Papers by Keyword: Subgrain Boundary

Abstract: Ultra-fine-grained (UFG) Al alloys have excellent mechanical properties such as high tensile strength without remarkable loss of elongation. Severe plastic deformation (SPD) process is an effective method for obtaining UFG microstructure. SPD-processed Al alloys has extremely high strength than the extrapolated from Hall-Petch relationship due to their microstructure with residual excess strain after dynamic recrystallization. Especially, on account of Al alloys have high stacking fault energy, the dislocation rearrangement in the dynamically recrystallized grain is difficult to form high angle grain boundary. As a result, there are substantial dislocation wall and low-angle grain boundary after SPD processing. These dislocations remain in the grain after recrystallization and partially form low-angle grain boundaries and subgrain boundaries. Consequently, the strength increases from Hall-Petch relationship, which is the degree of extra-hardening, was measured up to 200 MPa in as-SPD processed Al-3%Mg alloy. The authors previously reported that the low-angle grain boundaries distributed in the microstructure after the repetitive equal-channel angular extrusion processing. The strength difference calculated by Bailey-Hirsch equations was not in accord with measured extra-hardened strength. In this study, the effect of grain boundary distributions on the extra-hardening was investigated by changing SPD-processing and subsequent annealing conditions.

Abstract: The effect of sub-grain on the yield stress of pure copper single crystals with the [253] orientation was investigated by using the etch pit technique. The single crystal plates were successfully prepared from the seed crystals, which were produced at the melting temperature of 1473
K by the Bridgeman method. The present investigation confirmed the Hall-Petch relation concerning the effect of sub-grain boundaries on the macroscopic yielding of pure copper. The result derived from the extrapolation of the relationship of critical resolved shear stress (CRSS) and the initial dislocation density and sub-grain size is in good agreement with the evaluation in high purity copper single crystals of low dislocation density.

Abstract: Subgrain rotation is a common mechanism of continuous dynamic recrystallization in minerals and some metals. The mechanism involves new grain boundary formation by progressive rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This paper reviews the status of our current knowledge of rotation recrystallization in minerals. In minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚. In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have formed. Current models assume that all subgrain boundaries increase in misorientation with strain. However, recent studies show that many different types of subgrain boundary develop in minerals. The formation of new high angle grain boundaries is only likely along some types of geometrically necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle network of subgrain boundaries that develop in many minerals may be incidental boundaries, as found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries formed by trapping of statistically stored dislocations. New and extended models for rotation recrystallization are needed that consider i) incidental subgrain boundaries as well as different types of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link between the development of subgrain misorientation and texture development.