Grain boundary migration in Σ=5 bicrystals of an Fe-3%Si alloy

Migration of differently oriented grain boundaries was studied in the =5, 36.9°[100] tilt bicrystals of an Fe-3mass%Si alloy by the modified reversed-capillary technique. The principles of this method are outlined and discussed in connection with the application of multiple annealing of a single sample. It is shown that the errors introduced by both heating and cooling periods and by possible existence of an incubation period do not exceed the scatter of experimental data. A linear dependence between grain boundary migration velocity and driving force was found in most cases. The measured values of the product of grain boundary mobility and energy for individual grain boundaries differ substantially. The values of activation energy of migration of 332 kJ/mol, 392 kJ/mol, and 97 kJ/mol were found for the, {01}, {02} and (001)/(0 4) grain boundaries, respectively.     

Intergranular Stress Corrosion Cracking of Pure Copper 〈111〉 Tilt Bicrystals

Behaviour of stress corrosion cracking (SCC) in a series of pure copper bicrystals with a symmetrical 111-tilt boundary has been investigated. Tests were performed by the slow strain rate technique (SSRT) in 1M NaNO₂ solutions. The small-angle tilt bicrystals fractured in both intergranular and transgranular manners accompanied by a large amount of plastic strain to fracture while the large-angle bicrystals fractured in almost intergranular manner with a smaller plastic strain. Susceptibility of SCC increases with increasing misorientation and becomes relatively constant in large-angle grain boundaries. The local minima appeared at the 7(321) and 3(211) boundaries, suggesting that the susceptibility was partially affected by grain boundary energy. Stress concentration generated by the pile-up of trapped dislocations at the grain boundary could account for the high susceptibility of the intergranular SCC in large-angle grain boundaries.     

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO₃ bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport.The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible dip at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded.The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.     

Microstructure Observations for Diffusion Induced Recrystallization and Diffusion Induced Grain Boundary Migration on Surfaces of Zincified Cu Bicrystals With [1 1 0] Twist Boundaries

The formation behavior of the fine-grain region alloyed with Zn due to diffusion induced recrystallization (DIR) in the Cu(Zn) system was experimentally examined for the surfaces polished in different manners using a Cu bicrystal containing a [1 1 0] twist boundary with a misorientation angle of = 46° zincified at 673 K for 2.88 × 10⁴ s with Cu-15 wt% Zn and Cu-30 wt% Zn alloys by a capsule zincification technique. The extent and morphology of the fine-grain DIR region vary depending on the surface conditions and the composition of the Zn-source Cu-Zn alloy. For the specimen with the surface electrolytically polished in an etchant consisting of 20 vol% of nitric acid and 80 vol% of methanol, no DIR region was formed on the whole surface when the Cu-15 wt% Zn alloy was used as a Zn source.In order to observe the morphology of the moving grain boundary owing to diffusion induced grain boundary migration (DIGM) without influences of DIR, Cu bicrystals with [1 1 0] twist boundaries of = 32 ( 27), 39 ( 9), 46, 51 ( 11) and 55° were electrolytically polished in the etchant mentioned above. The polished Cu bicrystals were zincified at 673 K for 2.88 × 10⁴ s using the Cu-15 wt% Zn alloy as a Zn source. Remarkable surface relief and clear slip bands were recognized on the surfaces due to DIGM for the specimens with the 32 ( 27) and 46° boundaries. The moving boundary became zigzag owing to the slip bands parallel to the moving direction. On the other hand, such surface relief and slip bands were not observed for the specimens with the 39 ( 9), 51 ( 11) and 55° boundaries. The moving boundary was considerably irregular for = 39° whereas rather smooth for = 51 ( 11) and 55°. The migration behavior of the grain boundary was not affected by the interruption of the zincification.     

