Papers by Keyword: Grain Boundary Migration

Abstract: It is shown that an externally applied mechanical stress field can change the kinetics of individual grain boundaries. Moreover, such mechanical stresses also have influence on grain growth and recrystallization kinetics and can strongly affect the microstructure evolution, so that the application of mechanical stresses during annealing can be used as a new approach in the field of grain boundary engineering.

Abstract: Uniaxial compression deformation is conducted on solid solution Al-3mass%Mg and Al-3mass%Mg-0.2mass%Sc with Al3Sc precipitates in the strain rates ranging from 1.0×10-4s-1 to 5.0×10-3s-1 at 723K. High temperature yielding is observed. Fiber texture is constructed in all the deformation conditions. While the main component of the fiber texture changes from {011} to {001} in Al-3mass%Mg alloy with an increase in strain rate, no big change in texture main component is seen for Al-3mass%Mg-0.2mass%Sc alloy with Al3Sc precipitates. It is experimentally shown that the development of {001} fiber texture can be attributed to the grain boundary migration.

Abstract: Orientation-controlled copper bicrystals containing symmetrical 70o [0 0 1] tilt boundaries were deformed in tension at 923 K and at three initial strain rates from 4.2 x 10-5s-1 to 4.2 x 10-3s-1. The load was applied parallel to the grain boundary so as to eliminate grain boundary sliding. The nucleation of dynamic recrystallization (DRX) was investigated using optical microscopy and orientation imaging microscopy methods. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply bulged grain boundary regions. The critical strain for nucleation was less than one-half of the peak strain and largely independent of the strain rate. At a fixed strain, nucleation is more frequent and the grain size finer as the strain rate is increased. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, most of the twinning plane traces were parallel to the inactive slip traces of the bicrystals. This indicates that twin variant selection is essentially unaffected by dislocation motion. The observed mechanism of nucleation of DRX is discussed in relation to the occurrence of GBM and twinning.

Abstract: Annealing is an important mechanism of microstructural modification both in rocks and metals. In order to relate directly changes in crystallographic orientation to migrating boundaries the researcher has the option to investigate either samples where the grain boundary motion can be directly tracked or a series of samples exhibiting successively higher degrees of annealing. Here we present results from rock samples collected from two well characterised contact aureoles (a volume of rock heated by the intrusion of a melt in its vicinity): One quartz sample in which patterns revealed by Cathodoluminescence (CL) indicate the movement of grain boundaries and a series of calcite samples of known temperature history. Electron backscatter diffraction (EBSD) analysis is used to link the movement of grain, twin boundaries and substructures with the crystallographic orientation / misorientation of a respective boundary. Results from the quartz bearing rock show: (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area, and (c) a gradual change in crystallographic orientation in the direction of growth. Observations are compatible with a growth mechanism where single atoms are attached and detached both at random and at preferential sites i.e. crystallographically controlled sites or kinks in boundary ledges. Strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. Calcite samples show with increasing duration and temperature of annealing: (a) systematic decrease of the relative frequency of low angle grain boundaries (gbs), (b) decrease in lattice distortion within grains, (c) development of distinct subgrains with little internal lattice distortion, (d) change in lobateness of gbs and frequency of facet parallel gbs and (e) change in position of second phase particles. These observations point to an increasing influence of grain boundary anisotropy with increasing annealing temperature, while at the same time the influence of second phase particles and subtle driving-force variations decrease. This study illustrates the usefulness of using samples from natural laboratories and combining different analysis techniques in microprocess analysis.

Abstract: The migration of individual special [ ] 0 1 10 tilt grain boundary (GB) with Σ =15 and misorientation angle 29° in Zn bicrystal have been investigated. The stationary shape of migrating GB has been studied and the migration rate has been measured by optical microscopy in situ between 558 and 683 K using polarized light. In certain experimental runs the migrating GB is faceted and moves thermally activated. Its kinetics follows the Arrhenius type dependence despite the fact that shape of moving GB depends on temperature. After detachment from impurity cloud the [ ] 0 1 10 tilt GB migrates activationless in the temperature interval 618÷683 K. The detachment temperature is 618 K. The non-physically high value of the apparent migration activation enthalpy can appear due to the change of GB shape from faceted to smooth and back.

Abstract: We present results of atomistic computer simulations of spontaneous and stress-induced grain boundary (GB) migration in copper. Several symmetrical tilt GBs have been studied using the embedded-atom method and molecular dynamics. The GBs are observed to spontaneously migrate in a random manner. This spontaneous GB motion is always accompanied by relative translations of the grains parallel to the GB plane. Furthermore, external shear stresses applied parallel to the GB and normal to the tilt axis induce GB migration. Strong coupling is observed between the normal GB velocity vn and the grain translation rate v||. The mechanism of GB motion is established to be local lattice rotation within the GB core that does not involve any GB diffusion or sliding. The coupling constant between vn and v|| predicted within a simple geometric model accurately matches the molecular dynamics observations.

Abstract: A strong static magnetic field (SSMF) of about 10 T was introduced to the aging process of AZ91 magnesium alloy. Comparing with conventional aging, in the first stage of aging with SSMF, discontinuous precipitation of Mg17Al12 at grain boundary was accelerated. The magnetically induced grain boundary migration might be responsible for this acceleration effect. The density of the Mg17Al12 continuous precipitates inside the grains was increased and the precipitation plates became thinner in SSMF pre-aged specimens, which might be ascribed to the retarded volume diffusion resulted from the SSMF.

Abstract: Grain growth simulations using the microstructure simulation system Elle have been performed in materials with a pre-existing grain shape preferred orientation. As might be expected, the microstructure is completely modified by the end of the experiment, and grain areas have increased by a factor of seven. The area of material swept by the migrating grain boundaries was monitored, and it was found that at every stage, virtually all of the grains which survived the grain growth process contain one and only one core of unswept material. Remarkably these remnant unswept cores preserve a useable record of the grain size and orientation of the original grains. This work suggests that it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the polycrystal, and perhaps even reconstruct the grain boundary kinematics. The identification of unswept cores also has the potential to help unravel the evolution of grain boundary chemistry during grain boundary migration.

Abstract: Molecular dynamics (MD) simulation was performed to study the stress induced grain boundary migration caused by the interaction of dislocations with a grain boundary. The simulation was carried out in a Ni block (295020 atoms) with a Σ = 5 (210) grain boundary and an embedded atom potential for Ni was used for the MD calculation. Stress was provided by indenting a diamond indenter and the interaction between Ni surface and diamond indenter was assumed to have a fully repulsive force to emulate a traction free surface. Results showed that the indentation nucleated perfect dislocations and the dislocations produced stacking faults in the form of a parallelepiped tube. The parallelepiped tube was comprised of four {111} slip planes and it contained two pairs of parallel dislocations with Shockley partials. The dislocations propagated along the parallelepiped slip planes and fully merged onto the Σ = 5 (210) grain boundary without emitting a dislocation on the other grain. The interaction of the dislocations with the grain boundary induced the migration of the grain boundary plane in the direction normal to the boundary plane and the migration continued as long as the successive dislocations merged onto the grain boundary plane. The detailed mechanism of the conservative motion of atoms at the grain boundary was associated with the geometric feature of the Σ = 5 (210) grain boundary.