The genesis of the cellular precipitation reaction
01 Nov 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science (Springer New York)-Vol. 3, Iss: 11, pp 2757-2767
TL;DR: In this article, the early stages of cellular precipitation in quenched and aged alloys were investigated using light and electron microscopy observations of the early stage of cellular formation. But the early development of cellular cells was not investigated.
Abstract: The morphology of cellular precipitation in a Cu-9.5 at. pet In alloy has been investigated by light and electron microscopy. Both cellular and general precipitation were observed to occur simultaneously in quenched and aged alloys while only cellular precipitation was observed to occur in isothermally aged alloys. Because of the presence of wide, solute rich, precipitate free zones in the vicinity of grain boundaries in the quenched and aged alloys, the early development of cellular precipitation was found to be identical for both types of heat treatment. From light and electron microscopy observations of the early stages of cellular precipitation a mechanism for the formation of cells was developed. At the start of aging, the unoccupied grain boundary begins to migrate under the influence of grain boundary migration forces as if it were in a single phase alloy. As the boundary migrates, solute segregates along it to form allotriomorphs which pin the boundary. The boundary continues to migrate and bows between the simultaneously forming allotriomorphs. With further aging, the allotriomorphs lengthen following the bowing boundary and the allotriomorphs become the initial precipitate lamellae of the developing cell as a steady-state lamellar structure develops. Assuming that the critical step in the development of a cell is the ability of the boundary to bow between the initial allotriomorphs, a criterion for the occurrence of cellular precipitation was developed.
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TL;DR: 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 as discussed by the authors.
Abstract: 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, ...
211 citations
TL;DR: This paper showed that grain boundaries and interphase boundaries provide high diffusivity paths in solids, but little bulk composition change can occur at temperatures where lattice diffusion is frozen out unless the boundaries migrate.
190 citations
188 citations
TL;DR: In this paper, the analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C.
Abstract: The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M23C6 carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M23C6 carbides.
177 citations
TL;DR: In this article, the crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni 3 Nb δ phase reported previously in Inconel 718 and Allvac 718Plus.
148 citations
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01 Jan 1958
11,521 citations
"The genesis of the cellular precipi..." refers methods in this paper
...It should be noted, however, that the criterion correctly predicts the occurrence of cellular precipitation in Cu-9.5 at. pct In at 420~ as a value of 14.1 was calculated for the left term in Eq. [ 14 ] using the same values for ~ala', pAFo, and V used previously and a value of 1 ~ for 2L....
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TL;DR: In this paper, a method has been developed for the study of grain boundary migration in high purity aluminum, which allows positive determination of the direction of the grain boundary movements, by means of which it was confirmed that the migration of grain boundaries resulting from surface energy takes place in the direction toward the centers of curvature of the boundaries.
Abstract: A method has been developed for the study of grain boundary migration in high purity aluminum, which allows positive determination of the direction of grain boundary movements. By means of this method it was confirmed that the migration of grain boundaries resulting from surface energy takes place in the direction toward the centers of curvature of the boundaries. On the other hand, strain induced grain boundary migration, such as occurs in recrystallization, proceeds in a direction away from the centers of curvature of the moving boundaries. It was also found that certain strained grains serve as nuclei for strain‐free grains of the same orientation which grow at the expense of neighboring strained grains. Most of the strain‐free grains formed during the annealing of high purity aluminum cold rolled up to about 40 percent are produced by this mechanism of strain‐induced boundary migration, without the formation of new nuclei.
379 citations
TL;DR: In this paper, a theory based on the known form of the recrystallization kinetics without necessarily introducing any anisotropy of grain boundary mobility is proposed, which is identical to the activation energy for grain boundary migration.
Abstract: Electron microscope observations on some polycrystalline metals suggest that after small to moderate deformation, recrystallization occurs by the migration of the original grain boundaries. A theory based on this mechanism can account for the known form of the recrystallization kinetics without necessarily introducing any anisotropy of grain boundary mobility. For this mechanism the so-called recrystallization activation energy is identical to the activation energy for grain boundary migration.
379 citations
TL;DR: In this paper, it was shown that at least two controlling kinetic processes are required to describe such a complex reaction, such as discontinuous precipitation and pearlite formation, in addition to diffusion control, and that the cell boundary moves with a velocity proportional to the net free energy decrease.
350 citations
TL;DR: In this article, the precipitation process and age hardening mechanism in a Mg-9 wt.% Al alloy were studied principally by transmission electron microscopy, and no evidence for the formation of either G.P. zones or a transition lattice of the Mg17Al12 precipitate was obtained.
309 citations