Recrystallization and Grain Growth

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Authors: P.J. Konijnenberg, Dmitri A. Molodov, Günter Gottstein
Abstract: In magnetically anisotropic materials a driving force for grain boundary migration can be induced by an external magnetic ¯eld. It is experimentally shown that annealing of locally deformed Zn single crystals in a suitably directed high magnetic ¯eld results in a growth of new individual grains. Velocities of some solitary moving grain boundaries were measured and their absolute mobilities were estimated at a single temperature. Results are discussed in terms of preferential grain orientation and boundary character.
763
Authors: Arnaud Lens, Claire Maurice, Julian H. Driver
Abstract: The migration rates (V) of “random” high angle grain boundaries (HAGB) during annealing of a cold deformed (e=1.3) high purity Al-0.1wt% Mn alloy were determined using a combination of in-situ annealing and EBSD in the SEM at temperatures between 200 and 330°C. The SEM heating stage used for these experiments is described and results on the local recrystallization kinetics of the Al-Mn alloy are presented. For this, the local stored energies (P) were determined by subgrain size and misorientation analyses to give the boundary mobilities (M) through the standard V = M.P equation. The solute drag “force” was analysed with the atomistic model of Lücke and Stüwe (1971) for a “loaded” boundary and used to estimate the diffusion rates in the above temperature range. The activation energies for boundary migration were found to be consistent with those of solute atoms moving behind the grain boundary, i.e. intermediate between the values for bulk and boundary diffusion of Mn.
771
Authors: D. Mattissen, D. Kirch, Dmitri A. Molodov, Lasar S. Shvindlerman, Günter Gottstein
Abstract: The motion and geometry of connected grain boundary systems with triple junctions in aluminium -10 ppm magnesium was investigated in-situ with a special designed SEM heating stage. The results show that triple junctions can have a marked influence on grain boundary motion. The grain area change with annealing time was from a hot stage in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (Von Neumann-Mullins relation). This is attributed to the effect of triple junction drag on grain growth.
777
Authors: Herbert M. Miller, David Saylor, Bassem S. El Dasher, Anthony D. Rollett, Gregory S. Rohrer
Abstract: Measurements of the grain boundary character distribution in MgAl2O4 (spinel) as a function of lattice misorientation and boundary plane orientation show that at all misorientations, grain boundaries are most frequently terminated on {111} planes. Boundaries with {111} orientations are observed 2.5 times more frequently than boundaries with {100} orientations. Furthermore, the most common boundary type is the twist boundary formed by a 60° rotation about the [111] axis. {111} planes also dominate the external form of spinel crystals found in natural settings, and this suggests that they are low energy and/or slow growing planes. The mechanisms that might lead to a high population of these planes during solid state crystal growth are discussed.
783
Authors: F. Papillon, P. Wynblatt, Gregory S. Rohrer
Abstract: Grain boundary (GB) segregation has been measured in Ca-doped MgO by examining intergranular fracture surfaces with Auger electron spectroscopy. The measurements reveal several interesting features. The composition of any given GB on the fracture surface is almost uniform, except for small variations due to deviations from planarity. There is a strong anisotropy of GB composition, which can amount to as much as a factor of six between low and high segregation GB's. Finally, although the compositions of opposite sides of a GB fracture are uniform, there are sometimes significant differences between the two sides, in agreement with a recently formulated model of GB composition as a function of GB character.
789
Authors: Svetlana Protasova, Vera G. Sursaeva
Abstract: The migration of the systems of tilt grain boundaries with a triple junction (<100>, <110> and <111>) in high-purity aluminium are presented. The experimental results demonstrate that the motion of grain boundary systems with triple junctions in Al at low temperature can be controlled by slowly moving triple junctions. In the high temperature range the triple junctions less affected the motion of the systems and the activation enthalpy was smaller than that at low temperature. The experiments revealed a drastic difference between activation enthalpy of grain boundary and triple junction motion at different temperatures. The compensation effect (linear dependence of migration activation enthalpy on preexponential mobility factor) at the migration of the systems with triple junction was observed. The compensation temperature was revealed to be close to the temperature for the triple junction mobility.
