Systematics of the cellular precipitation reactions
01 Nov 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science (Springer New York)-Vol. 3, Iss: 11, pp 2717-2727
TL;DR: In this article, a systematic description of cellular reactions with special account to the discontinuous precipitation is given, including the constitutional, nucleation, and other conditions for its occurrence; the role of grain boundaries, dislocations, and vacancies for the mechanism; the effect of continuous precipitation before discontinuous rainfall; multiple discontinuous reactions; effects of grain boundary structure, third elements, and external stress; reactions in highly defect crystals and amorphous solids.
Abstract: A systematic description is given of cellular reactions with special account to the discontinuous precipitation. The following special aspects of this reaction are discussed: The constitutional, nucleation, and other conditions for its occurrence; the role of grain boundaries, dislocations, and vacancies for the mechanism; the effect of continuous precipitation before discontinuous precipitation; multiple discontinuous reactions; effects of grain boundary structure, third elements, and external stress; reactions in highly defect crystals and amorphous solids. It is shown that all phenomena can be understood either by effects on driving force or mobility of the reaction front.
Citations
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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: In this article, a review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded), and it is considered that fuller understanding requires increased application of the latest electron-optical techniques such as high-resolution microanalysis and in situ experiments in the highvoltage microscope.
Abstract: Discontinuous precipitation results in the formation of a two-phase lamellar structure behind moving grain-boundaries. The reaction is of interest because it generally has a deleterious effect on the mechanical, physical, and chemical properties of many commercial alloys. The combination of heterogeneous boundary precipitation and concurrent migration is complex and has resulted in a diverse range of proposed reaction mechanisms, several models of the growth kinetics, and many empirical observations of the effects of ternary additions and lattice strain. This review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded). It is considered that fuller understanding requires increased application of the latest electron -optical techniques such as high-resolution microanalysis and in situ experiments in the high-voltage microscope. Furthermore, to...
145 citations
TL;DR: In this paper, the orientation relationship between the precipitate types and the matrix has been determined by selected-area diffraction and using a computer analysis of electron diffraction patterns, the orientation relationships between precipitate type and matrix have been determined.
127 citations
TL;DR: In this paper, a discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy, which transforms the two-phase γ-γ′ parent microstructure into a three-phase cellular structure with a γ′ matrix containing Re-rich P-phase and agglomerated γ lamellae.
Abstract: A discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy. The reaction transforms the two-phase γ-γ′ parent microstructure into a three-phase cellular structure with a γ′ matrix containing Re-rich P-phase and agglomerated γ lamellae. The reaction has been studied in polycrystalline material and in bicrystals with varying degrees of boundary misorientation at temperatures in the range of T/Tm=0.78 to 0.85. The early stages of the reaction are characterized by heterogeneous nucleation of P-phase precipitates and migration of the grain boundary. At low-angle, near-tilt boundaries misoriented by less than 10 deg, nucleation of P-phase particles was observed, but the cellular reaction did not occur, due to limited boundary mobility and diffusivity. The high degree of supersaturation of Re and W in the initial γ-γ′ alloy appears to be the primary driving force for the reaction. Small amounts of creep deformation did not significantly influence the extent of the transformation. The diffusivity of Re associated with the moving boundary was calculated to be 5×10−8 cm2 s−1 at 1093 °C, which is approximately four orders of magnitude greater than the bulk lattice diffusivity of tungsten.
106 citations
TL;DR: In this paper, the first observation of a discontinuous reaction in dilute Al-Li alloys is reported, where a pre-existing dispersion of fine precipitates is converted to lamellae at the reaction front so that it is essentially a coarsening process.
88 citations
References
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TL;DR: In this paper, a quantitative atomistic theory of boundary migration was developed, where it is assumed that interaction forces exist between the foreign atoms and the boundary, increasing the concentration of foreign atoms in the boundary.
500 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, it was shown that grain boundary precipitation would vary in magnitude according to the degree of disorder existing at the boundary, and with position in the boundary between two given crystal grains if the boundary changed in direction.
Abstract: ACCORDING to the transition lattice theory, since in pure metals and single-phase alloys the only difference between the two grains which meet at a boundary is one of direction, the atoms at the boundary take up positions representing a compromise between the two crystal lattice directions. There thus exists a region, a few atomic diameters in thickness, over which a state of disorder exists, the extent of which will be expected to depend upon the relative orientation of the two lattices; there will exist angles for which this disorder is a minimum, an obvious example being the twinning angle. In addition, the direction of the boundary itself relative to the grains would be expected to produce an effect. Lateral misalignment of crystal planes would also be expected to produce ‘lack of fit’ at the grain boundaries, but this effect is most probably smoothed out by the imperfections in the crystal structure. It would be expected, therefore, that any phenomenon depending upon the degree of disorder existing at the boundary would vary in magnitude according to the relative orientations of the grains meeting at the boundary, and with position in the boundary between two given crystal grains if the boundary changed in direction. It is reasonable to suppose that where precipitation of a second phase occurs from the supersaturated solid solution, it occurs more readily in those regions where disorder is greatest, and it would, therefore, be expected that grain boundary precipitation would vary in this manner.
229 citations
TL;DR: In this article, the free energy of forming a special nucleus taking account of the boundary deformation was formulated. But this model was not applied to the case of tin microbicrystals, and no interconnection between tin lamellae was observed.
209 citations