Co-deformation of a two-phase FCC/BCC material

Abstract

<p>The drive to produce materials with novel or beneficial combinations of properties has prompted research into a range of new materials and processing routes. In many applications one of the important design variables is the mechanical strength. Exceptional strengths can be achieved in certain materials consisting of two deformable phases when they are drawn into fine wires or rolled into thin sheets, the common example being pearlitic steel wire which can achieve strengths in excess of 5 GPa. The mechanisms that permit co-deformation and result in the observed strengthening are, however, not well understood. In this thesis an approach was adopted whereby co-deformation of a well characterized model material has been studied primarily using uniaxial tensile tests. Directional solidification of a Cu-1.56at%Cr eutectic alloy has been used to produce material consisting of submicron diameter single crystals of Cr embedded within a polycrystalline Cu matrix. It has been shown that these two phases exhibit preferred crystallographic orientation relationships, habit planes and growth directions the same as those found for solid state precipitates of Cr in Cu. On deforming this material it is found that the Cr fibres yield at stresses close to the theoretical limit. However, their are able to continue to co-deform with the Cu matrix to large plastic strains. This process of co-deformation is observed to cause a rate of nearly constant work hardening that results in both high strength and high ductility. This behaviour has been attributed to the fact that the Cr fibres continue to carry increasing elastic strain beyond their yield thereby contributing to an increasing level of internal stress in the material. It is suggested that this mechanism may play an important role in other co-deformed two phase materials. In particular, it is suggested that this may provide one mechanism for the continued high rate of work hardening in heavily co-deformed two phase materials.</p>