Excess Volume of the Solid/Liquid Interface in Fe-6 at.%Si Bicrystals Wetted by Liquid Zinc

The pressure effect on grain boundary wetting in Fe-6 at.%Si bicrystals of different misorientation angles but constant misorientation axis has been studied. The wetting agent was liquid zinc. It was found that the pressure for the dewetting transition is higher for the near 5 boundary than for the other general boundaries, where is the inverse density of the coincidence sites in the two misoriented crystal lattices. This result was explained assuming a thinner liquid film wetting the near 5 boundary than in the case of nonperiodic grain boundaries. Furthermore, the wetting angle increased with increasing pressure. The wetting angle dependence on pressure could be understood assuming a excess surface volume of the solid/liquid (S/L) interface higher than 0.2 nm. This is considerably higher than the estimated excess volumes of grain boundaries based on computer simulations. To explain this result, it was postulated that in the system studied, where diffusion of Zn, Fe and Si perpendicular to the S/L interface takes place, the S/L interface is relatively thick and the interaction between the two crystals separated by the melt extends over more than 2 nm distance. This long-range interaction was rationalized in terms of clusters of several atoms, detaching from the solid and dissolving in the melt at some distance from the bulk.     

Theory of Triple Junctions Mobility in Crystals with Impurities

We consider the steady state migration of the triple junction in the tricrystal with impurities which segregate strongly at the grain boundaries. If the mobility of impurities inside grain boundaries is much higher than the rate of impurity atoms jumps from the grain boundary into the bulk, the triple junction migration causes the divergence of the impurity content at the triple point. We show that this divergence can be relaxed either by the non-equilibrium segregation at the growing grain boundary or by the formation of the inclusion of the impurity-rich phase at the triple point. In the former case the dihedral angle at the triple point differs considerably from its equilibrium value and is strongly temperature-dependent. However, the triple junction cannot be described as an individual object with its own mobility. In the latter case of the cavity formation at the triple point the triple junction can be characterized by its own mobility. It is shown that the dependence of the triple junction migration rate on the driving force is approximately linear at the low migration rates and highly nonlinear at high migration rates. Moreover, there is the maximal allowable steady-state migration rate of the system triple junction-inclusion. For the higher migration rates the jerky motion of the triple junction occurs. Both models are in a good agreement with the experimental data.     

Molecular-Dynamics Method for the Simulation of Grain-Boundary Migration

A molecular-dynamics method for the simulation of the intrinsicmigration behavior of individual, flat grain boundaries is introducedand validated. A constant driving force for grain-boundary migrationis generated by imposing an anisotropic elastic strain on a bicrystalsuch that the elastic-energy densities in its two halves aredifferent. For the model case of a large-planar-unit-cell, high-angle(001) twist boundary in Cu we show that an elastic strain of1%–4% is sufficient to drive thecontinuous, viscous movement of the boundary at temperatures wellbelow the melting point. The driving forces thus generated (at thehigh end of the experimentally accessible range) enable aquantitative evaluation of the migration process during the timeframe of 10^-9 s typically accessible bymolecular-dynamics simulation. For this model high-angle grainboundary we demonstrate that (a) the drift velocity is, indeed,proportional to the applied driving force thus enabling us todetermine the boundary mobility, (b) the activation energy forgrain-boundary migration is distinctly lower than that forgrain-boundary self-diffusion or even self-diffusion in the melt and(c) in agreement with earlier simulations the migration mechanisminvolves the collective reshuffling during local disordering(melting) of small groups of atoms and subsequentresolidification onto the other crystal.     

Wetting transition on grain boundaries in Al contacting with a Sn-rich melt

The contact angle at the intersection of a grain boundary in Al bicrystals with the solid Al/liquid Al–Sn interphase boundary has been measured for two symmetric tilt <011> {001} grain boundaries with tilt angles of 32° and 38.5°. The temperature dependencies (T) present the evidence of the grain boundary wetting phase transition at T_w. The observed hysteresis is consistent with the assumption that the wetting transition is of first order. The determined discontinuity in the temperature derivative of the grain boundary energy is–5.6 J/m²K (T w1=617°C) for the boundary with a low energy (=38.5°) and –17 J/m²K (T w2=604°C) for the grain boundary with a high energy (=32°).     

Coupled motion of [10−10] tilt boundaries in magnesium bicrystals

Magnesium bicrystals were grown with symmetric and asymmetric tilt boundaries about the [10–10] axis using the vertical Bridgman technique. Isothermal constant load tensile tests were conducted on these bicrystals in the temperature range 300–500°C and relative displacements of the two grains were measured to obtain an appreciation for grain boundary motion characteristics. Coupled grain boundary motion was noted in almost all cases with the degree of tangential motion versus migration changing with tilt misorientation, temperature and applied stress. Specifically, within the family of symmetric bicrystals evaluated, a minimum in grain boundary displacement in the specimen plane was observed at a tilt misorientation of 20°. In specific stress/temperature regimes, rigid body sliding was observed for the particular case of a 35° asymmetric tilt misorientation. The ease of basal and prism slip in magnesium at the temperatures considered and the consequential impingement of intragranular dislocations on the bicrystal boundary and their decomposition and motion along the boundary are thought to play an important role in the observed coupled motion of these tilt boundaries.     

