ON THE NATURE OF ICOSAHEDRAL PHASES IN AL-(FE, V,SI) ALLOYS

Icosahedral phases have been produced in a variety of Al-(Fe,V,Si) alloy ribbons by planar flow casting of melts with constituent elements of high purity. Differential scanning calorimetry, X-ray diffraction and transmission electron microscopy have been used to probe these phases in detail. It is found in the binary Al-Fe and Al-V alloys that the icosahedral edge length a is independent of composition over a wide composition range; a is about 0.508 nm and 0.526 nm in Al-Fe and Al-V, respectively. The value for a in ternary Al-Fe-V alloys is intermediate between these two values, and is seen to be a function of the alloy chemistry. This dependence is ascribed to a solid solution type effect wherein the Fe:V ratio in the icosahedral phases varies continuously. The addition of silicon to the ternary alloys has a minimal effect on a, but a pronounced effect in terms of reduction of stability of the icosahedral phase. The icosahedral phase transformation temperature is seen to be reduced by approximately 25 K per at.% Si.