Discontinuous Dissolution Research Articles

Discontinuous Dissolution Mechanism of Olivine Deduced from a Topography Observation Method.

Olivine dissolution plays an important role in environmental science and technology, from controlling global element circulation to carbon capture for climate change mitigation. Most studies have been focused on investigating its dissolution rates by monitoring chemical effluent changes under various conditions. However, only by observation of surface reactivity can we unravel the actual mechanism (s) of dissolution. Here, we studied the dissolution of an olivine (010) plane in a flow-through reaction cell with an acidic solution, a surface-controlled regime, and far-from-equilibrium conditions. Direct mineral surface topography measurements using vertical scanning interferometry and atomic force microscopy allowed for quantitative analyses of the spatial and temporal changes in the dissolution rate. The (010) plane dissolved discontinuously in time for different surface sites, resulting in a heterogeneously distributed rate map. Pits with different depths showed opposite dissolution rate distributions from the dislocation center to further out from the etch pit. Based on the step-wave model, we propose a mechanism of dissolution that is governed by the competition between Gibbs free energy of the dissolution process, ΔG, and the critical free energy of the opening of etch pits, i.e., ΔGcrit. The migration of step waves, the distribution of surface defects, the strain field of etch pits, and other dynamic elements, resulting in the instantaneous change of ΔGcrit on the surface, are important factors leading to the discontinuous dissolution of crystal materials. This discontinuous dissolution provides new insight into the guidance of crystalline mineral applications and the prediction of material properties regarding mineral dissolution variation.

Jerky Motion of the Reaction Front during Discontinuous Dissolution in a Fe-13.5 at.% Zn Alloy.

This paper studies the go- and -stop movement of a receding reaction front (RF) during a discontinuous dissolution (DD) process. A special simulation procedure was applied for the DD reaction to predict a jerky motion of the RF. The Fe-13.5 at.% Zn alloy was selected in which go- and -stop behaviour was revealed in the form of characteristic lines (called “ghost lines”) showing successive positions of receding RF. The results presented for the DD process are quite different from those relevant for the DP reaction at the same Fe-13.5 at.% Zn alloy in terms of go- and -stop motion and movement distance. For the presented case, the go- and -stop periods are relatively long and obtain an order of several dozen seconds, while for the DP reaction, it was only a few seconds. A similar conclusion was formulated after a comparison of the movement distance which, for the DD reaction, is usually longer by 1–2 orders of magnitude. The simulation results of the DD reaction indicate a good agreement with the experimental data presented in the literature for the same dissolution rate. It is necessary to emphasize that the simulation is the only source of data for z parameter changes during the -stop period of the DD reaction.

Electropulsing induced evolution of grain-boundary precipitates without loss of strength in the 7075 Al alloy

The influence of electropulsing on the peak-aged (T6) microstructures in the 7075 Al alloy was investigated in this study. The results show that parts of the intragranular precipitates dissolve in the matrix during electropulsing, and the electropulsing treated sample exhibits discontinuous dissolution of the grain-boundary precipitates (GBP). After re-aging, the mechanical properties of the re-aged (RA) sample are nearly the same as that of T6 temper, meanwhile, the grown discontinuous GBP were obtained. Hence it is a promising method to change the distribution of GBP without loss the strength in the 7075 Al alloy.

Discontinuous Precipitation and Dissolution under the Influence of the Plastic Strain and Temperature in Al-15 at.%Zn and Al-30 at.%Zn Alloys

The discontinuous precipitation and dissolution in the alloy Al-Zn system has been the subject of many theoretical and experimental investigations that have contributed to the understanding of the different mechanisms which control them. However, many questions remain unanswered because of the complexity of the constituted phases which are affected by the speed of the quenched, deformation, the temperature of homogenization and ageing effect. The purpose of this work is to clarify the effect of temperature and deformation on the mechanisms of these two reactions during ageing of Al-15 at.% Zn and Al-30 at.% Zn alloy. The techniques of analysis used in this respect are the optical microscopy, the X-ray diffraction and the hardness Vickers.

Complex reaction of the DIGM and discontinuous precipitation and dissolution of phases

The complex reaction of diffusion-induced grain-boundary migration (DIGM), discontinuous precipitation, and dissolution of phases in a precipitation-hardened lead-antimony alloy has been studied. The concentration profile of Sb in front of the migrating boundary has been measured. At the reaction front, there is observed an intense bulk diffusion, which leads to the formation of precipitate-free zones in front of the migrating boundary. Driving forces of the boundary migration have been analyzed. The activation energy of the reaction has been determined. A comparison of the theoretical and experimental values of the rate of the complex reaction of the discontinuous precipitation of phases and DIGM has been carried out.