Observation of structural units at symmetric [001] tilt boundaries in SrTiO3

Incoherent Z-contrast imaging in the scanning transmission electron microscope allows atom column positions to be deduced directly from the experimental image, including locations where the column separation is less than the resolution limit. Maximum entropy analysis applied to the incoherent image locates the high-Z columns to an accuracy of ±0.2 Å. Oxygen coordination at the boundary plane can be deduced by high spatial resolution electron energy loss spectroscopy, and approximate column positions determined by simple bond-valence sum calculations. Observations of 25° (=85), 36° (=5) and 67° (=13) [001] symmetric tilt grain boundaries in SrTiO₃ bicrystals show that half columns are a ubiquitous feature of grain boundary structural units. The observed structural units can be combined to produce structural models for symmetric tilt boundaries over a 0–90° range. The =17 (410), =5 (310), and =5 (210) are found to be favored boundaries and the structures of all the other tilt boundaries are comprised of these units combined with =1 (100) and =1 (110) structural units. All the proposed boundary models show continuity of grain boundary structure over the entire misorientation range. The =17 (410) structural unit is asymmetric which induces microfacetting on all boundaries less than the =5, 36.87° misorientation.Work Supported by the U.S. Department of Energy under contract DE-Ac05-84OR21400.     

Atomic structure and electronic properties of the ∑37(610) and ∑29(520) [001] tilt grain boundaries in Ge

400 kV high resolution electron microscopy (HREM), deep level transient spectroscopy (DLTS) and steady state electrical measurements have been applied to 37(610) and 29(520) [001] tilt grain boundaries (GBs) in germanium bicrystals. The atomic boundary structures were revealed by experimental HREM images taken under different defocus conditions. Later, structure models were refined by means of a trial-and-error method applying alternatively the image simulation and the molecular static calculation of relaxed structures. The structures were shown to be consistent with the modified structural unit model. Although the structures are different for the two GBs studied, DLTS data and steady state measurements were found to be quite similar for both GBs. Thus, the results point to the extrinsic origin of localized deep states at the GBs. The analysis of DLTS spectra indicates the impurity segregation at the boundary, e.g., the formation of vacancy-type oxygen complexes of a donor-like state at E _c-0.21 eV, which results in the fluctuation of the potential barrier. Defects in the GBs—like facets, atomic steps and secondary grain boundary dislocations—which are characteristic of both boundaries can act as nuclei to the impurity segregation.Presented at the Workshop on High-Voltage and High-Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Grain boundary cracking in fatigued bicrystals

Compact tension (CT) shaped copper bicrystals containing the 3(111), 9(221), 41(338) or random [110] tilt boundaries either perpendicular or parallel to the tensile axis (TA) were tested to examine the effect of grain boundary (GB) structure on fatigue crack initiation and propagation. With both types of bicrystals, the GB resistance to intergranular cracking increases with the decrease in the value. The effect of air on fatigue crack propagation is found to be of primary importance. It is shown that the GB is not necessarily a preferential site for crack propagation in vacuum. A crack introduced intergranularly in air may assume a stransgranular mode of growth in vacuum. The initial GB structure greatly affects the susceptibility of GBs to corrosion in air and this determines their resistance to crack growth. The relative importance of slip band cracking as compared to GB cracking in vacuum is examined. The dominant mechanism of fracture is seen to be determined by two factors: the GB structure in the non-equilibrium state and the geometry of slip with respect to GB orientation.     

