Microstructures and their stability in rapidly solidified Al-Fe-(V, Si) alloy powders

Microstructures and their stability in as-atomised Al-6.5Fe-1.5V and Al-6.5 Fe-1.5V-1.7Si powders have been investigated using transmission electron mi croscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDXS), s canning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. It was observed that microstructure s of the as-atomised powder particles showed a close relationship with powd er particle sizes. The as-atomised powders exhibited three types of microst ructures, namely 'zone A', 'zone B' and 'zone C'. The 'zone A' type microst ructure consisted of very fine and homogeneous distributed precipitates in the alpha -Al matrix. The 'zone B' microstructure represented the regions c onsisting of microcellular structures whereas the 'zone C' microstructure r epresented the regions consisting of coarse cellular structures and globula r quasi-crystalline phase particles. Fine powder particles exhibited both ' zone A' and 'zone B' microstructures. The size of 'zone A' decreased with i ncreasing powder particle sizes. The intercellular phases in 'zone B' of bo th Al-Fe-V and Al-Fe-V-Si were very fine, randomly oriented microquasi-crys talline icosahedral particles. Microstructures of coarse powder particles e xhibited both 'zone B' and 'zone C'. The intercellular phases in 'zone C' o f Al-Fe-V powders could be Al6Fe, whereas in Al-Fe-V-Si powders they were p robably silicide phase. Formation of powder microstructures may be explaine d by the interactions between the growing alpha -Al fronts with the freely dispersed, primary phase particles or the solute micro-segregation. Studies using DSC techniques have revealed the microstructural stability of as-ato mised powders. There were three DSC exotherms observed in the as-atomised A l-Fe-V powders. The 'zone A' was stable at elevated temperatures and the ex otherm peak corresponding to the transformation reactions occurring in 'zon e A' was at 360 degreesC. The exotherm peak, which might correspond to the transformation of the globular clusters of microquasi-crystalline icosahedr al phase to single-phase icosahedral particles, was at 450 degreesC. The ex otherm peak, which may correspond to the formation of Al13Fe4 and Al-45(V, Fe)(7) phases, was at 500 degreesC. In the as-atomised Al-Fe-V-Si powders, only one exotherm was observed with a peak at 400 degreesC. This exotherm m ay correspond to precipitation of silicide phase particles. (C) 2001 Kluwer Academic Publishers.