Drawing deformation induced fcc phase precipitates in the titanium sheath of an MgB 2 superconducting wire

Abstract

Titanium was selected as the metal sheath for MgB2 superconducting wires for its light weight, limited interdiffusion with the magnesium diboride and good mechanical properties. The starting commercial tubes exhibited a hexagonal α-phase crystal structure and presented a drawn tube crystallographic texture. After packing the superconducting MgB2 powder into the titanium tube, the resulting composite was subjected to cold drawing with intermediate thermal treatments, up to a total cross sectional area reduction of 97%. Electron backscatter diffraction (EBSD) of the final wire revealed the presence of acicular precipitates within the α-phase grains, which were indexed with an fcc crystallographic structure. These precipitates are consistent with the hcp to fcc phase transformation generally reported in association with high levels of plastic deformation. To confirm the presence of this phase and investigate further structural details, a thin lamella was cut from the composite sample sheath by FIB-SEM technique with Ga ions and subsequently examined via transmission electron microscopy (TEM). The electron diffraction pattern confirmed that the acicular precipitates observed in bright field mode correspond to an fcc phase, exhibiting a lattice parameter in agreement with that of the reported fcc titanium phase. This induced hcp to fcc transformation is believed to provide an additional deformation mechanism to accommodate mechanical strains, thereby improving both the ductility and toughness of the titanium sheath. This would significantly contribute to the mechanical stability of the wires besides promoting a better densification of the magnesium diboride core.