Rapidly solidified Al–Cr alloys: Crystalline and quasicrystalline phases

Abstract

Rapidly solidified Al–Cr alloys up to 20 at. % Cr were studied to delineate the extent of crystalline and quasicrystalline phase formation in these alloys in comparison with as-cast alloys by using transmission electron microscopy and x-ray diffraction technique. The icosahedral quasicrystals are observed from 7 to 15 at. % Cr alloys, while equilibrium η–Al₁₁Cr₂ phase is completely absent. Both rapid solidification and subsequent thermal decomposition studies indicate that the main competing phase is θ–Al₂Cr up to 15 at. % Cr. Beyond this composition –Al₄Cr is the dominant phase together with a small amount of γ₄–Al₇Cr₃. We have shown that the electron diffraction patterns of Al–Cr quasicrystals are often associated with a diffuse intensity distribution, indicative of short-range order. The change in quasilattice constant with composition suggests the existence of structural vacancies. Further, a sudden change from coarse to ultrafine quasicrystalline grain structure in Al-7 at. % Cr alloy points to a change in nucleation mechanism from heterogeneous to homogeneous mode during the rapid solidification.