ON THE FLOW-STRESS OF [100]-ORIENTED AND [111]-ORIENTED CU-MN SINGLE-CRYSTALS - A TRANSMISSION ELECTRON-MICROSCOPY STUDY

Cu-Mn single crystals oriented for multiple slip with [111] and [100] axis have been deformed in tension at room temperature. The Mn content was 0-8 at.% for the [111] and 3 at.% for the [100] crystals. The observed decrease in the work-hardening rate with increasing reduced shear stress tau - tau-0, coincides with the linear decrease in pure Cu for tau - tau-0 > 15-20 MPa (tau-0 is the critical resolved shear stress). The total dislocation density measured for tau - tau-0 between 5 and 58 MPa increases proportional to (tau - tau-0)2 with a slope which increases with increasing Mn content. The higher dislocation density in the alloys is stored in a cell structure of higher volume fraction of the walls compared with pure Cu. For tau - tau-0 exceeding 15-20 MPa, the dislocation density within the walls depends only on tau - tau-0 and not on orientation and Mn content. Furthermore, it alone determines, in the pure and alloyed crystals, the flow stress, which is additively composed of contributions from solid-solution and strain hardening. In the symmetrically oriented crystals the flow stress is successfully interpreted by a forest model. The results do not support existing composite models.