Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses

Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for non-coincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.     

Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses

Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar 〈10$ \bar 1 $ \bar 1 0〉 tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for noncoincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.     

Grain boundary sliding during superplastic stretching of AA7475 cylindrical cups

Topographic and microstructural examinations were performed at the deformed surface and also in the bulk superplastically stretched 7475 aluminum alloy cups. Sliding of grain groups at shear surfaces spaced at approximately four grain diameters was observed at strain level, ε=0.6. The spacing between shear surfaces of such a cooperative grain boundary sliding (CGBS) decreases when the strain increases; it is approximately three grain diameters at ε=0.85. The pattern of the CGBS surfaces is consistent with that predicted using slip line field theory. Long range correlated migration of sliding grain boundaries, the formation of dispersoid free zones and fibres have also been observed. These processes can be rationalized in terms of CGBS. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electromigration of grain boundaries in aluminum

The electromigration of grain boundaries has been investigated with aluminum wires (99.999% purity, 1 mm in diameter) in the temperature range 340°–540°C under a current stress of 6×10³ A cm^-2. Grain boundary migration in aluminum is found to be reduced or enhanced by the currents stress when the grain boundary migrates in or against the current direction, respectively. The effects of additions of copper, silicon and zirconium to aluminum on the electromigration of the grain boundaries have also been examined. The contribution of electromigration to normal grain boundary migration is found to be increased by the addition of solutes. The results are discussed on the basis of the impurity drag mechanism of grain boundary migration.     

Microstructure evolution and electrical properties of yttria and magnesia stabilized zirconia

The paper contains the results of microstructure and chemical composition analyses of micrograins, which were formed on initial grain boundaries in ZrO₂-Y₂O₃ and ZrO₂-Y₂O₃-MgO systems. It has been found that the micrograins appear in the process of diffusion induced by grain boundary migration (DIGM). The observed processes can be described as both liquid film migration (LFM) and chemically induced grain boundary migration (CIGM). New micrograins had an increased content of Y₂O₃ and a cubic symmetry. Zirconia-yttria solid solutions with magnesia particulate addition showed an increased amount of migration nuclei and bigger size of new grains. However, no change in the chemical composition of the grains has been detected. The ionic conductivity measurements have shown that the activation energy (E_a) of conductivity at lower temperatures is connected to a DIGM-like process and to the distance of grain boundary migration. In the case of materials with dominating LFM process an increased grain boundary migration distance leads to a lowering of the activation energy of conductivity. Contrary to that, in the materials with dominating CIGM process an increase of migration zone causes increase of E_a values. The data obtained with respect to the type of DIGM process (LFM or CIGM) indicate that the grain boundary conductivity contribution increases with the CIGM distance.     

Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy

The adiabatic shear band (ASB) was obtained by split Hopkinson pressure bar (SHPB) technique in the hat-shaped specimen of a near beta-Ti alloy. The microstructure and the phase transformation within the ASB were investigated by means of TEM. The results show that the elongated subgrains with the width of 0.2-0.4 μm have been observed in the shear band boundary, while the microstructure inside the ASB consists of fine equiaxed subgrains that are three orders of magnitude smaller than the grains in the matrix. The β → ω_(althermal) phase transformation has been observed in the ASB, and further analysis indicates that the shear band offers thermodynamic and kinetic conditions for the ω_(althermal) phase formation and the high alloying of this alloy is another essential factor for this transformation to take place. The thermo-mechanical history during the shear localization is calculated. The rotational dynamic recrystallization (RDR) mechanism is used to explain the microstructure evolution mechanism in the shear band. Kinetic calculations indicate that the recrystallized fine subgrains are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Molecular dynamics simulation of copper bicrystal response to shear loading

The behavior of a large-angle grain boundary of the Σ = 5 (210)[001] special type in a copper bicrystal under shear loading conditions has been computer simulated. It is established that, simultaneously with the relative slippage of grains in the direction of applied load, the grain boundary shifts in the direction perpendicular to that of shear straining. This motion of the grain boundary exhibits a discrete character and leads to a growth of one grain at the expense of another. The mechanism of this displacement is analyzed and the influence of the loading rate and direction on the character of grain boundary motion is studied. The obtained results provide better understanding of the atomic mechanisms of plastic strain development in polycrystalline materials.     