Discontinuous Precipitation and Dissolution in Cu-4, 6 at% In Alloy under the Effect of Plastic Deformation and the Temperature

The study of the discontinuous precipitation reaction and the lamellar precipitate dissolution in the alloy Cu-In system provoked a considerable benefit and has been the subject of many theoretical and experimental investigations. The aim of this work is to make the evidence on the one hand the effect of the plastic deformation on the mechanism of the discontinuous precipitation reaction such as nucleation, growth and lamellar coarsening and in other hand the effect of temperature on the characteristics and front behavior movement of the opposite reaction (discontinuous dissolution). Different techniques of analysis have been used in this respect such as the optical microscopy, the differential thermal analysis and the microhardness Vickers. The obtained results confirm various works achieved in this field.

The Loss of Dissolved Zirconium in Zirconium-Refined Magnesium Alloys after Remelting

Magnesium–rare earth–zirconium (Mg-RE-Zr) materials are a high value-added class of speciality alloys used chiefly for aerospace applications, where the use of zirconium ensures the critically needed structural uniformity and consistency in performance through potent grain refinement. However, the manufacture of these alloy components has proved to be problematic and volatile. A viable approach preferred by manufacturers is to simply remelt fully grain-refined prealloyed ingot materials and cast. Unfortunately, the dissolved Zr content in these alloys decreases appreciably after remelting, leading to significantly increased grain sizes and inhomogeneity. This work investigates the mechanisms responsible for the loss of dissolved Zr in Zr-refined magnesium alloys after remelting. It was revealed that the phenomenon stems from the unique Zr-rich cores existing in these alloys. Precipitation of zirconium from these supersaturated cores during heating reduces the dissolved Zr content in the α-Mg phase. On the other hand, once precipitated out, the zirconium precipitates show very limited dissolution in molten magnesium due to their discontinuous dissolution behavior in the absence of agitation. The loss of dissolved Zr was found to be reversible by agitation when remelted in an iron-free environment while irreversible in steel crucibles due to the interference from iron.

The effect of plastic deformation and temperature on the mechanism of discontinuous precipitation and dissolution in the Al-15 at.%Zn alloy

Discontinuous precipitation and dissolution in the Al-Zn alloy system has been the subject of many theoretical and experimental investigations that have contributed to the understanding of the different mechanisms which control them. However, many questions remain unanswered because of the complexity of the constituted phases which are affected by the rate of quenching, the amount of deformation, the temperature of homogenization and the ageing effect The purpose of this work is to clarify the effect of temperature and deformation on the mechanisms of these two reactions during ageing of the AI-15at.%Zn alloy. The techniques of analysis used in this respect are optical microscopy, X-ray diffraction and Vickers microbardness.

Morphology and kinetics of discontinuous precipitation and dissolution in an Fe-8.5Al-27Mn-1.0Si-0.92C alloy

The morphology and growth kinetics of discontinuous precipitation (DP) and discontinuous dissolution (DD) in an Fe-8.5Al-27Mn-1.0Si-0.92C alloy are investigated. The results indicate that the solid-solution-treated austenite phase decomposes into lamellar DP after aging at temperatures ranging from 900 to 1050 K. After the specimens of a preaged DP structure go through further aging at temperatures ranging from 1118 to 1173 K, the lamellar DP falls into dissolution because of the DD reaction. The lamellar spacings of the DP structure, as well as the reaction-front migration rates during DP and DD reactions, are measured. Based on Aaronson and Liu’s simplified kinetics model, the grain-boundary diffusivities are estimated. They are found to be slightly lower than the grain-boundary diffusion data of Mn in Fe and Fe in Fe as reported by Aaronson and Liu.

A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy

The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution.

Discontinuous reactions in solids

Discontinuous reactions are solid state moving boundary phase transitions characterised by a discontinuous or abrupt change in orientation and composition between the matrix phases in the reactant and product aggregate across the migrating boundary or reaction front that provides a short circuit path of diffusion. The reactions include discontinuous precipitation, discontinuous coarsening, discontinuous dissolution, and diffusion induced grain boundary migration. All these reactions may account for a substantial change in microstructure, composition, and material properties, and hence, deserve adequate scientific attention for a better understanding. The present review provides a comprehensive discussion on the current status of understanding about nucleation and growth mechanisms, genesis and driving force, product morphology and distribution, kinetic growth models, and related experimental techniques, and above all, the unresolved questions concerning these discontinuous reactions. In addition, exhaustive lists have been provided to document the important literature on the concerned subjects, whenever possible. Finally, a particular emphasis has been placed on analysing the recent findings about dynamic behaviour of grain boundaries, scope of determination of Arrhenius parameters by kinetic analysis, and orientation/structural dependence of boundary migration and diffusion.

