On the Phase Transformation
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quasicrystalline grains. Low-magnification dark-field images, on the other hand, suggest twinning deformation mechanisms for the annealed-induced phase transformation. In the past, few investigations were carried out on the transitions between quasicrystalline phases to crystalline structures or amorphous phases. Most of these reports were mainly concerned with the binary A1-Mn system. t2-Sj Quasicrystalline phases in ternary alloys have also been widely studied in the literature; however, very few investigations on the transformation quasicrystallinecrystalline structures have been carried out. 16'7"sl Recently, it was found that both quasicrystalline phases, decagonal and icosahedral, coexist in quaternary alloys of A1-Cu-Co-Fe. tlj The results presented are based on electron diffraction patterns and high-resolution elec~on microscopy (HREM) image characteristics from quasicrystalline phases obtained in quaternary alloys produced using a normal casting technique and also from subsequently annealed specimens. The A165Cu22Co6.sFe6.5 alloy was obtained by melting high-purity elements (99.999 pct of purity) in an arc furnace under an argon atmosphere. The resulting ingot was slowly cooled. For annealing, the specimens were encapsulated in vacuum and subsequently annealed at 800 ~ for 24 hours. The observations were carried out in a JEOL 4000EX microscope operated at 20 KV. For electron microscope observations, the specimens were thinned using an Ar-ion beam machine. Deviations from perfect icosahedral symmetry were reported in the past. Thus, for example, in A1-Mn-Si al-
Fig. 1 --Diffraction patterns from the icosahedral phase of A1-Cu-Fe-Co alloy: (a) along the fivefold axis; (b) along the twofold axis; (c) along the fivefold after annealing at 800 ~ for 24 h; and (d) along the twofold after annealing. METALLURGICAL TRANSACTIONS A
VOLUME 23A, NOVEMBER 1992--3161
Fig. 2--Diffraction patterns from the icosahedral phase of A1-Cu-Fe-Co alloy: (a) along the pseudo-twofold axis; (b) along the threefold axis; (c) along the pseudo-twofold after annealing at 800 ~ for 24 h; and (d) along the threefold after annealing.
loys, pronounced elongated spots along different zone axis diffraction patterns have been found, t91 Asymmetries frequently observed in the overall intensity distribution of the spots were also obtained, t1~ These diffraction features are partially explained assuming the presence of fields of phasons in the icosahedral phases. There have also been reports on particular morphological shapes of the diffraction spots in the icosahedral phase of A1-Mn. Triangular shapes have already been reported in A1-Mn alloys and have been explained in terms of the super-
Fig. 3--Diffraction pattern from the icosahedral phase along the twofold axis. Periodicity is indicated by the arrows. 3162--VOLUME 23A, NOVEMBER 1992
position of morphological effects different than those previously mentioned, tgJ The decagonal and icosahedral phases found in normal casting alloys of A1-Cu-Co-Fe also show diffraction patterns with parti
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