Al–Cu approximants and associated B2 chemical-twinning modes

Abstract

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=0.8166, b=1.4149, c=0.9995nm) and body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a=0.4083, b=0.7074, c=0.9995nm). 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-Al36Cu48□12 and oI-Al8Cu12□4 (□: vacancies). They both exist in twinning variants of the types 120°/[001] and 180°/[310]. Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase ϵ2-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°/〈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.