Electron diffraction characterization of a new metastable Al 2 Cu phase in an Al–Cu friction stir weld

The space group of a new metastable orthorhombic Al 2 Cu phase, located in the Al‐rich interfacial region of an Al–Cu friction stir weld, was unambiguously identified as Ic 2 m 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‐Al 2 Cu counterpart (tetragonal, I 4/ 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‐Al 2 Cu phase by in situ irradiation within the transmission electron microscope.