Relative interfacial free energy measurements in Cu and Cu-Al alloys

Measurements of γtb/γgb for Cu and Cu-Al alloys utilizing transmission electron microscopy and selected-area electron diffraction techniques, and measurements of γSF for Cu-Al alloys from observations of extended dislocation nodes have shown that both γtb/γgb and γgF decrease with increasing atomic percent Al. Values of γtb = 26 erg/cm2 and γgb = 730 erg/cm2 at 700°C were estimated by correcting previous experimental values of γgb for Cu. By assuming the value of γgb in Cu to be reduced by approximately 10% upon the addition of 16 at % Al, the minimum value of the coherent twin boundary free energy over the range of alloying studied was estimated to be γtb = 12 erg/cm2. Mean values of γtb/γgb = 0.031, 0.029, and 0.018 were measured for the Cu-5, 11, and 16 at % Al samples respectively corresponding to an equilibrium temperature of 700°C. By plotting values of γtb/γgb as a function of misorientation, θ, between 〈110〉 directions for rotations about [100] for Cu and [110] for the Cu-Al alloys, a definite variation of grain boundary free energy with misorientation was established. The spread of histograms of γtb/γgb was also shown to be related to this variation in γgb with θ. Energy cusps were observed in the grain boundary free energy curves.