Load partitioning during compressive loading of a Mg/MgB2 composite

A composite, consisting of 68 vol.% superconducting continuous MgB₂ fibers aligned within a ductile Mg matrix, was loaded in uniaxial compression and the volume-averaged lattice strains in the matrix and fiber were measured in situ by synchrotron X-ray diffraction as a function of applied stress. In the elastic range of the composite, both phases exhibit the same strain, indicating that the matrix is transferring load to the fibers according to a simple iso-strain model. In the plastic range of the composite, the matrix is carrying proportionally less load. Plastic load transfer from matrix to fibers is complex due to presence in the fibers of a stiff WB₄ core and of cracks produced during the in situ synthesis of the MgB₂ fibers from B fibers. Also, load transfer behavior was observed to be different in bulk and near-surface regions, indicating that surface measurements are prone to error.