TEM Investigation of Random Grain Boundary Migration in Pure Aluminum

This research investigated the migration of random grain boundaries in recrystallized high-purity Al using static and in situ transmission electron microscopy (TEM). The driving force for grain boundary motion in the experiments was grain boundary curvature. The random orientation relationship between the grains across the boundary and the absence of dislocations or steps in the boundary were established by TEM. As the random grain boundaries were heated in situ in the TEM, parts of the boundaries were observed to rumple, and the magnitude of the rumpling increased with temperature. Segments of the rumpled boundary were seen to move forward without any apparent defects at approximately 0.7 of the melting temperature in two sets of experiments performed. The driving force for motion was determined to be 2.1 and 13.6 MPa in the two experiments, based on the local curvature of the rumpled boundaries. The lower limit of the migration velocities obtained from frame-by-frame analysis of video recordings were 160 and 80 nm/s, respectively, for the two cases, and the corresponding reduced mobilities were of the order 10 -14 and 10 -15 m 2 /s, respectively. The mobilities compare reasonably well with values for other types of grain boundaries in Al reported in the literature at similar temperatures, although the time resolution of the video (1/30 s) did not allow precise determination of the migration velocity. These results provide new insight into the mechanisms of random grain boundary motion.

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