On the strain accommodation of β1 precipitates in magnesium alloy WE54

The distribution arrangements of β1 precipitate plates in Mg–Y–Nd alloy WE54 are systematically examined using high-angle annular dark-field and bright-field scanning transmission electron microscopy. It is found that the β1 plates often form with a triadic configuration that is the building block of a honeycomb structure. In a triad, three β1 variants are separated by 120° pointing outward with an equilateral-triangular Mg phase isolated in the triadic centre. The isolated Mg phase is related to the surrounding Mg matrix by a unique 10.5° rotation about [0001]α. The interface between the isolated Mg phase and the three surrounding β1 variants is (2¯24¯)β1)//(22¯00)α-iso, which is fully coherent and identical to that between the broad side of β1 variants and surrounding Mg matrix (2¯24)β1//(22¯00)α. It is shown that the rotation of the isolated Mg phase is necessary to minimise the shear strain energy and the interfacial energy of the triadic precipitates.