Electron diffraction characterization of a new metastable Al₂Cu phase in an Al–Cu friction stir weld

The space group of a new metastable orthorhombic Al₂Cu phase, located in the Al-rich interfacial region of an Al–Cu friction stir weld, was unambiguously identified as Ic2m by a recently developed systematic method combining precession electron diffraction and convergent-beam electron diffraction. This metastable phase has the same tetragonal lattice as its stable θ-Al₂Cu counterpart (tetragonal, I4/mcm, No. 140). The tetragonal-to-ortho­rhombic symmetry lowering is due to slight modifications of the atomic positions in the unit cell. This metastable phase can be transformed into the stable θ-Al₂Cu phase by in situ irradiation within the transmission electron microscope.