In situ investigations of phase transformations in Fe - sheathed MgB 2 wires

The phase evolution inside Fe-sheathed wires containing precursor powders consisting of a mixture of Mg and B has been studied in situ by means of x-ray diffraction with hard synchrotron radiation (90 keV). Mg was found to disappear progressively during the heating stage. At 500 ◦C, the intensity of the Mg diffraction lines is reduced by about 20%. This effect is partly attributable to MgO formation. The MgB2 phase was detected from 575 ◦C. Fe2B was forming at the interface between the sheath and the ceramic core at sintering temperatures of 780 and 700 ◦C, but not at 650 ◦C. The formation rate of this phase is strongly dependent on the heat treatment temperature. Its presence can be readily detected as soon as the average interface reaction thickness exceeds 150–200 nm.