Structural evolution and phase formation in cold-rolled aluminum-nickel multilayers

Multilayer samples of nickel and aluminum with the compositions of Al-20 at % Ni and Al-25 at% Ni were Prepared by the repeated folding and cold rollin g (F&R) of elemental foils. Characterization by X-ray diffraction, scanning electron microscopy and transmission electron microscopy/selected-area ele ctron diffraction reveals that the rolling procedure results in a decrease in thickness of the elemental layers to below 0.1 mum and a reduction in gr ain size to below 50 nm, but does not induce Formation of an intermediate p hase. Differential scanning calorimetry (DSC) measurements on samples subje cted to different numbers of F&R cycles show double exotherms related to an initial nucleation and lateral growth and a subsequent thickening of react ively formed AI,Ni. The DSC data agree qualitatively with predictions of an existing kinetic model fur multilayer reactions, but a decisively improved quantitative fit is obtained when a distribution of layer spacings is cons idered, rather than a well-defined modulation wavelength. The modified kine tic model can be inverted to yield information on the evolution of the dist ribution of layer sizes during deformation from DSC data. (C) 2001 Acta Mat erialia Inc. Published by Elsevier Science Ltd. All rights reserved.