Tensile strength-ductility properties of face-centered cubic deformation twin/matrix structures
摘要
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, M1, M2, M3, were introduced to describe the deformation properties of the bimodal T/M structure. The “zero” tensile ductility (εT) accompanied by the tensile strength σT about 550 MPa or above 900 MPa was established when the T/M structure was loaded almost parallel (M3) or perpendicular (M1) to the T/M interface. Consequently, the T/M structure revealed the minimal (“zero”) tensile strength-ductility combination (σT × εT), when plastic yielding was initiated by M1 or M3 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 σT × εT of 850 MPa, projected to about 1500 MPa for an analogous T/M structure of high manganese steels. All the “non-zero” cases of σT × εT were determined by activity of M2 mechanism, i.e., T/M lattice shear parallel to the T/M interface. The σT × εT values depended on the length of M2 deformation path terminated by a sudden entry of M3 (M1) 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.