Effect of surface orientation on the plasticity of small bcc metals

The basic dislocation processes responsible for plasticity of fcc and bcc metals are fundamentally different. Dislocation based deformation in fcc metals is not very sensitive to temperature whereas in bcc metals, deformation is strongly temperature dependent and controlled by the low mobility of screw dislocations. In bcc metals it has been observed that the mobility of screw dislocations can be enhanced in the proximity of the sample surface. In order to investigate this effect, a comparative study of small scaled samples with high surface to volume ratios and selected geometries was performed. The systems that were tested were made of the bcc metals Ta, Mo and Fe. Cu served as a reference material for fcc metals. Microcompression experiments were carried out on focused ion beam (FIB) machined samples. In the FIB machining process, the surfaces of cuboidal pillars were oriented relative to the expected active slip systems. For Ta and Mo the flow stress depended on the orientation of the pillar. This observation is interpreted in terms of the surface enhanced screw dislocation mobility of bcc metals.