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

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-foldIQC//[0 0 0 1]H//[1 1 1]W and 2-foldIQC//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.