ISSN:0005-9021    

DOI:10.1002/bbpc.19981020902

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe initial stage of a first order transformation reactions is controlled by a nucleation process. In fact, the kinetic constraints provided by nucleation and growth difficulties of the new phase provide the necessary condition to expose metastable reaction pathways. The resulting hierarchy of metastability is viewed in terms of the severity of the imposed kinetic constraints. The identification of nucleation control has been extended to cover solid state reactions such as those involved in solid state amorphization reactions (SSAR) of so‐called driven systems. Some of the fundamental characteristics of alloy metastability under kinetic constraints are illustrated by experimental observations and model analyses that highlight the current understandin

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020903

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractStarting around 1970, two methods have been developed that may give information about the lattice stabilityHα/βHα–Hβ, i.e. the enthalpy difference between two phases, α and β, of a pure element or compound. The ab initio method is purely theoretical and relies on electron structure calculations. The CALPHAD method derives a Gibbs energy functionGHTSfrom experimental thermodynamic data, including phase diagrams. A long‐standing problem has been that the two methods gave very different lattice stabilities for some transition metals, e.g., W. The reason for this discrepancy has recently been clarified, when the role of dynamical lattice instabilities was realised. This paper gives an account of how the discrepancy emerged and is now resolved. The interpretation and practical applicability of the Gibbs energy function in the CALPHAD approach to systems containing dynamical instabiliti

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020904

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe ternary system Pb‐Sn‐Zn displays a large miscibility gap in the liquid phase which extends from the binary system Pb‐Zn. The calculated isothermal sections at 714, 757, 805, 842 and 877 K, in which two liquid phases are shown to exist in equilibrium with each other, will be presented and compared with experimental data. The calculated activities of Zn in the liquid phase for different isopleth sections are in good agreement with experimental data.The appearance of GP Zones in Al‐Cu‐Mg alloys may be described by a Gibbs energy function which predicts a miscibility gap in the fcc (Al) phase. The new function was first obtained for the binary Al‐Cu system and then extended into the ternary to describe the precipitation of GPZB zones. The Thermo‐Calc calculation package was used for

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020905

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractAnalysis of the whole complex of the electrophysical and thermodynamic properties of semiconductors allowed us to conclude that melting entropy can be presented by three basic components, i.e. by the position, vibration and electron contributions. Possible methods of calculating the above contributions have been discussed, e.g. it has been shown that the electron contribution to the melting entropy of semiconductors that are metallized makes up a considerable part of the total melting entropy, which accounts for the anomalous values of the melting entropy of the substances discussed.

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020906

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe BWG type of approximation describing chemical ordering has been available in thermodynamic calculation‐optimisation software. Developments were done for special cases of symmetry, e.g. the γ‐γ′ ordering, based on the fcc lattice, in Ni‐based superalloys or the β‐β′ ordering in the Cu‐Zn system, based on the bcc lattice.The technological importance of materials presenting order‐disorder transformations, demands a versatile Gibbs energy modelling, able to describe the changes taking place during ordering as functions of composition and temperature. Recently practical use of Gibbs energy descriptions of ordered phases are becoming a daily procedure.In this communication we present a general method for deriving the constraints in the compound energy formalism (CEF) for the model parameters describing an order‐disorder transformation. The cases discussed include, for example, the oxygen ordering in its interstitial s

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020907

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThis paper describes an attempt to introduce a treatment of short range order within the compound energy formalism. The attempt is limited to binary alloys and phases with fcc or hcp structures.

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020908

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe estimation of thermodynamic functions of crystallization of amorphous alloys was carried out using a two‐stage procedure. The main approximation for this consists in the consideration of amorphous alloys as a supercooled metastable liquid solution. The first stage describes the thermodynamic properties of the melts Co‐Si‐B and Fe‐Si‐B by means of the ideal association model. A method of mathematical optimization was used to get the most reliable model parameters. All experimental thermodynamic data (integral and partial enthalpies, entropies and chemical potentials received by different experimental methods) available in the literature for both the ternary system and all composed binary systems were taken into account for this purpose. The second stage of estimation consists in the extrapolation of the calculated temperature and concentration dependences of the thermodynamic functions of the melts to low temperatures, using known models for the heat capacities of undercooled melts, and their comparison with appropriate thermodynamic functions of crystalline phases. The estimated values for the enthalpy of crystallization of the Fe‐B, Co‐Si‐B and Fe‐Si‐B amorphous alloys are in good agreement wit

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020909

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe velocity of sound in the molten As‐Te has been measured as a function of temperature, which shows up to 60 at.% As a pronounced minimum like in pure liquid Te. In an As‐ rich alloy, only a high temperature branch of the wholevs‐Tcurve that the sound velocity decreases rapidly with lowering temperature was obtained, because solidification took place before a minimum could be observed. The results suggest that rapid structural changes occur with varying temperature over the whole composition and confirm the recent experiments indicating that liquid As undergoes rapid structural changes with raising temperature like in supercooled Te. An extrapolation predicts a minimum in the sound velocity in As to occur at 736±6°C. The Peierls distortion mechanism and stability of structure of the liquid sp‐bonded elements are

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020910

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractWe present a numerical study of simple models of liquid ionic alloys and metal‐molten salt solutions. The microscopic structure is modeled using the primitive model of charged hard spheres, the electronic structure is described by a right‐binding Hamiltonian including the crucial component of Madelung potential fluctuations. We discuss the nature of the nonmetal‐metal crossover as a function of composition in these systems and present a computation of thermodynamic properties within the screened mean spherical approxim

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020911

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY