Microstructure and strength of Cu-Fe in situ composites after very high drawing strains

Composites consisting of a Cu matrix and a body centred cubic (b.c.c.) phase of fibre morphology have been prepared from rapidly-solidified powders or spray-deposited billets by extrusion and heavy cold drawing. During deformation the b.c.c. phase elongates into ribbons and the microstructure refines, leading to large increases of tensile stresses. The rapid solidification techniques give a fine initial microstructure so that significantly increased strength is observed after a relatively small amount of cold working, whilst after heavy working the material becomes very strong with the microstructure reaching nanoscale dimensions. The dependence of strength on the extent of prior cold work has been analysed in relation to the evolution of microstructure. There are considerable residual stresses in the materials after drawing which induce a strong Bauschinger effect. The very fine scale of the Fe ribbons at large drawing strains makes them very strong, such that they are still elastically loaded when the composite begins to deform plastically. The yield stress saturates at very high strains as the nanoscale Fe ribbons achieve their ideal strength.