Extrinsic Grain Research Articles

The effect of grain boundary precipitation on the stability of extrinsic grain-boundary dislocations in austenitic steel and aluminium alloy

The effect of grain boundary precipitation on the stability of extrinsic grain boundary dislocations (EGBD) was studied in alloys in which the occurence of depletion of definite alloy elements in grain boundary region is well documented, namely Cr-Ni-austenitic steel and Al-Zn-Mg alloy. It was found that depletion of grain boundaries of alloying elements that occurs in the process of precipitation on grain boundaries results in substantial decrease of EGBDs stability. Results are interpreted in terms of changes in grain-boundary diffusion coefficients.

Effet de la structure des joints de grains sur la déformation plastique des polycristaux

The interaction of lattice dislocations with high angle grain boundaries is considered from the point of view of recently discovered process of spreading of extrinsic grain boundary dislocations (EGBDs). The Lojkowski and Grabski model for the spreading kinetics is explained and the new experimental evidence is provided, showing that the spreading rate is a most sensitive available indicator of the state of grain boundary structure, allowing for the accurate determination of g.b. self diffusion coefficients and g.b. energy. With use of spreading kinetics measurements it is shown that as a result of spreading the grain boundary structure deviates from the equilibrium state, and that high temperature annealing transforms the structure back towards the equilibrium one. This process of transformation is called the recovery of grain boundary structure.The proof is delivered that the changes of grain boundaries structure produced by the spreading process exerts a strong effect on the value of the yield stress, and that the annihilation of dislocations through the spreading or EGBDs is an important mechanism of dynamic recovery, affecting the plastic flow at high temperatures. The recent results on Hall-Petch relation in stainless steel and aluminium and on Piobert-Luders bands formation in pure aluminium are presented and discussed in terms of changes in grain boundaries structure. It is concluded, that the changes in grain boundary state can influence the mechanical properties of polycrystal much more strongly than changes in grain size.

Grain boundary diffusion and free energy during the recrystallization of type 316 stainless steel

Experimentally obtained pairs of values of the temperature and time at which extrinsic grain boundary dislocations spread, i.e. the contrast of the extrinsic grain boundary dislocations disappeared, have been used to calculate the activation enthalpy ΔH b and grain boundary free energy γ GB in type 316 stainless steel cold rolled to 82% reduction in thickness and then recrystallized at 757°C. The calculated ΔH b value of 177.52 ± 2.08 kJ mol −1 is in excellent agreement with the value reported very recently by Patil and Sharma (1982) for type 316 stainless steel annealed at higher temperatures (900–1200 °C). Such agreement indicates that grain boundaries as well as recrystallization fronts in partially recrystallized type 316 steel are fully equilibrated in spite of the fact that they have incorporated a large number of dislocations from the heavily deformed matrix during recrystallization. This fact is accounted for on the basis of the molybdenum-enhanced lattice diffusivity which in turn causes the time necessary to attain complete equilibrium of a grain boundary structure to be significantly shorter.

Stability of extrinsic grain boundary dislocations in relation with intergranular segregation and precipitation

The present investigation emphasizes the conditions and mechanisms of stabilisation of EGBD's in grain boundaries of some alloys (Fe-9%Cr, Fe-4%Ni, Fe-1%Mo, Fe-1%V) with various low carbon contents. Our results show that EGBD's may be maintained in grain boundaries after a high temperature anneal under two conditions: a cosegregation of the substitutional element and the interstitial one (carbon), which requires a strong interaction between these two elements, and a sufficient level of cosegregated species, depending on whether the boundary is special or general. These species and especially the substitutional ones, which can hinder climb processes by decreasing the intergranular coefficient of diffusion D b , can strongly influence the accommodation processes of EGBD's. Stabilised EGBD's may then act as nucleation sites for carbide precipitates by a diffusion of segregated species along the dislocation or along the boundary towards the dislocation line. We then notice a déstabilisation of EGBD's, which is explained by the fact that accommodation of extrinsic dislocations does occur in grain boundaries.

Non-equilibrium state and recovery of grain boundary structure. II. Energetic Analysis

The energy of grain boundary (GB) with non-equilibrium structure caused by trapped lattice dislocations (TLD's) and TLD dissociation products — extrinsic grain boundary dislocations (EGBD's) — are estimated. The process of GB structure recovery, i.e. transition of non-equilibrium GB structure into equilibrium state, is analysed on the basis of the model of TLD dissociation into EGBD's and EGBD uniform distribution in GB. A process of this type appears to be energetically favoured for GB's of any type. The kinetics of the recovery of GB structure which is of diffusional character is considered. The differences in TLD stability in various GB's are caused by kinetic reasons. In the process of TLD absorption GB's possess non-stationary structure, which is of great importance for GB behaviour during deformational processes. A comparison between the results obtained and known experimental data is made. [Russian Text Ignored].