Disconnections and other defects associated with twin interfaces

The general topological model for interfacial defects is reviewed and expanded, and the role of these defects in the coupled shear - migration of interfaces is explored. We focus on twinning in hexagonal metals for many defect examples. The definition of shuffles within the topological model is presented. The concept of partitioning of the rotational component of elastic distortions at a grain boundary or interphase interface has recently been elucidated. This work shows that rotational coherency has an important role in twinning. The role of disconnections in type II twins is presented.     

Texture competition during thin film deposition – effects of grain boundary migration

In this paper, we describe an implementation of grain boundary migration in the atomistic simulator of thin film deposition (ADEPT), and apply the simulator to study effects of the grain boundary migration on texture evolution. In the implementation, atoms are classified into two categories: those belong to a single grain and those at grain boundaries. An atom is defined as one at a grain boundary if it has more than half of its neighbors occupied and not all of the neighboring atoms are in the same grain. The grain boundary atom is attempted to re-align with neighboring grains to represent the grain boundary migration; the attempt probability is defined by the grain boundary migration coefficient. Our studies show that grain boundary migration does not always assist formation of texture with a top surface of the lowest energy. At the nucleation stage of thin film deposition, high migration coefficient of grain boundaries may enhance the formation of grain nuclei with top surfaces of higher energy, and therefore effectively may suppress formation of textures with a top surface of the lowest energy. This effect may provide an extra dimension to engineer textures of thin films.     

Sliding and Migration of Tilt Grain Boundaries in a Mg–Zn–Y Alloy

Sliding and migration of tilt grain boundaries in a Mg–Zn–Y alloy have been investigated on the atomic scale using aberration‐corrected scanning transmission electron microscopy. Grain boundary sliding is accommodated by non‐basal dislocations moving along the grain boundary; grain boundary migration is induced by the motion of grain boundary dislocations with synchronized grain boundary diffusion. Simultaneous sliding and migration of tilt boundaries take place in both Mg matrix and long period stacking ordered phases. These results provide evidence for occurrence of grain boundary motion, which may play a role in plasticity of this kind of Mg alloys.     

Effect of faceting on twin grain boundary motion in zinc

Faceting and migration of incoherent twin grain boundary in Zn [112?0] flat single crystals has been investigated. The stationary shape of the slowly migrating incoherent twin grain boundary of the twin plate tip was studied and migration velocity was measured in situ in the range from 473 K to 692 K using polarized light. Below 623 K the incoherent twin grain boundary represents the facet which forms at a 43° angle to the coherent twin grain boundary. Above 623 K the incoherent twin grain boundary represents the facet, whose position changes from the initial to 75° to the coherent twin grain boundary as temperature increases. Below 623 K the incoherent twin, grain boundary moves at very low experimentally determined activation enthalpy 19.3 kJ/mol of facet migration. Above 623 K the experimentally determined activation enthalpy for the facet migration is 154.4 kJ/mol, which is higher than the activation enthalpy of grain boundary diffusion in Zn. These results clearly indicate that there is a strong effect of the grain boundary shape on the migration velocity of the twin tip. We suppose that there is a grain boundary structural phase transition in this system: facet with high coincidence site lattice transforms to facet with low coincidence site lattice with disordered structure at 623 K.     

The effect of solutes on grain boundary mobility during recrystallization and grain growth in some single-phase aluminium alloys

The effect of solutes (Si, Mn, Mg) in quantities typical of commercial aluminium alloys, on grain boundary mobility in aluminium, has been investigated with in situ annealing and electron backscattered diffraction in the SEM, and grain growth experiments. The in situ experiments provided information on the migration of the high mobility tilt boundaries of misorientations close to 40°〈1 1 1〉. Grain growth experiments were used to investigate boundary migration in alloys of high solute content (1-5wt%Mg), and a comparison between the in situ and bulk experiments is made. The relationship between boundary velocity and driving pressure was found to be linear in all cases, and the activation energies for boundary migration were higher than those controlled by lattice diffusion of the solutes at higher solute concentrations.     

