Kinetics Of Spinodal Decomposition Research Articles

Unravel the Spinodal Decomposition Kinetics in (FeCoCrNi)85(AlCu)15 Alloy through Small-Angle Neutron Scattering

Unravel the Spinodal Decomposition Kinetics in (FeCoCrNi)85(AlCu)15 Alloy through Small-Angle Neutron Scattering

Phase-field simulation of the spinodal decomposition process near moving grain boundaries

Many investigations show that the grain boundary can significantly affect the pattern formation and kinetics of spinodal decomposition, however, these works rarely consider the effects of the kinetic properties of grain boundaries. In this work, the spinodal decomposition process near moving grain boundaries was investigated by the phase-field model. The simulation results indicate that the grain boundary-directed spinodal pattern is anisotropic, and with the increase of atom mobility inside the grain boundary, the pattern of the decomposed phase changes from parallel to the original grain boundary to perpendicularity. Moreover, the spinodal decomposition also affects the migration process of the grain boundary in turn.

Phase-Field Simulation of Spinodal Decomposition in Mn-Cu Alloys

The spinodal decomposition was studied in the aged Mn-40 at. %Cu, Mn-30 at. %Cu, Mn-20 at. %Cu alloys using a phase-field model based on the Cahn–Hillard equation, considering a subregular solution model and the energy contribution of the magnetic behavior. The simulations were performed at aging temperatures of 300, 400, and 500 °C for times from 1 to 240 min. The growth kinetics of the Mn concentration profiles with time indicated clearly that the phase decomposition of the supersaturated solid solution γ into a mixture of Mn-rich γ′ and Cu-rich γ phases occurred by the spinodal decomposition mechanism. Moreover, the phase decomposition at the early stages of aging exhibited the characteristic morphology of spinodal decomposition, an interconnected and percolated microstructure of the decomposed phases. The most rapid growth kinetics of spinodal decomposition occurred for the aging of Mn-20 and 30 at. %Cu alloys because of the higher driving force. The presence of the phase decomposition is responsible for the increase in hardness, as well as the improvement of the damping capacity of Mn-Cu alloys.

Spinodal Decomposition of an Alloy with a Strong Concentration Dependence of the Mutual Diffusion Coefficient

It is shown that the kinetics of decomposition of an alloy with a strong concentration dependence of the mutual diffusion coefficient is qualitatively different from the widely known kinetics of spinodal decomposition, since it includes the formation of metastable (in the kinetic sense) precipitates of the intermediate composition. Concentration dependence of the mutual diffusion coefficient is particularly due to the difference in impurity diffusion coefficients, as well as component self-diffusion coefficients, and it increases as temperature decreases. The calculations use an expression for the flow of component atoms in the absence of the substance flow, obtained previously under the kinetic theory of vacancy diffusion ("hole gas method"). Keywords: spinodal decomposition, mutual diffusion coefficient, precipitates of intermediate composition.

Evolution of the role of molybdenum in duplex stainless steels during thermal aging: From enhancing spinodal decomposition to forming heterogeneous precipitates

Thermal aging of ferrite in duplex stainless steels leads to a complex microstructural evolution that can give way to large deleterious changes in mechanical properties. The microstructural evolution mechanisms that happen concurrently are strongly linked to minor changes in composition. By characterizing duplex stainless steels with various compositions over a range of aging conditions, it was revealed that adding a modest amount of Mo to cast austenitic stainless steel (such as changing from grade CF3 to grade CF3M) not only increases the kinetics of spinodal decomposition and Ni-Si-Mn-rich G-phase precipitation, but it also forms a separate Mo-rich phase. At shorter equivalent aging times, Mo segregates from Fe and towards Cr accelerating spinodal decomposition. At longer aging times, Mo enriches in the Ni-Si-Mn clusters before phase separating into an adjoined Mo-rich cluster. These phase changes and precipitation events result in a rather steep drop in fracture and impact toughness. These results will inform the design of similar alloys for use at high temperatures.

Effect of Ni, Mo and Mn content on spinodal decomposition kinetics and G-phase precipitation of aged model cast austenitic stainless steels

• Effect of Ni, Mo and Mn content on ferrite decomposition. • Thermal ageing of cast austenitic stainless steels (CASS) and model alloy. • Spinodal decomposition kinetics and G-phase precipitation. During ageing, the ferritic phase of Cast Austenitic Stainless Steels (CASS) undergoes decomposition by spinodal decomposition and G-phase precipitation, which leads to increasing hardness and decreasing impact toughness. Mo-bearing steels are more prone to ageing than Mo-free steels. To study the effects of Ni, Mo, and Mn, duplex model alloys were thermally aged at 350 °C and 400 °C. The microstructure evolution during ageing was characterized by atom probe tomography. Only Ni additions seem to affect the spinodal decomposition kinetics. Ni accelerates the spinodal decomposition kinetics regardless of the presence of Mo or Mn. Additions of Mo and Mn promote the precipitation of G-phase.