On the structure of tilt grain boundaries in cubic metals. III. Generalizations of the structural study and implications for the properties of grain boundaries

The results of atomistic calculations of long-period tilt boundaries, which were reported in the preceding parts I and II, are generalized and represented concisely by using two-dimensional lattices, called decomposition lattices. The basis vectors of a decomposition lattice characterize the two fundamental structural elements composing all boundaries in a continuous series of boundary structures. Conversely, the governing condition on the basis vectors is that the boundary structure can change continuously throughout the misorientation range between the boundaries represented by the basis vectors. On assuming that no discontinuous changes in boundary structure occur at non-favoured boundary orientations, and that all boundaries considered are stable with respect to faceting, the governing condition may be used to deduce selection rules for adjacent favoured boundaries and the existence of other favoured boundaries in the misorientation range between two given favoured boundaries. The necessary condition for a discontinuous change in boundary structure to be possible at a non-favoured boundary orientation is formulated. Various aspects of intrinsic and extrinsic grain boundary dislocations (g.b.ds) are treated. It is first shown that the observation of intrinsic g.b.d. networks in the transmission electron microscope does not necessarily imply that the reference structure, preserved by those g.b.ds, is a favoured boundary. Secondly, it is argued that extrinsic g.b.ds provide imperfect steps with Burgers vector components parallel to the boundary that do not exist in equilibrium high-angle tilt boundaries. Finally, an explanation of the physical basis of plane matching dislocations is proposed. A general classification of grain boundary properties is introduced that is based on the results of this investigation of grain boundary structure. It is argued that only properties, such as grain boundary diffusion, that depend exclusively on the atomic structure of the boundary core may be used to detect favoured boundaries. Favoured boundaries exist at those misorientations where such a property is continuous but its first derivative, with respect to misorientation, is not. Grain boundary diffusion, the energy against misorientation relation and grain boundary sliding and migration are then discussed.

On the Relation Between Temperature and Time of the Spreading of Extrinsic Grain Boundary Dislocations and the Grain Boundary Energy in an Austenitic Steel

On the Relation Between Temperature and Time of the Spreading of Extrinsic Grain Boundary Dislocations and the Grain Boundary Energy in an Austenitic Steel

On the Validity of Interpretation of Some Grain Boundary Processes Based on the Assumption about the Existence of Unrelaxed Extrinsic Grain Boundary Dislocations

On the Validity of Interpretation of Some Grain Boundary Processes Based on the Assumption about the Existence of Unrelaxed Extrinsic Grain Boundary Dislocations

Secondary grain boundary dislocations in [001] twist boundaries in MgO II. Extrinsic structures

Abstract Extrinsic secondary grain boundary dislocation (GBD) structures have been observed by weak-beam transmission electron microscopy in a variety of [001] twist boundaries in MgO. These structures were derived from segments of lattice dislocations embedded in the boundaries and could be interpreted as the result of the decomposition of the lattice dislocations into extrinsic GBDs and the subsequent interaction of the product GBDs with the intrinsic boundary structure. The results demonstrate that lattice dislocations in MgO are attracted to grain boundaries over a wide range of conditions and tend to remain embedded in the boundaries as extrinsic GBD structures. All observations could be rationalized on the basis of the CSL model for grain boundaries in cubic materials and were consistent with the intrinsic boundary structures described in Part I of the present work (Sun and Balluffi 1982). Furthermore, the results were similar in many respects to earlier results obtained with [001] twist boundaries in gold.

The effect of extrinsic grain boundary dislocations with unrelaxed and relaxed cores on the state of random boundaries in an austenitic steel

A transmission electron microscopic investigation employing in situ heating of thin foils has been carried out to determine the effect of extrinsic grain boundary dislocations (EGBDs) with unrelaxed as well as relaxed cores on the spreading temperature (Td) of EGBDs on random grain boundaries in an austenitic stainless steel. The results show that while Td is not measurably affected by the presence of unrelaxed EGBDs up to a density of a least 5.6 × 107/m, its value is observed to decrease with increasing density of EGBDs with relaxed cores. These changes in Td are discussed in terms of the possible changes in the structure and the energy of random grain boundaries which may be caused by the presence of these two types of EGBDs.

Spreading of extrinsic grain boundary dislocations on grain boundaries migrating in thin foil

Spreading of extrinsic grain boundary dislocations on grain boundaries migrating in thin foil

Investigation of extrinsic grain boundary dislocations in iron, iron - chromium and iron - nickel alloys

Investigation of extrinsic grain boundary dislocations in iron, iron - chromium and iron - nickel alloys

A model for stress-induced migration of tilt grain boundaries in crystals of NaCl structure

We present a qualitative model for stress-induced migration of tilt grain boundaries which we have observed during creep of sodium chloride. The elementary mechanism of the model consist in the propagation of the ledge associated to an extrinsic grain boundary dislocation. The model appears as an extension of the mechanism for growth of deformation twins and is compatible with but more general than Read's and Shockley's model for migration of low angle boundaries.

