Attributing this enthalpy release exclusively to the removal of grain boundaries in stage B, a specific grain boundary energy(2)γ=Hρ3dini-1-dfin-1=0.85±0.08Jm-2is estimated using the initial and final crystallite diameters of stage B, as given above (dini=222nm, dfin=764nm), as well as the Cu bulk value of 8.92gcm-3 for the mass density ρ.