A nanocalorimetric study of the effect of composition gradients on crystallization in amorphous Cu-Zr thin films

Abstract

Nucleation, the initial formation of a new phase from a parent phase, plays an important role in the eventual microstructure and properties of materials. Theories and models of nucleation have been integral to materials science for close to a century. These models assume that the parent material is compositionally homogeneous on length-scales relevant to nucleation. However, in certain materials – such as thin films or reactive nanolaminates – sharp gradients in the composition may influence nucleation. Models and theories exploring these impacts are based on little direct experimental data. Here we present means of producing and characterizing samples with composition gradients to measure the impacts of gradients on nucleation. We fabricate amorphous Cu-Zr films with known composition gradients through their thicknesses; we perform isochronal nanocalorimetry to measure the impact of the gradients on nucleation and growth; and we characterize the samples before and after reaction. We see evidence of phase separation of the vapor-quenched Cu-Zr amorphous films. While we measure differences between the samples with gradients and those without, the gradients relax sufficiently during heating such that nucleation (the onset of crystallization) occurs at the same temperatures. For both sets of samples we find three distinct regions of heat release: the first we attribute to local ordering, the second to extended phase separation and interdiffusion, and the third to nucleation and growth of the Cu10Zr7 crystalline phase. This work represents a first step towards investigating the impact of gradients on nucleation, as well as growth.