Dynamic behavior of grain boundaries with misorientations in the vicinity of Σ3 coherent and incoherent twin boundaries in Al bicrystals

The migration behaviour of different grain boundaries with misorientations close to the Σ3 CSL orientation relationship in high purity Al bicrystals under the capillary driving force and applied mechanical stress was investigated. The experiments were performed by an in-situ technique to observe and measure the boundary migration with a scanning electron microscope. The ability of the nearly Σ3 60° 〈 111 〉 incoherent {110} and {112} boundaries to move under capillary driving force was found to depend critically on the initial boundary inclination. While boundaries with inclinations near {112} can easily assume a curved shape and migrate, boundaries with an initial {110} plane remain stationary or form non-mobile facets. This is attributed to the essential anisotropy of the inclination dependence of the energy γ ( ψ ) of 60° 〈 111 〉 tilt boundaries with differently high torque d γ / d ψ around {112} and {110} inclinations, as revealed by atomistic simulations of the respective boundaries. The Σ3 70.5° 〈 110 〉 tilt boundary with the geometry corresponding to the coherent {111} twin boundary, was found to be immobile under both driving forces applied. The 59.2° 〈 111 〉 tilt grain boundary with geometry near the Σ3 {110} incoherent twin boundary was found to be quite mobile under applied shear stress. The measured migration activation enthalpy H = 0.45 eV for this boundary is the lowest among the values obtained in previous experiments for any other stress driven grain boundary in Al bicrystals of the same purity. Moreover, this boundary migrated with a zero coupling factor, i.e. without producing any measurable shear parallel to the boundary plane.