Diffusion induced grain boundary migration in thin gold/copper films

Au/Cu thin film bimetallic couples containing well defined tilt grain boundaries have been annealed in situ to observe the phenomenology and kinetics of DIGM and DIR. The discontinuous nature of the migration process has been directly observed and correlated with changes in the mobility of the grain boundaries with misorientation. The variations in the phenomenology of DIGM/DIR with increasing temperature are discussed, and some results of other workers reinterpreted in the light of new concepts of the nucleation process of DIR, and the possibility of high temperature DIGM.     

Diffusion driven grain boundary migration

If the grain boundary diffusion coefficient of the two species in a binary alloy differ, diffusion driven grain boundary motion should lead to predictable surface relief and shape changes. This leads to predictions and equations which would allow the determination of the separate diffusion coefficient of each species as well as the chemical diffusion coefficient for the boundary, D_b. For a thick sample the stress gradients built up by differences in the fluxes of the two species can lead to a reduced apparent D_b, especially if the solute boundary diffusion coefficient, D′₂, is appreciably greater than that of the solvent. D′₁. For Zn diffusion into Fe it appears that D′_Fe ⪢ D′_Zn. The stress built up will also slow the boundary motion.     

Metastable grain boundary segregation in copper containing tellurium

Evidence for metastable grain boundary segregation has been experimentally obtained in Cu alloyed with 25 to 100 ppm Te. The solution-treated material fractured in a ductile intergranular mode. After aging for a short time; i.e., one day at 540°C, the fracture is intergranular and is attributed to the observed intergranular segregation of Te. The magnitude of the tellurium segregation and its local nature are established using Auger electron spectroscopy. Further aging results in precipitation of a stable phase throughout the grains and grain boundary fracture no longer takes place.     

Mechanism for diffusion induced grain boundary migration

Grain boundaries are found to migrate under certain conditions when solute atoms are diffused along them. This phenomenon, termed diffusion induced grain boundary migration (DIGM), has now been found in six systems. The observed phenomenon and empirical data are used to discard certain concepts for the driving force and the mechanism. We propose a mechanism in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.     

The role of grain boundary migration in reheat cracking

Prior austenite grain boundary (PAGB) reheat cavitation has previously been shown to be controlled by a fine dispersion of incoherent PAGB particles and by the concentration of the overall deformation into deformation zones adjacent to the PAGB. A series of low-alloy baintic CrMoV steels have been investigated to establish the microstructural basis for PAGB deformation zones. It is shown that the fundamental cause of reheat cavitation in low alloy bainites is stress induced Grain Boundary Migration, which initiates at temperature immediately upon application of a load. The migrating boundary sweeps out a volume of material that is both particle and dislocation-free, relative to the secondary hardened grain interiors. This creates a mechanically weak zone at the PAGB. The overall strain during deformation is subsequently concentrated into these regions, producing extensive plastic deformation leading to failure by cavity growth and crack propagation within the PAGB deformation zone. The presence of large, stable carbides in high chromium CrMoV steels prevents PAGB migration and thus eliminates the PAGB deformation zones. Also matrix carbide coarsening in the high chromium steels leads to an increase in matrix recovery rates, which further permits more uniform deformation of the material.     

Diffusion driven grain boundary migration in iron during zincification

At temperatures where only grain boundary diffusion is active in a diffusion couple, solute diffusion along the boundary can cause the boundary to migrate, mixing solute into the lattice swept by the moving boundary. This phenomenon—diffusion driven boundary migration (DDBM) — was investigated in the iron-zinc system. The (∼40 μm) iron foils and zinc vapor sources were sealed together in evacuated Vycor tubes and annealed at 550 °C to 630 °C for different times. The migration rate of grain boundaries was nearly constant throughout the transformation of the foil to alloy. Migration rates were about the same in pure iron and decarburized steel sheet. The apparent activation energy of DDBM (269 kJ/mol) was well above that expected for boundary diffusion of zinc in iron and is felt to reflect the role of creep in accommodating the local expansion that accompanies the addition of zinc. This expansion is in the plane of the sheet and indicates that D′_Zn in the boundary greatly exceeds D′_Fe. Inbulk samples annealed under the same conditions boundaries did not move (in 16 hours). This is consistent with a marked difference in the boundary mobility of Zn and Fe.     

Cyclic grain boundary migration and sliding in pure aluminum

Reverse bending fatigue tests were conducted on high purity polycrystalline aluminum in the temperature range from 298 to 773 K. Observations show that the grain boundaries migrate and slide at temperatures above ≈500 K. The activation energy for migration is estimated as 76.1 ± 5.0 kJ mol⁻¹ and this is consistent with other measurements on high purity Al. Migration occurs cyclically but there is a small proportion of the boundaries, of the order of 10%, which are immobile and exhibit no measurable migration. Grain boundary sliding occurs at both the migrating and immobile boundaries, but at the migrating boundaries the total sliding offset, measured by a projection of interference fringes across the migrated region, tends to be small. Careful inspection shows that migration usually precedes sliding at the migrating boundaries, and the migration of triple junctions leads to the formation of very diffuse triple point folds.     