Effect of stress-induced grain growth during room temperature tensile deformation on ductility in nanocrystalline metals

In the present study defect-free nanocrystalline (nc) Ni-Co alloys with the Co content ranging from 2.4–59.3% (wt.%) were prepared by pulse electrodeposition. X-ray diffraction analysis shows that only a single face-centred cubic solid solution is formed for each alloy and that the grain size reduces monotonically with increasing Co content, which is consistent with transmission electron microscopy (TEM) observations. In the nc Ni-Co alloys, both the ultimate tensile strength and the elongation to failure increase as the Co content increases. The TEM observations reveal that stress-induced grain growth during tensile deformation is significantly suppressed for the nc Ni-Co alloys rich in Co in sharp contrast to those poor in Co. We believe that sufficient solutes could effectively pin grain boundaries making grain boundary motions (e.g. grain boundary migration and/or grain rotation) during deformation more difficult. Thus, stress-induced grain growth is greatly suppressed. At the same time, shear banding plasticity instability is correspondingly delayed leading to the enhanced ductility.     

Role of diffusional coherency strain theory in the discontinuous precipitation in Mg-Al alloy

Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. It is called discontinuous precipitation because precipitation occurs on prior matrix grain boundaries followed by grain boundary movement. The DP nodule consists of alternate lamellae of the precipitate and the matrix respectively. The chemical driving force for DP is one of solute supersaturation. Although solute supersaturation is responsible for precipitation, it has to be coupled with another driving force to explain grain boundary migration. This coupling driving force has been identified to be diffusional coherency strain which has been verified to be active in diffusion induced grain boundary migration and liquid film migration. To test diffusional coherency strain theory for discontinuous precipitation Mg-7Al and Mg-7Al-1Pb alloys were studied. While the fraction transformed was high at 6% in Mg-7Al alloy, dit was significantly low at 2% in Mg-7Al-1Pb alloy. The velocity of DP nodules decreased by half in alloy with Pb as compared to the alloy without Pb. Theoretical calculations also predict that the misfit parameter δ_th decreases with the addition of Pb. These observations are an evidence to the fact that diffusional coherency strain is the most active driving force for the movement of the grain boundaries of the DP nodules during discontinuous precipitation in Mg-Al alloy.     

Thermally assisted deformation of structural superplastics and nanostructured materials: A personal perspective

Optimal structural superplasticity and the deformation of nanostructured materials in the thermally activated region are regarded as being caused by the same physical process. In this analysis, grain/interphase boundary sliding controls the rate of deformation at the level of atomistics. Boundary sliding develops to a mesoscopic level by plane interface formation involving two or more boundaries and at this stage the rate controlling step is boundary migration. In other words, grain/interphase boundary sliding is viewed as a two-scale process. The non-zero, unbalanced shear stresses present at the grain/interphase boundaries ensure that near-random grain rotation is also a non-rate controlling concomitant of this mechanism. Expressions have been derived for the free energy of activation for the atomic scale rate controlling process, the threshold stress that should be crossed for the commencement of mesoscopic boundary sliding, the inverse Hall-Petch effect and the steady state rate equation connecting the strain rate to the independent variables of stress, temperature and grain size. Beyond the point of inflection in the log stress-log strain rate plot, climb controlled multiple dislocation motion within the grains becomes increasingly important and at sufficiently high stresses becomes rate controlling. The predictions have been validated experimentally.     

Grain boundary migration: misorientation dependence

The ability of grain boundaries (GB) to move has been found to be strongly dependent on crystallography, i.e. misorientation of the adjacent grains and orientation (inclination) of the GB in a crystal. Boundary mobility is rate-controlling in recrystallization and grain growth and thus, affects microstructure evolution and texture formation. This paper deals with recent advances in our understanding of misorientation and inclination dependence of grain boundary migration.     

