  The microstructures of MgB2 wires prepared by the powder-in-tube technique
and subsequent hot isostatic pressing were investigated using transmission
electron microscopy. Large amount of crystalline defects including small angle
twisting, tilting, and bending boundaries, in which high densities of
dislocations reside, were found forming sub-grains within MgB2 grains. It is
believed that these defects resulted from particle deformation during the hot
isostatic pressing process and are effective flux pinning centers that
contribute to the high critical current densities of the wires at high
temperatures and at high fields.
