Al-Cu approximants and associated B2 chemical-twinning modes.

The Al3Cu4 alloy, with an e/a ratio of 1.86 being close to ternary Al-Cu-TM (transition metal) quasicrystals, has been chosen for the search of Al-Cu approximants. Phase structures and compositions were studied using TEM, X-ray diffraction and EPMA techniques. Two new phases were found: face-centered orthorhombic oF-Al43.2Cu56.8 (a = 0.816(6), b = 1.414(9), c = 0.999(5) nm) and body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a = 0.408(3), b = 0.707(4), c = 0.999(5) nm). Their e/a ratios are the same as that of the Al-Cu-Fe icosahedral quasicrystal. Both are B2 superstructures and their unit cell components can be expressed approximately as oF-Al36Cu48vacancies12 and oI-Al8Cu12vacancies4. They both exist in twinning variants of the types 120 degrees/[001] and 180 degrees/[310]. Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase epsilon2-Al2Cu3, the atomic structure of which shows pentagonal atomic arrangements. Further analysis on the twinning modes of oF and oI leads to the recognition of the chemical-twinning mode of the basic B2 structure as 180 degrees/(111)B2. This kind of chemical twinning mode is responsible for the pentagonal atomic configuration in the Al-Cu approximants as well as for the pseudo-5-fold B2 twinning.