Grain boundary and triple junction enthalpies in nanocrystalline metals

We calculate the contribution to the total enthalpy of nanocrystalline computer-generated samples coming from grain boundaries, (GB's) and triple joints (TJ's). We show that the excess enthalpy per unit volume (excess enthalpy density) at the TJ is essentially the same as that found in the GB. This implies that TJ's and GB's are kinds of matter with equivalent departures from a perfect crystal structure, at least in the energetic aspect. By a proper account of the amount of GB's and TJ's, we show that the reported observations on decreasing GB energy with decreasing grain size in nanocrystallized amorphous Se [K. Ln and N. X. Sun, Philos. Mag Lett. 75, 389 (1997)] and negative TJ line tension from computer simulation results [S. G. Srinivasan et al., Acta Mater. 47, 2821 (1999)] are consequences of neglecting the relation between the grain boundary width delta and the grain size d, which in the nanophase regime may be of the same order of magnitude.