Towards predictive simulations of spinodal decomposition in Fe-Cr alloys

Simulations of spinodal decomposition in an Fe-36 wt%Cr alloy at 773 K are performed by solving the non-linear Cahn–Hilliard equation, and the results are compared with atom probe tomography measurements. The influence of gradient energy coefficient, atomic mobilities and initial structure on the kinetics of spinodal decomposition is studied. It is shown that a proper initial structure, accounting for the thermal history above the miscibility gap, is crucial and enables predictive simulations of spinodal decomposition in Fe-Cr alloys.

Concentration-gradient-driven anisotropic spinodal decomposition kinetics: nitriding of metallic alloys

ABSTRACT The effect of crystallographically oriented, unidirectional concentration gradients on spinodal decomposition in cubic crystalline solids with elastic and interfacial energy anisotropy is discussed. Phase-field simulations reveal that the kinetics of spinodal decomposition occurring in such systems is dependent on the degree of misorientation between the direction of composition gradient and the preferred crystallographic orientation for growth of spinodal fluctuations; the larger is the misorientation, the slower the kinetics. This phenomenon has been used to explain the well-known grain-orientation-dependent N-uptake kinetics observed during nitriding of metallic alloys. Several plausible causes have been proposed in the literature for the grain-orientation-dependent N-uptake kinetics during nitriding. However, this study reveals that this phenomenon is observed exclusively and without exception in alloy systems having a spinodal instability. The N-uptake kinetics in such systems is known to be dependent on the kinetics of spinodal decomposition. Consequently, anisotropic spinodal decomposition kinetics occurring owing to the presence of a surface-directed N-composition gradient in poly-crystalline metals has been shown to be a more fundamental cause for the phenomenon.

Two-dimensional Cahn-Hilliard simulations for coarsening kinetics of spinodal decomposition in binary mixtures.

The evolution of the microstructure due to spinodal decomposition in phase separated mixtures has a strong impact on the final material properties. In the late stage of coarsening, the system is characterized by the growth of a single characteristic length scale L ∼ Ctα. To understand the structure-property relationship, the knowledge of the coarsening exponent α and the coarsening rate constant C is mandatory. Since the existing literature is not entirely consistent, we perform phase field simulations based on the Cahn-Hilliard equation. We restrict ourselves to binary mixtures using a symmetric Flory-Huggins free energy and a constant composition-independent mobility term and show that the coarsening for off-critical mixtures is slower than the expected t1/3-growth. Instead, we find α to be dependent on the mixture composition and associate this with the observed morphologies. Finally, we propose a model to describe the complete coarsening kinetics including the rate constant C.

Effect of cold rolling on phase separation in 2202 lean duplex stainless steel

The effect of plastic deformation (cold rolling) on the thermal aging of a lean 2202 duplex stainless steel at intermediate temperatures (280 °C–450 °C) was studied using a combination of atom probe tomography and thermoelectric power measurements. In the undeformed condition, spinodal decomposition of ferrite was shown to be the prime aging process with an activation energy of 206 kJ/mol. Plastic deformation enhanced the aging and the austenite partly transformed into martensite. When no precipitation or phase transformation was observed in austenite, both ferrite and martensite decomposed. The spinodal decomposition kinetics of ferrite increased with increasing deformation rate. The FeCr-rich martensite also decomposed at 450 °C, via a nucleation and growth process, owing to a decrease of the Cr content in the matrix. Chromium nitrides, G-Phase, and Cu-rich precipitates were observed in the decomposed martensite.

Long‐Term Heat Treatment of Phase Separating Sodium Borosilicate Glass

The kinetics of spinodal decomposition of glasses with a composition of (in mol%) 8 Na2O, 26 B2O3, and 66 SiO2, are investigated as a function of time (up to 64 d) and temperature (between 630 and 750 °C) of heat after leaching in diluted HCl and NaOH solution to generate porosity. The glasses as well as porous materials are analyzed by dilatometry, mercury intrusion porosimetry, nitrogen sorption, X‐ray diffraction, X‐ray fluorescence spectroscopy, helium pycnometry, and scanning electron microscopy (SEM). The development of microstructure parameters like pore sizes are evaluated to characterize the kinetics of microstructure formation. Two different growth stages are identified via their size growth kinetics and the analysis of the obtained microstructures. In both stages the pore sizes follow size time laws dp ∝ tβ with an exponent β of 0.33 or 0.5. Both processes are running parallel in the second stage. This lead to the conclusion that the dominating mechanism of microstructure formation in the first identified stage is Ostwald ripening. This process is followed by a new stage which is concluded to be diffusion limited with a calculated activation energy of 220 ± 10 kJ mol−1. The observed size time laws do not limit the pore sizes.