Observation of intrinsic and extrinsic secondary grain boundary dislocations in [001] high angle twist boundaries in MgO

The report describes a systematic electron microscope investigation of possible GBD structures in (001) high angle twist boundaries in MgO using bicrystals of controlled geometry. Boundaries of this type were selected because they are relatively easy to prepare, and because GBD structures have been extensively observed and studied (3,4,7) in boundaries of this type in the metal gold.

Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

We have found that hydrogen uranyl phosphate tetrahydrate HUO 2PO 4·4H 2O has a high proton conductivity. The ac conductivity was 0.4 ohm −1 m −1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole −1. The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H + conductivity thus inferred is attributed primarily to the high concentration of H +, which exists as H 3O + in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H 3O + ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.

Spreading of extrinsic grain boundary dislocations in austenitic steel

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Grain boundaries as point defect sources or sinks—Diffusional creep

The rate of emission and absorption of point defects at high angle grain boundaries in single phase materials is considered by means of an (intrinsic) boundary dislocation model. The emission (absorption) rate is found to depend on the boundary structure and the applied stress and/or point defect supersaturation. Ideal low energy boundaries act as point defect sources (sinks) only above a threshold stress that is proportional to the slope of the energy misorientation curve of the boundary. Boundaries deviating from ideal low energy misorientation relatíonships emit (absorb) point defects at all stresses with a rate that increases with increasing angular deviation from the ideal low energy misorientation relationship. At high stresses and/or large crystal sizes and also in the case of pure twist boundaries, extrinsic grain boundary dislocations may be important for the emission and absorption of vacancies. The results seem to agree with the experimental observations available.

The effect of creep deformation on the stability of intergranular carbide dispersions in an austenitic stainless steel

The stability of intergranular TiC in a 20% Cr-30% Ni, Ti stabilized stainless steel and the transformation of TiC to M23C6, has been investigated as a function of creep deformation over a wide range of stresses at 800° C. It was found that diffusion creep does not make a significant contribution to the general ageing process or to the transformation of TiC to M23C6. However, dislocation creep strongly accelerates this transformation and increases the general rate of coarsening of intergranular carbides. It is concluded that this acceleration occurs through the combined action of an increase in the number of available nucleation sites (extrinsic grain boundary dislocations) and dislocation enhanced diffusion.

Spreading of extrinsic grain boundary dislocations in plastically deformed aluminium

Electron microscope is used to study the dependence of the densities of grain interior dislocations and extrinsic grain boundary dislocations (EGBD's) in plastically deformed aluminium. The experiments show that the grain interior dislocation density increases with strain but the random high-angle grain boundaries are free from EGBD's which can be observed only on special boundaries (i. e. coincidence or low and medium angle). This behaviour of different types of grain boundaries can be explained in terms of EGBD core width after spreading which cannot exceed the grain boundary period of special boundaries. Die Abhangigkeit der Dichte von inneren Kornversetzungen und auseren Korngrenzenversetzungen (EGBD) in plastisch verformtem Aluminium wird elektronenmikroskopisch untersucht. Die Experimente zeigen, das die Dichte der inneren Kornversetzungen mit der Dehnung zunimmt, jedoch sind statistische Groswinkelkorngrenzen frei von EGBD, die nur auf besonderen Grenzen beobachtet werden konnen (d. h. Koinziden oder kleine und mittlere Winkel). Dieses Verhalten der verschiedenen Typen von Korngrenzen last sich erklaren mit einer EGBD-Kernbreite nach der Ausbreitung, die nicht nicht die Periode der Korngrenze der speziellen Grenzen uberschreiten kann.

The nature of dislocations in high-angle grain boundaries

Abstract The width of dislocations in grain boundaries is calculated by minimizing the interfacial energy of the boundary and the strain field energy of the dislocations. The calculations indicate that models, assuming equilibrium grain boundaries to consist of characteristic low energy structures with, if necessary, superimposed' arrays of localized misfit dislocations are of physical significance only for those boundaries whose energy depends strongly on the orientation relationship. For all other boundaries, dislocation models, although geometrically correct, seem to be of limited physical significance. Localized misfit dislocations are found to be unstable in these boundaries in the sense that their cores spread over the entire boundary area. The same applies to extrinsic grain boundary dislocations. They are found to be localized only in boundaries whose energy depends strongly on the mis-orientation. Measurements of the widths of misfit dislocations and observations on the width of extrinsic grain boundary dislocations agree with the results of the calculations. External forces (externally applied mechanical forces, thermal stresses, chemical forces) may result in the localization of dislocations in grain boundaries. This effect may be important for grain boundary sliding, phase transformations and the generation of dislocations from grain boundaries.