Toward Optimization of the Grain Boundary Character Distribution in OFE Copper

Abstract

The authors used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM) has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, special fraction, was {approximately} 70% in the starting recrystallized material. High, moderate, and low temperature processing conditions were employed. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction. The authors approach is aimed at testing a hypothesis for the mechanism for GBCD optimization in Cu, i.e., it is possible to optimize the GBCD in Cu by selectively removing random grain boundaries through a strain-annealing process. Theymore » begin with a fully recrystallized material, deform it by a crucial amount, then apply a specific heat treatment schedule. The deformation is not intended to be sufficient to induce full recrystallization upon heating, but is intended to localize the deformation energy at random boundaries (since random boundaries are expected to be less efficient at transmitting dislocations than special boundaries). Upon heating, the stored energy near these random boundaries is expected to provide sufficient driving force to rearrange these boundaries into special types.« less