Effect of pressure on grain boundary migration in aluminium bicrystals

The temperature and pressure dependencies of 〈100〉, 〈110〉, and 〈111〉 tilt grain boundaries in bicrystals of pure aluminum were measured, and the corresponding activation enthalpies and activation volumes were determined. While the activation enthalpy strongly depended on orientation, in particular close to low Σ coincidence orientation relationships, the activation volume was found independent of orientation except for 〈110〉 tilt boundaries. The absolute values of the activation volume for 〈100〉 and 〈111〉 tilt boundaries were about the same as for bulk self-diffusion, which agrees with previous measurements on polycrystals. The activation volume for migration of 〈110〉 tilt boundaries substantially exceeded the activation volume for bulk self-diffusion. It is concluded that at least 〈110〉 tilt boundaries migrate by a group mechanism rather than by diffusive jumps of single atoms.     

Observations on the analysis of grain boundary migration data

Abstract

Stress is laid upon the importance of first analyzing the velocity/driving force relationship exhibited by experimental data, as against the frequently employed procedure of assuming a specific form.

The contributions of the intrinsic and solute drag contributions to the macroscopic drag are discussed in terms of the formulations of Turnbull, Cahn, Lücke and Stüwe. The advantage of considering experimental data in terms of drag parameters is indicated, with special reference to the low velocity/low driving force regime. Grain growth data from the literature for tin with dilute additions of Pb, Bi and Ag, zinc with additions of aluminum, and high purity lead have been analyzed in detail, employing the procedures described.

Résumé

Contrairement à la méthode fréquemment utilisée et qui suppose une forme spécifique, les auteurs soulignent l'importance d'une première analyse de la relation vitesse-force motrice telle que révélée par les résultats expérimentaux.

Ils discutent, selon les formulations de Turnbull et de Cahn, Lücke et Stüwe, des contributions respectives du traînage macroscopique, du traînage intrinsèque et de celui dù aux solutés. L'avantage de considérer les résultats expérimentaux en termes de paramètres de traînage est souligné spécialement pour le régime faible vitesse-faible force motrice. En utilisant la méthode décrite, les auteurs font une analyse détaillée, en fonotion des résultats expérimentaux publiés, de la croissance du grain pour l'étain contenant de faibles additions de plomb, bismuth et argent, pour le zinc contenant des additions d'aluminium et pour le plomb de haute pureté.     

Bicrystal studies of diffusion-induced grain boundary migration in Cu/Zn

In this article, we present results of systematic studies of the effects of grain boundary parameters upon diffusion-induced grain boundary migration (DIGM). It is shown that grain boundary velocity, solute penetration depth, concentration of deposited solute, and activation energy are all strongly and reproducibly dependent upon the grain boundary parameters. The effects are also somewhat different in the cases of symmetrical and asymmetrical boundaries, indicating the importance of the boundary inclination. It is shown that the observed differences can be rationalized in terms of the variations of the driving force for DIGM and the grain boundary mobility. Fu-Sen Chen and Girish Dixit, formerly Graduate Students, Department of Materials Science and Engineering, State University of New York at Stony Brook This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Morphological changes due to diffusion induced grain boundary migration

The evolution of the morphological changes that are caused by diffusion induced grain boundary migration (DIGM) has been studied on Fe exposed to Zn vapor at 600°C. The changes can be divided into four types; the first three involve pre-existing grain boundaries while the fourth is the growth of zinc-rich hillocks on the surfaces of grains. Those changes that occur at grain boundaries comprise:

• 1.(a) normal DIGM in which boundaries merely migrate,
• 2.(b) a combination of normal DIGM and the formation of new grains which grow in the other direction
• 3.(c) the nucleation, along a boundary, of two sets of new grains which grow in opposite directions and then coalesce giving rise to four orientations where there was initially two. Types b and c are interpreted as arising from diffusion induced boundary dissociation. Dezincification experiments showed that the boundaries return to their former positions without reversing topographic and compositional changes produced by their earlier migration.

     

A metallographic study of diffusion-induced grain boundary migration in the Fe-Zn system

Diffusion-induced grain boundary migration has been studied by zincification of pure iron. The temperature dependence of the rate was measured at 460–650°C A change in morphology was observed between 460 and 650°C and may explain the fact that the phenomenon changed character at higher temperatures. The temperature dependence of v/D^hδ may be decisive, v being the rate of migration. D^h the grain boundary diffusivity and δ the thickness of grain boundaries. The concentration behind the migrating boundaries was studied as function of the distance from the surface by microprobe measurements and allowed the grain boundary diffusivity to be evaluated as function of temperature.Fine zinc-rich grains of ferrite sometimes nucleate on the surface a form and complete fine-grained surface layer. After a while it disappears by abnormal grain growth but the increased zinc content remains.     

Chemically induced grain boundary migration and recrystallization in Al2O3

The chemically induced grain boundary migration in Al₂O₃ has been observed during depletion of Fe₂O₃ from Al₂O₃Fe₂O₃ solid solution as well as additon of Fe₂O₃ in Al₂O₃. Many migrating boundaries are faceted. In some receding grains, the faceted planes of different grain boundaries advancing into the same grain are parallel to each other. The faceting is attributed to the anisotropy of driving force in thin diffusion layer of the receding grain; this result provides an experimental support for coherency strain energy as the major driving force for the migration. When the supply of the solute is excessive during alloying, recrystallization occurs in Al₂O₃ grains.     

Characterization of diffusion induced grain boundary migration in the Ag/Cu system

The phenomenon of diffusion induced grain boundary migration (DIGM) when silver diffuses along copper's grain boundaries was confirmed through the characterization of the microstructure and the morphology of the migrated boundaries, the discontinuity and asymmetric character of the concentration profile, the dislocation configuration and the kinetics of migration by means of SEM, EPMA, TEM and AEM. The experimental results were discussed to prove the existence and the characteristics of DIGM in Ag/Cu system.