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.
Highlights
The fabrication of many metallic alloys – both thin film and bulk – relies on the nucleation and subsequent growth of intermetallic phases from a solid solution
While we measure differences between the samples with gradients and those without, the gradients relax sufficiently during heating such that nucleation 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
We present the first results of what is, to the best of our knowledge, a first attempt to directly control initial composition gradients in order to measure their effects on nucleation and growth
Summary
The fabrication of many metallic alloys – both thin film and bulk – relies on the nucleation and subsequent growth of intermetallic phases from a solid solution. Due to the initial structure of these films, significant composition gradients still exist within the films at the point of nucleation, even after intermixing has occurred Both theoretical treatises and recent experimental work imply that these compositional gradients can inhibit nucleation when the gradient is above a critical threshold. This gradient effect is not captured in classical nucleation theory (CNT), which assumes that the parent material is compositionally homogenous
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