Thermodynamics Of Phase Transformations Research Articles

Liberalizing the effects of Al and Cr in coatings for enhanced interface stability with Mo-rich Ni3Al-based superalloys

The interfacial behaviors between protective coatings and substrate single-crystal superalloys significantly impact their service performances. This work focuses on the elements interdiffusion and microstructural evolution at the interface between a NiCrAlY coating and a series of high-Mo Ni3Al-based single-crystal superalloys, through the coupled phase-field simulations and DICTRA kinetics calculations of their model systems. The critical roles of Al and Cr played in the microstructural evolution at coating/superalloy interface have been confirmed, which requires the appropriate content decreases of Al and Cr in the coating at 1100 °C. However, the addition of Mo reduces the driving force of element diffusion from coating to substrate by increasing the activities of Al and Cr in superalloy. Moreover, increasing the Mo content in superalloy from 8 wt% to 10 wt% could also counteract the promoting effect of Al on Ni mobility and mitigate the γ′ coarsening and detrimental TCP precipitation. Therefore, the effects of Mo enable the reasonable increase of Al content in coating to better balance the interfacial stability and oxidation resistance of coated superalloy, besides its advantage in reducing the anisotropy of TCP precipitate. The obtained interfacial evolution mechanisms resulting from the phase transformation thermodynamics and element interdiffusion kinetics are expected to aid the coating design for advanced single-crystal superalloys servicing at ultra-high temperature.

Thermodynamics of Phase Transformation and Microstructure Evolution of 18Cr2Ni4WA Carburized Steel during High‐Temperature Tempering

The microstructural evolution and phase transformation mechanism in 18Cr2Ni4WA carburized steel during high‐temperature tempering is investigated using a thermodynamic model. The experimental results demonstrate that the retained austenite in the matrix decreases from the surface to the core, and martensite transform from acicular to lath‐type. Furthermore, the hardness distribution of the carburized layer increase initially, and then decrease. Based on thermodynamic calculations, the driving forces for the decomposition of martensite decomposition and nucleation of cementite in martensite are larger than those for the decomposition of austenite during tempering. Furthermore, cementite preferentially precipitate in martensite. The driving force for the overall phase transformation of the decomposition of the retained austenite is negative, and the driving force for the nucleation of cementite in the low‐carbon retained austenite is positive, indicating that cementite can nucleate only in high‐carbon retained austenite. The low‐carbon retained austenite transform into secondary martensite on cooling after the tempering process while the driving force gradually increase. The decomposition of the initial quenched martensite during the tempering process leads to a decrease in the overall hardness of the carburized layer. The secondary martensite obtained from the retained austenite contribute to the increase in the surface hardness.

Statistical Thermodynamics of Phase Transformations in Lithium Alanates with Release of Hydrogen

Based on the molecular and kinetic concepts, the paper presents the theory of phase transformations in lithium alanates with the release of hydrogen. The calculations are given for free energies of phases and their dependences on pressure, temperature, hydrogen concentration, and energy parameters are determined. The equations are derived for the thermodynamically-equilibrium states which determine the Pressure-Temperature-Concentration diagram and estimate the energy parameters with the use of experimental results taken from the literature. The investigation of the detected temperature/concentration dependence in crystals shows the impossibility of a complete hydrogen release from alanates. The paper contains isotherm and isopleth plots. A possibility is established for the hysteresis effect. A comparison is given to the theoretical and experimental results.

Modeling High-Alloy Chromium-Nickel Alloy Nanostructuring with Treatment by Highly-Concentrated Energy Sources

A mathematical model is provided for phase transformation thermodynamics making comprehensive study possible for the equilibrium phase composition of chromium-nickel welded joints at different temperatures. According to the model, with a chromium content of more than 22 wt.% the temperature for the start of intermetallic precipitation exceeds the solidus temperature for Cr–Ni alloy, i.e., in this case precipitation of primary Ni3Al intermetallic is thermodynamically possible. Primary Ni3Ti precipitates are probable with a chromium content of more than 36 wt.%. Model calculations are confirmed by microstructural studies. In order to improve welded joint strength for the alloy in question, slow joint cooling in the range below 1000°C is recommended, depending on welded structure operating temperature, and ageing in the range of intense γ′-phase precipitation, i.e., 900–600°C. The mathematical model developed for phase transformation thermodynamics may be used for studying the phase composition of other chromium-nickel alloys.

Evolution of host‐inclusion systems: a visco‐elastic model

AbstractA visco‐elastic model is suggested to recover the P−T−t paths of spherically symmetrical host‐inclusion systems from residual pressure patterns. For a coesite–garnet, aragonite–garnet and diamond–garnet system with partial transition of high pressure polymorphs to low pressure ones, the model predicts that the exhumation history of ultra‐high pressure metamorphic rocks from depths below 90 km began with pressure decrease to ∼1 GPa (∼40 km depth), while the temperature remained high (>800 C); then the system cooled and decompressed to ambient conditions. Non‐spherical inclusion shapes in the 3‐D model lead to heterogeneities in residual pressure and shear stress distributions as well as to significant gradients of shear stress, which may affect the kinetics and thermodynamics of phase transformation in the inclusions.