Discontinuous dissolution of a frozen matrix liquid phase in Ba(Zn 1/3Nb 2/3)O 3 ceramics

Grain-boundary migration during the dissolution of a liquid phase in Ba(Zn 1/3Nb 2/3)O 3 (BZN) ceramics has been investigated. When the BZN specimens sintered at 1550°C are further heat-treated at lower temperatures, the liquid phase formed during sintering is dissolved discontinuously and grain boundaries migrate. With the increase of heat-treatment temperature, grain-boundary migration becomes much less pronounced. These results are consistent with the coherency strain energy model proposed for the driving force for diffusion-induced grain boundary migration (DIGM).

A Comparative Study of the Kinetics of Interface Diffusion Controlled Transformations In Fe-Zn Alloys

A comparative study of the kinetics of discontinuous precipitation, coarsening and dissolution, diffusion induced grain boundary migration and grain growth has been carried out in Fe-Zn alloys. Except for cells of the discontinuous coarsening reaction, rates of migration of the cell boundaries of precipitation and dissolution and diffusion induced grain boundary migration (DIGM) rates lie within an order of magnitude of each other. The rate of cell boundary migration during discontinuous coarsening is two orders of magnitude smaller than those for the four transformation processes mentioned above. The kinetics (of minimum rate) of grain growth which occurs after long annealing times is comparable to the discontinuous precipitation and coarsening in the Fe-Zn alloy. The kinetics (of maximum rate) of grain growth, which corresponds to short annealing times, are comparable to the discontinuous dissolution and DIGM. The diffusivity values lie within an order of magnitude of the boundary diffusivity during grain growth when considered at the same temperature (extrapolated). Although the driving force differs by four orders of magnitude from as low as 0.08 Jmol−1 for grain growth to 1420 Jmol−1 for DIGM at 923 °K, the small difference in the diffusivity (kDbδ) predicts that the conduit for solute transport is the same for the above transformations.On a effectué une étude comparative de la cinétique de précipitation discontinue, de grossissement et de dissolution, de migration de joints de grain induite par diffusion (DIGM) et de croissance de grain, d'alliages de Fe-Zn. À l'exception des cellules de réaction de grossissement discontinu, le taux de migration des joints de cellules de précipitation, de dissolution ainsi que le taux de migration de joints de grain induite par diffusion se situent à un ordre de grandeur l'un de l'autre. Le taux de migration du joint de la cellule lors du grossissement discontinu est de deux ordres de grandeur plus petit que celui des quatre procédés de transformation mentionnés ci-dessus. La cinétique (taux minimum) de croissance de grain qui se produit après une longue durée de recuit est comparable à la précipitation discontinue et au grossissement de l'alliage Fe-Zn. La cinétique (taux maximum) de croissance de grain qui correspond à de courtes périodes de recuit est comparable à la dissolution discontinue et à “DIGM”. Les valeurs de diffusivité se situent à un ordre de grandeur de la diffusivité du joint lors de la croissance de grain lorsque considérée à la même température (extrapolée). Bien que la force directrice diffère par quatre ordres de grandeur, allant de 0.08 Jmol−1 pour la croissance de grain jusqu'à 1420 Jmol−1 à 923 °K pour “DGIM”, la petite différence de diffusivité (kDbδ) prédit que le conduit pour le transport de soluté est le même pour les transformations ci-dessus.

Discontinuous Dissolution of a Liquid Phase in Ba(Ni1/3Nb2/3)O3 Ceramics

Discontinuous dissolution of a liquid phase accompanied by diffusion‐induced grain‐boundary migration (DIGM) in Ba(Ni1/3Nb2/3)O3 (BNN) ceramics has been studied. When the liquid‐phase‐sintered BNN specimens are heat‐treated at low temperatures, the liquid phase formed during sintering is dissolved discontinuously and during the reaction grain boundaries migrate. As the heat‐treatment temperature is lowered, the degree of grain boundary migration and dissolution of the liquid phase increase. These results are qualitatively consistent with the coherency strain energy model proposed for the driving force for DIGM.

AEM of the Solute Redistribution During Discontinuous Dissolution

High spatial resolution X-ray microanalysis of thin foils in a transmission electron microscope has been used for the first time to examine the residual solute content profiles resulting from the discontinuous dissolution reaction in a Ni-4 at. % Sn alloy. The sharp change in the Sn content, detected performing the line scans across the reaction front, indicated that the solute redistribution occurred just at the reaction front. The Sn profiles measured along the reaction front were the reverse of those typical for the discontinuous precipitation reaction, with a relatively higher excess amount of tin stored close to the position where the Ni3Sn lamellae had previously existed. The obtained results also suggest that the chemistry and shape of profiles are not only dependent on the dissolution temperature, but that the initial state of the single cell just before the beginning of the discontinuous dissolution reaction plays an important role.

