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The theory of transformations in metals and alloys

TL;DR: In this paper, the authors present a general introduction to the theory of transformation kinetics of real metals, including the formation and evolution of martensitic transformations, as well as a theory of dislocations.
Abstract: Part I General introduction. Formal geometry of crystal lattices. The theory of reaction rates. The thermodynamics of irreversable processes. The structure of real metals. Solids solutions. The theory of dislocations. Polycrystalline aggregates. Diffusion in the solid state. The classical theory of nucleation. Theory of thermally activated growth. Formal theory of transformation kinetics. Part II Growth from the vapour phase. Solidification and melting. Polymorphic Changes. Precipitation from supersaturated solid solution. Eutectoidal transformations. Order-disorder transformations. Recovery recrystalisation and grain growth. Deformation twinning. Characteristics of martensic transformations. Crystallography of martensitic transformations. Kinetics of martensitic transformations. Rapid solidification. Bainite steels. Shape memory alloys.
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TL;DR: In this article, the growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions, and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour.
Abstract: An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT ΔT* (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT* strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.

49 citations

Journal ArticleDOI
TL;DR: In this paper, Nd3+-doped transparent oxyfluoride glass ceramics containing CaF2 were prepared through controlled crystallization of melt-quenched glass.
Abstract: Nd3+-doped transparent oxyfluoride glass ceramics containing CaF2 were prepared through controlled crystallization of melt-quenched glass. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) observation revealed the precipitation of CaF2 crystallites with about 13 nm in size among the glass matrix after heat-treatment at 650 °C for 2 h. Non-isothermal investigation on CaF2 crystallization kinetics showed that the apparent activation energy Ea and Avrami exponent n are about 310 kJ/mol and 1.23, respectively, indicating the crystallization a diffusion controlled growth process with zero nucleation rate. Fluorescence measurements exhibiting a stronger emission in the glass ceramics than in the precursor glass, together with the fluorescence decay results and energy dispersive X-ray spectroscopy (EDS) analysis, evidenced the incorporation of Nd3+ into CaF2 during crystallization.

49 citations

Journal ArticleDOI
TL;DR: In this article, an order-of-magnitude criterion for the initiation of damage-induced surface topography on clean and well-polished metal single crystals is proposed based on a model of preferential erosion of parts from the sample surface due to a local increase of sputtering yield above "extensive defects".
Abstract: A first approach to the establishment of an “order-of-magnitude” criterion for the initiation of damage-induced surface topography on clean and well-polished metal single crystals is proposed. It is based on a model proposing a mechanism of preferential erosion of parts from the sample surface due to a local increase of sputtering yield above “extensive defects”. These “extensive defects”, resulting of the migration of the point-defects, created during the collision-cascade started by the incoming ions, can indeed influence the sputtering yield if they are located close under the surface, i.e. when the surface has regressed, due to sputtering, to the level where these “extensive defects” have been formed. The condition implies that the formation of “extensive defects” must have taken place before the surface reaches them. The condition is written in terms of the irradiation parameters and quantities characteristic for the migration of point-defects due to the presence of a potential gradient rela...

49 citations

Journal ArticleDOI
TL;DR: In this article, a series of experiments under mild hydrothermal conditions (140-220°C, vapour saturated pressures) were conducted to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation.
Abstract: The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions (140–220 °C, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O2(aq) environments, some of the replacement still occurred via Fe2+ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe2+ (and the oxidant) to and from the reaction front. Reaction rates are fast at ∼200 °C, and in nature transport properties of Fe and, in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol−1. This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions.

49 citations

Journal ArticleDOI
TL;DR: In this article, the Johnson-Mehl-Avrami equations were applied to a series of power plant steels to explain the properties of the steels as a function of their chemical composition.
Abstract: The ability of steels to resist creep deformation depends on the presence in the microstructure of carbide and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. Kinetic theory has recently been developed which enables such sequences to be modelled using an adaptation of the classical Johnson-Mehl-Avrami equations. Our modification permits the treatment of more than one precipitation reaction occurring simultaneously, a feature which is essential for the reactions observed experimentally in a wide range of secondary hardening steels. In this paper, the model is applied to a series of power plant steels. It is possible to explain some interesting differences between the steels as a function of the chemical composition.

49 citations