Design and application of differential thermal analysis apparatus in high magnetic fields

The differential thermal analysis (DTA) apparatus has been developed for a commercial superconducting magnet. The DTA apparatus could detect the kinetics and thermodynamics of phase transformation with and without a magnetic field. Preliminary results for Al-Al2Cu eutectics are presented. The DTA curves indicate the similarity at several rates regardless of a magnetic field; however, at the same rate, melting transformation seems not to be influenced by a magnetic field, while solidification could be delayed via suppressing nucleation and crystal growth in a magnetic field. It will be believed that the DTA apparatus can be used to investigate the phase transformation of substances of interest in a magnetic field.

Progress in Research on Solidification in a Strong Static Magnetic Field

Strong magnetic fields available from superconducting magnets are opening a way to new phenomena that could lead to new methods in materials processing including solidification. The principal research involving solidification in strong static magnetic fields is emphasizing four aspects: control of crystal orientation, convection damping, thermoelectric magnetohydrodynamics (TEMHD) and change in thermodynamics. Under high magnetic intensity, aligned structural textures are induced in both magnetic and non-magnetic materials. Since in strong magnetic field the melt flow is suppressed by convection damping, the microstructure being formed during solidification is affected heavily; this phenomenon applies to eutectic, monotectic and peritectic alloys as well as to dendritic morphologies typical of directional solidification. If strength and orientation of a magnetic field are controlled appropriately, this strong damping effect will generate more homogeneous crystals as a result of achieving diffusion-controlled solute transport conditions. TEMHD more easily occurs in strong magnetic fields, resulting in equiaxed crystals even under directional solidification. It is evidenced experimentally and theoretically that the thermodynamics of phase transformation and nucleation are changed by strong magnetic fields.

Solid-solid phase transformation via internal stress-induced virtual melting: Additional confirmations

Recently, we predicted a mechanism of solid-solid phase transformation (PT) via virtual melting at 121K below the melting temperature. We report additional experimental and theoretical results for PTs among three polymorphs of the energetic material HMX, α, β, and δ that support this mechanism. In particular: (a) the predicted velocity of interface propagation for β→δ PT and overall kinetics of δ→β PT are in agreement with experiment; (b) the energy of internal stresses is sufficient to reduce the melting temperature from 520to400K for δ→β PT; (c) the nanocracking that appears during solidification does not change the PT thermodynamics and kinetics for the first and the second β↔δ PT cycles; (d) δ→β PT starts at a very small driving force; (e) δ→α and α→δ PTs do not occur above 400K and below 461K, respectively.

Continuum mechanical aspects of phase transformations in solids

Some aspects of “classical” thermodynamics of phase transformations are discussed. Then typical solid state transformations as the displacive and diffusional transformation in metals are explained. A general formulation of the Gibbs free energy is presented including all energy terms required. Based on the “classical” nucleation, the “triggering-off” and the “dissipation” condition, various transformation conditions are formulated taking into account the elasto-plastic deformation of both phases. Transformation conditions presented in the literature over the last 40 years are reviewed and compared to the transformation conditions derived here. The transformation conditions for a spherical region growing under a certain volume change in an elasto-plastic matrix are studied as an example. The relevant analytical expressions are presented and discussed.

Morphology of polymers

Abstract

Influence of grain size and ordering degree of the parent phase on Ms in a cuznal alloy containing boron

In this paper, the influence of the average grain size and ordering degree of the parent phase on the starting temperature of thermoelastic martensitic transformation in a Cu-25.62 Zn-3.97 Al0.0018 B (wt%) shape memory alloy is studied. Based on the thermodynamics of phase transformation, a linear relationship between the starting temperature of martensitic transformation and the reciprocal of the square root of grain size is obtained, i.e. M s temperature decreases with increasing grain size, concurrent exactly with the result of electric resistance measurement. Application of Landau's theory gives a quantitative relationship between M s temperature and the ordering parameter of the parent phase, which is well confirmed by the results of X-ray diffraction and electric resistance measurement. Besides, the activation energy of the ordering process in the parent phase of the alloy employed is calculated to be 46 kJ/mol.

Thermodynamics of phase transformations in steels and alloys

Thermodynamics of phase transformations in steels and alloys

Some peoblems concerning the thermodynamics of phase transformation

The de Londe method is applied to phase transformation processes in multicomponent systems. It is shown that these processes can be described by a single universal coordinate (the transformation level) and one generalized force (the mixing affinity).

The specific heat and other thermodynamic functions of rhombic and monoclinic sulphur and their application to the kinetics and thermodynamics of phase transformation

The specific heat and other thermodynamic functions of rhombic and monoclinic sulphur and their application to the kinetics and thermodynamics of phase transformation