Microanalytical study of discontinuous precipitation and dissolution in Ni–4 at.% Sn: local and global characterization of the reactions

The morphology and kinetics of the discontinuous precipitation (DP) and discontinuous dissolution (DD) reactions have been studied in a Ni–4 at.% Sn alloy. High spatial resolution energy-dispersive X-ray microanalysis has been used to determine the Sn concentration profiles left behind the moving reaction front for the individual cells of the Sn-depleted α lamellae and Ni 3Sn compound. These data, combined with the local values of the reaction front velocity and the thickness of the α lamellae, have been used to evaluate the local sδD b values ( D b is the grain-boundary chemical diffusion coefficient, δ is the grain-boundary thickness and s is the segregation factor). The obtained results have been compared with those calculated by the global approach to the DP and DD reactions, which is relevant for the whole population of the cells. It has been shown that the application of the local characterization of the DP and DD reactions removes essentially the differences between the sδD b values calculated by the Petermann–Hornbogen equation and the equations of Cahn and Zięba–Pawłowski. Moreover, both sets of data do not show any substantial differences from the sδD b values obtained from measurements of the tracer diffusion of tin along stationary grain boundaries in nickel.

Growth of Lamellar Products During Discontinuous Solid State Reactions

AbstractThe nucleation, growth and dissolution of lamellar precipitates formed due to discontinuous solid state reactions like: discontinuous precipitation, coarsening and dissolution, are reviewed. Emphasis is given on recent studies based on analytical electron microscopy in describing the microchemistry, andin situelectron microscopy for revealing the morphological features of the reactions.

The kinetics of the discontinuous precipitation and dissolution in Mg-rich Al alloys

The morphology and growth kinetics of the discontinuous precipitation and dissolution reactions in supersaturated Mg-Al solid solutions containing 7.3, 9.1 and 10.9 at % Al have been investigated by optical and scanning electron microscopy and X-ray measurements. The volume fraction of regions transformed by the discontinuous precipitation reaction, the reaction front velocity, the interlamellar spacing and the average composition of the solute-depleted lamellae were determined as a function of the temperature. For the first time, the kinetics of the discontinuous dissolution reaction has been studied in the Mg-Al system. It has been shown that the transport of the solute atoms during both reactions is governed by grain boundary diffusion.

Local values of the diffusivities at the migration front of the discontinuous precipitates in Al-22 at.% Zn alloy

Diffusion along migrating grain boundaries is a rate-controlling factor in many solid-solid phase transformations like discontinuous precipitation (DP), discontinuous dissolution (DD) and diffusion-induced grain boundary migration. In this paper it will be shown that some divergence between the tracer diffusivities obtained by the sectioning technique and the chemical diffusivity data obtained from DP and DD processes can be removed by applying a local concept for the determination of the kinetic parameters for these reactions. It means that the solute concentration in the {alpha} solution will be measured for the same individual cell whose solute profile across the {alpha} lamella was determined in the former DP process. An Al-22 at.% Zn alloy was chosen for the investigation for which reliable data for the tracer diffusivity of stationary grain boundaries of Al-Zn solid solutions over a wide range of zinc content were previously reported. As the DP and DD reactions are also well documented, the Al-Zn system is particularly useful for studies of the mutual relation between the diffusivities of stationary and moving grain boundaries.

Kinetics of discontinuous precipitation and dissolution in Cu–Ag alloys

The kinetics of discontinuous precipitation and dissolution of the cellular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag alloys. The growth rates of the cells were measured using optical microscopy. The interlamellar spacings of the primary cells and compositions of the depleted matrix were measured using scanning electron microscope and X-ray diffraction, respectively. The Cu–Ag alloys were observed to decompose into a lamellar structure consisting of alternate lamellae of the α (Cu-rich) and β (Ag-rich) phases when a solid solution of the alloy was aged below the solvus temperature. The rod shaped morphology of the β phase dispersed in the matrix of α was observed at all temperatures. The primary cell growth data were analysed using the theories of Cahn, Hillert, Sundquist, Turnbull and Petermann and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the grain boundaries. The results are consistent with the diffusivity values reported in the literature in the same temperature range. The discontinuous dissolution of the primary cells occurred above the solvus temperature as well as 30–40 K below it. The dissolution occurred primarily at the prior positions of grain boundaries as well as at the primary cells intersection, at least in the initial stages of dissolution. The discontinuous dissolution occurred by the diffusion of Ag along the interface boundary between the primary and dissolution cells. The diffusivity and mobility values obtained during dissolution are one order of magnitude smaller than those obtained for the discontinuous precipitation. This has been explained by volume diffusion ahead of the interface and not all of the free energy being used to drive the interface boundary.