795
Authors: Vera G. Sursaeva
Abstract: When a bicrystal or polycrystal are subjected to a change in temperature, the individual responses of the two adjoining crystals may differ in a manner, which tends to produce a dilatational mismatch along grain boundaries. If compatibility is to be retained along the interface, an additional set of stresses must then be generated in order to conserve this compatibility. ‘Compatibility stresses’ will also be generated whenever a polycrystal is heated or cooled and the thermal expansion coefficients of the individual grains are different due to thermal expansion anisotropy. In such cases adjacent grains will attempt to change dimensions and develop mismatches by amounts controlled by the parameter Δa*ΔΤ, where Δa is the difference between the thermal expansion coefficients in the appropriate directions, and ΔΤ is the temperature change. These ‘compatibility stresses’ may be relieves if grain boundary motion, triple junction migration and grain growth are possible. These ‘compatibility stresses’ may play important role in the kinetic behavior of the microstructure ranging from influencing the behavior of lattice dislocations near the grain boundaries to promoting grain boundary and triple junction dragging or moving. The motion of the ‘special’ grain boundaries, triple junctions with ‘special’ grain boundaries and twins under the influence of internal mechanical stresses is the main subject of this paper.
801
Authors: Naoki Takata, Kenichi Ikeda, Fusahito Yoshida, H. Nakashima, Hiroshi Abe
Abstract: In the present study, grain boundary energy and atomic structure of <110> symmetric tilt boundaries in copper were evaluated by molecular dynamics (MD) simulation. From the simulations, the grain boundary energy of <110> symmetric tilt boundaries depended on misorientation angle and there were large energy cusps at the misorientation angles which corresponded to (111) S 3 and (113) S 11 symmetric tilt boundaries. It was found that the atomic structure of each <110> symmetric tilt boundary was described by the combination of three kinds of structural units which consisted of (331) S 19, (111) S 3 and (113) S 11 symmetric tilt boundaries and two single crystal units which consisted of (110) S 1and (001) S 1 single crystals. From the the analysis of the excess free volume in each grain boundary, it was found that the energy of structural units depended on the excess free volume of the units and that the misorientation dependence of grain boundary energy agreed with that of the free volume in grain boundaries.
807
Authors: D.N. Wasnik, Vivekanand Kain, I. Samajdar, Bert Verlinden, P.K. De
Abstract: Thermo-mechanical processing of type 304 and type 316L stainless steels done by (a) cold rolling to a reduction in thickness of 20 to 80 percent and (b) solution annealing to obtain a medium size of grains led to a considerable improvement in resistance to both sensitization and intergranular corrosion. The nature of the resultant grain boundaries was examined in a scanning electron microscope using orientation imaging microscopy in electron back scattered diffraction mode. Fraction of random and special grain boundaries were established for each set of thermo-mechanical processing. After appropriate sensitization treatments, the degrees of sensitization of these stainless steels were evaluated by double loop electrochemical potentiokinetic reactivation tests. Standard ASTM tests were used to evaluate susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). These studies showed that a particular combination of thermomechanical processing led to formation of over 75 percent random grain boundaries in the steels and this imparted resistance to sensitization and to IGC and IGSCC. This opens a new concept in grain boundary (GB) engineering of a high fraction of random GB increasing the resistance to localized corrosion like IGC and IGSCC. Textural studies were carried out with the help of X-ray and MTM-FHM software. It showed significant change of texture in type 304 stainless steel, while no change in the texture of type 316L stainless steel after cold rolling and annealing.
813
Authors: Tadao Watanabe, Kouichi Obara, Sadahiro Tsurekawa
Abstract: In-situ observations of a/g phase transformation were made to study the effect of grain boundary microstructure of the generation of a new phase and the migration of a/g interphase boundaries in an Iron-4.2at.%Cr alloy. It was found that triple junctions with more random boundaries could be the primary nucleation sites, while triple junctions with low angle and low S coincidence boundaries did not play a role as preferential sites. The migration of a/g interphase boundaries during heating across the transformation temperature showed the two stage behaviour characterized first by a stage with a migration velocity of 0.33-0.75µm/s and secondly a stage with 3.7-7.6 µm/s. It was also found that abnormal grain growth and a high density of S3 coincidence boundaries could occur in the a/bcc phase after cycling of a/g/ a phase transformation.
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