transport by vacancy and interstitial diffusion in NiFe alloys

In this article, we report the results of extended atomistic modeling of intrinsic mobility of point defects and associated atomic transport in NiFe model binary alloys. We consider the effects of composition and temperature and present evidence of the sluggish and chemically biased diffusion, and percolation effects occurring in atomic transport via the vacancy and interstitial migration mechanisms. The results are analyzed and discussed in the light of previous studies and some experimental observations. It is demonstrated that the sluggish diffusion, the chemically biased diffusion, and the percolation are interlinked phenomena that are defined by the chemical complexity of particular alloys. Methods for predicting these phenomena in multicomponent alloys are discussed.We report a fundamental understanding of sluggish diffusion, chemically-biased diffusion, as well as percolation phenomena, in NiFe random alloys for vacancy and interstitial atom migration mechanisms.