Grain boundary chemistry and grain boundary dislocations in bulk YBa2Cu3O7-δ

We report the results of our microchemical analyses of low- large-angle grain boundaries in bulk YBa₂Cu₃O_7- using nanoprobe energy-dispersive-X-ray spectroscopy (EDX) and electron-energy-loss spectroscopy (EELS). We observed periodic variation in the concentration of Cu along the boundaries, and oxygen depletion at the boundaries. We found that the chemistry of the grain boundary is very sensitive to grain boundary dislocations (GBDs), while, in turn, the configuration of the GBDs is very sensitive to the boundary misorientation and the boundary plane normal. The strain field associated with closely spaced GBDs reduced the density of mobile holes at the boundary, which is expected to be detrimental to the superconducting properties of the boundary. The possible structural transition of the grain boundaries from an oxygen-deficient state to a fully oxygenated state near a coincidence orientation is discussed, based on the reduction of the elastic strain energy of the boundaries.     

HRTEM Study on Σ7 Grain Boundary in Aluminium Bicrystals with and without Ga Doping

A HRTEM investigation was carried out on the 7{213} tilt grain boundary in bicrystals of both high purity aluminium and same aluminium doped with 10 ppm gallium. It was determined by image simulation that the bright contrast in the TEM images show the positions of atom columns. By measuring contrast profiles it was found that in the specimen of aluminium-10 ppm gallium, the atoms which are closest to the grain boundary, are anomalously shifted along the ‹121› direction compared to equivalent atom positions of the pure aluminium sample. This anomalous shift makes the disparity in the grain boundary structure of both samples. It is assumed that this anomalous shift is most likely the reason for the higher mobility of the grain boundary of the aluminium-10 ppm gallium sample.     

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO₃ grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped 5 SrTiO₃ grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.     

Triple Junction Mobility: A Molecular Dynamics Study

We present a molecular dynamics simulation study of the migration of individual grain boundary triple junctions. The simulation cell was designed to achieve steady state migration. Observations of the triple junction angle and grain boundary profiles confirm that steady state was achieved. The static, equilibrium grain boundary triple junction angles and the dynamic triple junction angles were measured as a function of grain size and grain boundary misorientation. In most cases, the static and dynamic triple junction angles are nearly identical, while substantial deviations were observed for low boundary misorientations. The intrinsic, steady-state triple junction mobilities were extracted from measurements of the rate of change of grain boundary area in simulations with and without triple junctions. The triple junction velocity is found to be inversely proportional to the grain size width. The normalized triple junction mobility exhibits strong variations with boundary misorientation, with strong minima at misorientations corresponding to orientations corresponding to low values of . The triple junctions create substantial drag on grain boundary migration at these low mobility misorientations.     

Twin boundaries with 9R zone in Cu and Ag studied by quantitative HRTEM

When highly inclined against the {111} plane of the coherent twin boundary, 3 110 tilt boundaries in Cu or Ag have a complex structure. As the boundary plane approaches the symmetrical {211} orientation, the grain boundaries decompose into two phase boundaries. Between these phase boundaries the metal adopts a rhombohedral crystal structure, denoted as 9R. Not the {211}-oriented boundary, but a boundary inclined by 8° against {211} has the minimum energy in this family of grain boundaries with 9R zone. Using high resolution transmission electron microscopy, we have studied the atomistic structure of this special boundary. An iterative structure refinement based on quantitative image analysis reveals the atomistic structure of the grain boundary at a well-defined level of confidence. Comparing the refined grain boundary structure with a model obtained by molecular statics calculations exposes small, but significant discrepancies. These probably arise because in the model the stacking fault energy is too small and the short distance repulsion is too weak. Grain boundaries of equivalent geometry in Ag and Al exhibit different widths of the 9R zone. Experimental observations support a theory relating the equilibrium width of the 9R slab to the stacking fault energy and the elastic properties of the material.Presented at the Workshop on High-Voltage and High Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Interdiffusion in two-layer Pd/Ag films. IV. Diffusion-induced Σ13 boundary migration

Fast growth of grains with homogeneous composition and 13 orientation was observed in Pd/Ag single-crystal thin films during annealing at 400°C. It was found that nucleuses of these orientation are contained in an initial structure. The grain growth obeys the mechanism of diffusion-induced grain boundary migration and the migration velocity was >-10^-7 m/s. The assumption was made that an elementary act of fast grain boundary migration is a transition of an atomic group determined by a Coinsidence Site Lattice, and the reconstruction occurs as correlated displacements of n atoms resulting in the transition of m atoms from one grain to another. A free activation energy of such a process and velocity of the special grain boundary were estimated in the frames of the model.