A new two-dimensional qnasicrystal - octagonal phase has been found in the quenched Mn_4Si alloy. This octagonal phase and related microdomain of β-Mn structure with eightfold symmetry have been studied by the electron diffraction and high revolution electron imagies, the difference between octagonal phase and micro-domain has been proved. And a new ordered Mn_3Si and a metastable Mn_(12)Si_5 have also been found in this alloy. The orientation relation-ships between ordered Mn_3Si and Metastable Mn_(12Si_5 and 37° rotation twins of Mn_(12Si_5 have been explained. We also indexed the electron diffraction of [001] zone of octagonal phase using the four-indexed numbers.
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