Influence of Mo Additions on the Mechanical Properties of Cast Duplex Stainless Steels before and after Thermal Aging

The influence of Mo additions on the mechanical properties of cast duplex stainless steel (CDSS) before and after thermal aging was investigated using a series of model alloys with different Mo contents ranging from 0 to 1.75 wt%. By increasing Mo content, the content, morphology, and distribution of ferrite in CDSS change significantly. After thermal aging at 400 °C for 3000 h, the impact properties of all CDSS specimens obviously decline, and their hardness values in ferrite significantly increase. The impact energies of the aged CDSS decline, and the proportion of cleavage features significantly increases with Mo content increasing. The spinodal decomposition kinetics in ferrite is not significantly affected by the Mo contents. High content and interconnected ferrite will lead to the severe embrittlement in CDSS after thermal aging.

Carbon diffusivity and kinetics of spinodal decomposition of martensite in a model Fe-Ni-C alloy

From direct observations and measurements at the atomic scale by atom probe tomography coupled with calculations using a mean-field approach, the kinetics of low temperature carbon redistribution in martensite of a Fe-Ni-C alloy was investigated for duration up to 4.5 years. The results suggest that the ordered phase α″-Fe16C2 forms via a spinodal decomposition mechanism, in the range of a few days at room-temperature. The approach proposed allows us to retrieve the diffusivity of carbon in a large composition range during decomposition kinetics of martensite into α-Fe and α″-Fe16C2.

Effect of aliovalent dopants on the kinetics of spinodal decomposition in rutile-type TiO2-VO2

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO2-VO2 system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as-prepared polycrystalline samples were annealed at 673k inside the spinodal. For undoped samples, phase decomposition occurred along the [001] direction, and a lamellar structure formed on the nanometer length scale. The phase decomposition kinetics in this system were sensitive to the type of aliovalent dopant; the decomposition rate in a sample doped with Al2O3 was more than one order of magnitude higher than a sample doped with Nb2O5. These observations could be explained on the basis of the premise that cation interstitial mobility is greater than cation vacancy mobility. The obtained results are consistent with those for a TiO2-SnO2 system, indicating that phase separation in both rutile-type oxide systems can be interpreted through a common framework.

Epitaxial Halide Perovskite Lateral Double Heterostructure.

Epitaxial III-V semiconductor heterostructures are key components in modern microelectronics, electro-optics, and optoelectronics. With superior semiconducting properties, halide perovskite materials are rising as promising candidates for coherent heterostructure devices. In this report, spinodal decomposition is proposed and experimentally implemented to produce epitaxial double heterostructures in halide perovskite system. Pristine epitaxial mixed halide perovskites rods and films were synthesized via van der Waals epitaxy by chemical vapor deposition method. At room temperature, photon was applied as a knob to regulate the kinetics of spinodal decomposition and classic coarsening. By this approach, halide perovskite double heterostructures were created carrying epitaxial interfaces and outstanding optical properties. Reduced Fröhlich electron-phonon coupling was discovered in coherent halide double heterostructure, which is hypothetically attributed to the classic phonon confinement effect widely existing in III-V double heterostructures. As a proof-of-concept, our results suggest that halide perovskite-based epitaxial heterostructures may be promising for high-performance and low-cost optoelectronics, electro-optics, and microelectronics. Thus, ultimately, for practical device applications, it may be worthy to pursue these heterostructures via conventional vapor phase epitaxy approaches widely practised in III-V field.

Martensitic anelasticity in 40Mn60Cu alloy

The martensite anelasticity effects such as internal friction and the shape memory effect in 40Mn60Cu alloy were studied. The kinetics of spinodal decomposition in the 40Mn60Cu alloy after during aging (400, 440 and 540°C) was studied by the mechanical spectroscopy methods. It is shown that the temperature of the martensitic transformation depends on temperature and time of aging within the metastable miscibility gap. The characteristics of the shape memory effect were determined at different pre-strains. It is shown that the extent of shape recovery K decreases exponentially with the increase of pre-strain (γr=1…5%). This is due to the increase of the proportion of irreversible slip deformation.

