THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF STRUCTURES AND ENERGIES OF SIGMA = 3, [112] TILT GRAIN-BOUNDARIES IN COPPER

Using atomistic computer simulations, symmetric and asymmetric Sigma = 3 [112] tilt grain boundaries in Cu were investigated. Equilibrium en ergies and structures were calculated by static and dynamic energy min imization. A semiempirical N-body potential served as a model of the i nteratomic forces in Cu. The atomistic structure of the grain boundary inclined at about 84 degrees to the {111} twin boundary was investiga ted by high-resolution transmission electron microscopy (HRTEM). Plott ed against the inclination angle phi(112) of the boundary plane, the c alculated grain boundary energies increase monotonically up to phi(112 ) approximate to 73 degrees. At larger inclination angles the data ind icate an energy minimum at about 80 degrees. The computer simulations predict that boundaries equilibrated at temperatures near T = 0K are p lanar for inclination angles phi 112 < 73 degrees, but consist of a th ree-dimensional layer of predominantly body-centred-cubic (bcc) Cu for inclination angles greater than 73 degrees. In all three-dimensional boundaries by bce layer is about 1 nm wide in the direction perpendicu lar to the boundary plane. By analysing the microfaceting of these bou ndaries and its influence on the misfit strain of the bcc layer, we ex plain the observation of a local energy minimum close to 80 degrees. F or the boundary inclined at about 84 degrees, the calculated atomistic structure and the HRTEM images display striking similarities, suggest ing that the bcc crystal structure is the equilibrium structure within this grain boundary at low temperatures.