APPROXIMANT CRYSTALS OF ICOSAHEDRAL MG-(GA,AL)-ZN ALLOYS

It has been found in Mg-39.5(Ga1-xAlx)(20.5)Zn-40.0 alloys that an ico sahedral phase produced by melt-spinning transforms to rational approx imant phases of the icosahedral quasicrystal on heating. In the alloy with the composition x = 0.2, the icosahedral phase transforms to a 3/ 2-2/1-2/1 orthorhombic approximant phase with a = 3.693 nm, b = 2.296 nm and c = 2.268 nm. In the alloy with x = 0.7, the icosahedral phase transforms to a 1/1 cubic approximant phase with a = 1.421 nm at 653 K and successively to a 2/1 cubic approximant with a = 2.300 nm al arou nd 668 K on heating. Structural relations of the approximant phases to the icosahedral phase are discussed, in the framework of the projecti on scheme, on the basis of X-ray diffraction data.