In situ transmission electron microscopy investigation of quasicrystal-crystal transformations in Mg–Zn–Y alloys

Highlights: • Quasicrystal-to-crystal transformation sequence in Mg–Zn–Y alloys was determined. • H phase Zn₃MgY and W phase Zn₃Mg₃Y₂ nucleated on icosahedral quasicrystal Zn₆Mg₃Y. • Growth of both H and W phase is controlled by diffusion. • The close relationship between building units plays a key role in the transformation. - Abstract: Evolution of icosahedral quasicrystals (IQC) in Mg–Zn–Y alloys during annealing was investigated by in situ transmission electron microscopy (TEM), in combination with differential thermal analysis and X-ray diffraction. In bulk samples, the IQC phase transformed to face-centered cubic phase W and hexagonal phase H at 720 K and 727 K, respectively. In TEM samples, IQC started to transform to W at 673 K and H at 688 K during heating, and H transformed to W at 623 K on cooling. Quantitative analysis of the in situ transformation process reveals that growth of both H and W is controlled by diffusion, agreeing with the Avrami's model. The transformed products have specific orientation relationships with the parent phases: 3-fold_IQC//[0 0 0 1]_H//[1 1 1]_W and 2-fold_IQC//112^¯0]_H//[01^¯1]_W. The specific orientation relationships are attributed to close structural correlation among icosahedron, distorted icosahedron and cuboctahedron in IQC, H and W phases. The close structure relationship among IQC, H and W phases on the unit-cell level plays an important role in the phase transformations