Impacts of High Field Magnetic Fields on Processing of Selected Materials

Various composite conductors and reinforcement materials are used for high field magnets in the USA National High Magnetic Field Laboratory.Typical composite conductors are Cu based metal-metal composites,whereas most of reinforcement materials have faced-center-cubic matrix and relatively low ductile-to-brittle transformation temperatures so they can perform at cryogenic temperatures.During the operation of the magnets,the mechanical stresses,magnetic fields and other extreme environments are imposed to the materials and materials are 'processed' during the service of the magnets.For instance,the conductors in the magnets are likely to experience higher temperatures than ambient during the operations if the electrical current density for producing high field is sufficiently high.Some of the conductors are fabricated by cold rolling or drawing that introduces lattice distortions and high densities of interfaces in unit volume.High temperature and high field exposure of the conductor may affect the characteristics of the lattice distortions and the interfaces.The lattice distortion and density of the interface affects the mechanical properties of the conductors,such as the tensile and yield strength,as well as the electric conductivity of the composites.Therefore,the materials after service are expected to have different properties compared to as-received conditions.The first portion of our paper will focus on relationship between the service and processing of the magnet materials in high field magnets.The high magnetic field can be used directly to process materials.In some cases,the material properties can be improved by more than 50%if the processing is undertaken in the high magnetic field.The improvement is due to the microstructure changes induced by high magnetic fields.Understanding the behaviors of the materials after they are exposed to high magnetic fields helps us to make good use of the high field processing approach efficiently to fabricate better materials,particularly when the magnetic fields are so high that the cost of building and operation of high field magnets cannot be ignored.In high field processing, phase transformation usually occurs in the magnetic fields.In such cases,one has to consider the impacts of high magnetic fields on critical points,which include transformation temperatures and chemistry,crystallographic structure and habit planes for nucleation and growth,and kinetics.The goal of this portion of research is to understand the microstructure evolution of the selected materials processed in high magnetic fields,and to relate such microstructural features to properties of the materials.