A multi-fluid model for microstructure formation in polymer membranes.

We develop a multi-fluid model for a ternary polymer solution using the Rayleighian formalism of Doi and Onuki, and give an efficient pseudo-spectral method for solving both the diffusion and momentum equations that result. Subsequently, we find that the numerical simulation is capable of describing systems at the micron length-scale and easily reaches millisecond time-scales. In addition, we characterize the model thermodynamics and kinetics including the (i) phase behavior, (ii) structure of the interfaces, (iii) mutual diffusion coefficients, (iv) bulk spinodal decomposition kinetics with and without hydrodynamics and (v) spinodal decomposition in the presence of an interface with a non-solvent bath. We obtain good qualitative agreement with the expected thermodynamic and kinetic behavior. We also show that a linear stability analysis of the diffusion equation quantitatively predicts the fastest growing mode obtained from simulation and gives insight into the phase separation process relevant for the evolution of microstructure in phase-separating ternary polymer solutions.

Effect of solution treatment on spinodal decomposition during aging of an Fe-46.5 at.% Cr alloy

Spinodal decomposition is a key phase transition in advanced materials and a significant effort is paid to the quantitative modeling of the phenomenon. The initial materials condition is often assumed to be random during modeling, but this may be an oversimplification. In this work, the effect of solution treatment above the miscibility gap, on spinodal decomposition during subsequent aging, has been investigated for an Fe-46.5 at.% Cr alloy. By atom probe tomography (APT), it is found that different extents of quenched-in Cr clustering exist after solution treatments at different temperatures. The clustering is pronounced at 800 °C but decreases significantly with increasing temperature to 900 °C. Thermodynamic Monte Carlo simulations show that there is a difference in atomic short range order between the different solution treatment temperatures. By APT, it is, furthermore, found that the kinetics of spinodal decomposition at 500 °C, i.e., within the miscibility gap, is enhanced in the initial alloy condition, where Cr was less randomly distributed. These observations are supported by kinetic Monte Carlo simulations, predicting a similar but less pronounced qualitative effect on spinodal decomposition kinetics. Other possible reasons for the enhanced kinetics could be related to clustering of interstitial elements and/or sigma phase, but neither was found in the experiments. Nonetheless, the observations in this work suggest that it is necessary to implement a modeling strategy, where the initial structure is properly accounted for when simulating spinodal decomposition.

Kinetics of G-phase precipitation and spinodal decomposition in very long aged ferrite of a Mo-free duplex stainless steel

Evolution of spinodal decomposition and G-phase precipitation in ferrite of a thermally aged Mo-free duplex stainless steel was studied by Atom Probe Tomography (APT). Kinetics was compared to kinetics observed in ferrite of some Mo-bearing steels aged in similar conditions. This paper shows that formation of the G-phase particles proceeds via at least a two-step mechanism: enrichment of α/α′ inter-domains by G-former elements followed by formation of G-phase particles. As expected, G-phase precipitation is much less intense in the Mo-free steel than in Mo-bearing steels. The kinetic synergy observed in Mo-bearing steels between spinodal decomposition and G-phase precipitation is shown to also exist in Mo-free steel. Spinodal decomposition is less developed in the ferrite of the Mo-free steel investigated than in Mo-bearing steels: both the amplitude of the decomposition and the effective time exponent of the wavelength (0.06 versus 0.16) are much lower for the Mo-free steel. Neither the temperature of homogenisation nor quench effects or Ni and Mo contents could successfully explain the low time exponent of the spinodal decomposition observed in the Mo-free steel. The diffusion mechanisms could be at the origin of the different time exponents (diffusion along α/α′ interfaces or diffusion of small clusters).

Kinetics of secondary phase precipitation during spinodal decomposition in duplex stainless steels: A kinetic Monte Carlo model – Comparison with atom probe tomography experiments

This paper presents the first simulations of the kinetics of spinodal decomposition and G-phase precipitation occurring in ferrite of duplex stainless steels. The kinetics was simulated using a simple but effective atomic kinetic Monte Carlo model in a ternary alloy. The simulations reproduced the α/α′ spinodal structure with precipitates at the α/α′ interface. The comparison of simulated results with experiments shows that the simulations quantitatively reproduce the kinetics of phase transformation and the synergy observed experimentally between the spinodal decomposition and G-phase precipitation. By following the vacancy pathway, we show that the coarsening of G-phase precipitates proceeds via diffusion along the α/α′ interfaces. The simulations made it possible to explain the origin of the kinetic synergy between spinodal decomposition and G-phase precipitation.