Discontinuous precipitation of M23C6 in austenitic steels

Grain boundary precipitation of M₂₃C₆ has been studied in a 20% Cr-35% Ni stainless steel with two grain sizes during creep deformation at 700°C as well as during an ordinary ageing treatment at 700°C. A special etching technique was applied which showed how the grain boundary precipitation gave rise to depletion of alloying elements in a zone of uniform thickness, independent of the carbide distribution, and with a gradual decrease of the depletion towards the grain interior. At some places the carbide precipitation and grain boundary migration co-operated and in these cases there was a sharp change in alloying content across the grain boundary. This process was more frequent in creep tested samples and the degree of co-operation was larger in the coarse-grained material where even a few cases of lamellar, eutectic-like precipitation was observed. Such a grain size dependence is expected theoretically and is caused by the large difference in diffusivity between carbon and the other alloying elements. It is proposed that the various degrees of co-operation between carbide precipitation and grain boundary migration are all examples of discontinuous precipitation. The various proposed mechanisms for grain boundary migration during discontinuous precipitation are discussed on the basis of the present results.     

Comparison of the effects of La- and Ce-rich rare earth additions on the microstructure, creep resistance, and high-temperature mechanical properties of Mg-6Zn-3Cu cast alloy

The effect of 1 wt.% La- and Ce-rich rare earth (RE) additions on the microstructure, creep resistance, and high temperature mechanical properties of the Mg-6Zn-3Cu alloy (ZC63) was investigated by impression creep and shear punch tests (SPT). Impression creep tests were performed in the temperature range 423-498 K and under punching stress in the range 150-700 MPa for dwell times up to 3600 s. The ultimate shear strength (USS) was measured by the SPT in the temperature range 298-498 K. The results showed that Ce-rich RE was more effective than the La-rich RE in refining the as-cast microstructure, increasing the number density of eutectic phases at grain boundaries, and producing thermally stable Mg₁₂RE and MgRE compounds. The creep strength of the base alloy was remarkably improved by addition of both types of RE elements, although the Ce-rich RE-containing alloy showed better creep resistance. The addition of La-rich RE increased the shear strength of the base alloy, whereas Ce-rich RE addition had detrimental effects on the shear strength. This was attributed to the formation of a grain boundary network of Mg(Zn,Cu) Laves phases in Ce-rich RE-containing alloy. This grain boundary network with a bulky morphology promoted the initiation and propagation of cracks, leading to an adverse effect on the strength. This was in contrast with its positive influence on inhibiting grain boundary sliding and migration, which enhanced the creep strength of the alloy.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

Abstract

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with ∑3(011), ∑5(001) and ∑9(011) coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}(001) (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.

The ∑5 001l and ∑9 011l twist boundaries showed higher migration speed than ∑3(011) twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (?θ) from the ideal orientation relationship for a coincidence boundary. The increase of ?θ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the ∑9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed. © 2000 Elsevier Science Ltd. All rights reserved.     

Lattice distortion and thermal stability of nano-crystalline copper

Molecular dynamics simulations of high temperature annealing of copper bicrystals with varying grain sizes in nano-meter range have been carried out. Planar 1 1 1-tilt CSL grain boundaries are set. An EAM potential of FS type is used for calculating inter-atomic forces in copper. For comparison, similar simulations for aluminum and tungsten have been conducted. The results show that in the copper bicrystals of present grain boundary geometry, mismatch between the {1 1 1} planes of the neighboring grains occurs at the grain boundary, resulting in a general shear lattice distortion within the grains. The shear strain is inversely proportional to the grain size. The energy of such mismatched grain boundary is found lower than that of the mismatch-free grain boundary. For aluminum such kind of mismatch is much smaller, and for tungsten no such mismatch appears. The nano-sized copper bicrystals with grains smaller than a critical size are found instable at high temperature, where grain boundary motion and atomistic reconstruction lead to annihilation of the grain boundaries after an incubation time.