Al-Co reactions in multilayer films: first stages and selective formation of new (metastable) phases

Reactive diffusion in Al/Co multilayers has been analyzed using differential scanning calorimetry, ramped resistance measurements and X-ray diffraction. Two major points appear. (1) the first phase formed is AI,Co,. This is in agreement both with the Cu,Au rule on phase selection and former studies on Al/Co bilayer films. However, we show that the formation of this phase proceeds in two distinct steps. The first one is limited to a thin interfacial layer. The second is the diffusion-controlled growth of this layer until total consumption of Al. The second phase formed is the metastable decagonal Al13Co4 quasicrystal, It is observed for Al/Co ratios ranging from 13/4 to 5/2. In this composition range several stable phases are reported. Reactive diffusion in multilayers appear thus as a very powerful tool to promote selective formation of new (metastable) phases. (C) 2000 Elsevier Science B.V. All rights reserved.