Nucleation, Growth, and Grain Structure Control of Electrodeposited Graded Density Alloys
Authors: Hooks Daniel E , McBride Michael , Stull Jamie , Johnson Donald R , Dervishi Enkeleda , Edwards Randall L 
Published: May 2020

Driven by an interest in dynamic high-pressure research, we are developing methods to create thick (mm) electrodeposited films of alloys with graded composition and density. We are designing deposition processes to dynamically control nucleation and growth to investigate correlations between grain structure and dynamic properties. We are using a combination of electrochemical techniques with in situ atomic force microscopy (AFM) complemented by phase contrast imaging (PCI) to isolate variables controlling nucleation and growth which lead to the grain structure of the deposit.
Gold is of interest as a model system in high-pressure and dynamic research because of its high compressibility and inert character, especially because purity, defects, and grain structure can be carefully controlled. Further, because of its potential as a graded density alloy, baseline characterization is important. Gold has complete solid solubility with silver, and similar deposition chemistries. Using several techniques, including control of several process variables, we have produced films of graded composition in silver and gold. To broaden the potential for high-pressure research, similar techniques can be applied to any material that can be electrodeposited, and we will discuss graded compositions varying from aluminum to rhenium.
The grain structure of the deposit can have an influence on dynamic properties, so in addition to composition, we have been researching deposition parameters to control this structure. Controlling nucleation and growth is critical to influencing ultimate grain structure in the deposit, so we have also determined and applied some basic nucleation and growth parameters for our systems. Although there are several well-known aspects, nucleation and growth are some of the most fundamental rate dependent processes in many chemical and physical transformations and remain among the most difficult to measure and predict. Assessing and predicting nucleation and growth, and rate-dependent processes, such as first-order phase transitions, are widely applicable to a multitude of practical applications. In the context of metal deposition for alloy systems, characterizing and modelling nucleation and growth can lead to an ability to design compositionally varied alloys with controlled grain structures. Therefore, we also performed some real-time dynamic investigations of nucleation and growth in our systems using complementary techniques of real-time, in situ AFM and x-ray PCI. AFM offers the ability to dynamically monitor growth processes, and through a method to isolate nucleation sites, estimate nucleation processes with spatial resolution sufficient to capture individual nuclei (nm or better). However, the technique is relatively slow (frame rate of several minutes), and dynamic processes are therefore only estimated. PCI, on the other hand, has very high time resolution (frame rate of ~60 ps), and can capture aspects of dynamics to complement AFM, even if the spatial resolution is lower (~2-3um).
We present a study of electroplated films with compositional gradients and varied microstructures up to thicknesses of several mm for gold and silver, with discussion of other alloys in development. We relate the composition and grain structure to dynamic response. Constant potential and pulsed plating techniques were used to create a variety of grain structures, including high aspect ratio columnar grains. These structures and their dynamic response were compared to cast and wrought microstructures. Microscopy was correlated with mechanical characterization of the films at several rates and scales, and impact characteristics are compared and related to growth techniques and their relation to nucleation and growth parameters.
LA-UR-19-31897