Tensile strength-ductility properties of face-centered cubic deformation twin/matrix structures

Large anisotropy of the tensile strength-ductility combination of the laminated twin/matrix (T/M) structure was found by studying the mechanical response of pre-twinned Cu-8at.%Al crystal structures, subjected to plastic tension under the different loading conditions. Three resolved shear stress-driven mechanisms, M 1 , M 2 , M 3 , were introduced to describe the deformation properties of the bimodal T/M structure. The “zero” tensile ductility (ε T ) accompanied by the tensile strength σ T about 550 MPa or above 900 MPa was established when the T/M structure was loaded almost parallel (M 3 ) or perpendicular (M 1 ) to the T/M interface. Consequently, the T/M structure revealed the minimal (“zero”) tensile strength-ductility combination (σ T × ε T ), when plastic yielding was initiated by M 1 or M 3 mechanism; both connected with T/M lattice shears occurring across the T/M interface. However, the T/M structure loaded at an angle about π/4 to the T/M interface revealed exceptionally large ductility (true strain equal to unity) accompanied by true stress of 850 MPa, i.e., the maximal combination σ T × ε T of 850 MPa, projected to about 1500 MPa for an analogous T/M structure of high manganese steels. All the “non-zero” cases of σ T × ε T were determined by activity of M 2 mechanism, i.e., T/M lattice shear parallel to the T/M interface. The σ T × ε T values depended on the length of M 2 deformation path terminated by a sudden entry of M 3 (M 1 ) mechanism. The entry, governed by the Schmid–Boas law, occurred always well before the Considère condition was met. The physical reasons of the observed anisotropy of the T/M structure are discussed.