Grain boundary motion under high temperature low cycle deformation : study of mechanism, kinetics and driving force

The present study was performed to investigate the mechanism and kinetics of grain boundary motion during high temperature low cycle deformation (HT-LCF) of high purity aluminium bicrystals under two distinct conditions of cyclic deformation. The first part of the study emphasises the high temperature cyclic stress response of tilt grain boundaries where motion was caused by a plastic stress of cyclic deformation, which generates a difference in dislocation density of the two grains, these causes a driving force for boundary motion. Grain boundaries with misorientation angle above 13.8° were found to be mobile under cyclic deformation conditions. The differences of slip density (Dfi) and Schmid factors between the two grains of the deformed structures of bicrystal were observed most accountable for indexing of driving force. Larger stress reduces the chance for grain boundary motion due to homogeneous deformation structure in the two grains. The second part of this work includes the mechanism and kinetics during the motion of and tilt and twist symmetrical grain boundaries under completely elastic amplitude of stress at elevated temperatures, which permits an estimation of the activation parameters for such cyclic motion. Irrespective of geometry, two sharp regions of misorientation angle dependence of activation energies were estimated during the motion and tilt boundaries. For instance, in the present study it was in the range of 13.6° to 15.5° for and 7.8° to 11.7° for boundaries, which is comparable to the previous experiments [30-32]. However, transition angle between low angle and high angle grain boundary was difficult to establish for twist boundary motion, especially with regard to the measured activation energy due to a limited number of samples. Driving forces for motion of and tilt boundaries were calculated by using dislocation dynamics approach, assuming that the grain boundary motion is basically controlled by the movement of structural edge dislocations. Formation of trace-marks as observed on the sample surfaces during cyclic motion was found strongly dependent on the direction of force acting on dislocations rather than the mode of deformation. Similarly, the mechanism of the twist boundary motion has been discussed with in terms of the movement of structural screw dislocations by cross slip. Mechanism models with respect to driving force for the motion of grain boundaries have also been discussed. A relation between normal stress and shear stress were established in terms of the dislocation arrangement of the grain boundary. The contribution of sliding part and movement part during the entire boundary motion can approximately be predicted by the force acting on dislocation during deformation