SEGREGATION TO SIGMA-5 [001] TWIST GRAIN-BOUNDARIES IN NI-CU ALLOYS

Atomistic simulations of segregation to the SIGMA-5 [001] twist boundary in Ni-Cu alloys have been performed for a wide range of temperature and composition all within the solid solution region of the phase diagram. In addition to the grain boundary segregation profile, grain boundary structures, free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. For all alloy bulk compositions (0.05 less-than-or-equal-to C less-than-or-equal-to 0.95) and temperatures (400 less-than-or-equal-to T(K) less-than-or-equal-to 1000) examined, Cu segregates strongly to the grain boundary. The width of the segregation profile is limited to approximately two atomic planes on each side. The resultant segregation profiles are shown to be in poor agreement with classical segregation theories. The grain boundary thermodynamic properties depend sensitively on the magnitude of the grain boundary segregation. The free energy, enthalpy, entropy of segregation and the change in grain boundary excess volume are shown to be linearly proportional to the magnitude of the grain boundary segregation.