Functionally Graded Nanocrystalline Alloys

The creation of a new class of materials, functionally graded nanocrystalline metals (FGNMs), offers the unique ability to optimize different material properties in separate locations.  In mechanical applications involving cyclic loading, for example, a gradient from small grains (which slow crack initiation at the surface) to larger grains (which slow crack propagation in the interior) could lead to the optimization of fatigue properties throughout a coating.  Current work in our group explores the fundamental processing science involved in creating FGNMs through periodic reverse pulse electrodeposition of Ni-W alloys.  By modifying the current waveform, this technique allows precise in-situ control of composition and grain size as shown in Figure 1.  The goal of this project is to understand the mismatch of composition, structure, and properties between adjacent layers in a graded nanocrystalline structure and to, in turn, define a window of processing variables in which stable, artifact-free, and stress-free FGNMs can be synthesized.  Profilometry methods are presently being used to characterize the residual stress inherent in these deposits while the interfacial structure is being investigated using scanning electron and transmission electron microscopy methods.  In addition, parallel studies of mechanical properties of these materials is underway.

Patterned nanocrystalline electrodeposits synthesized using the periodic reverse pulsing technique.  Nanoindentation hardness (circles) and grain size (squares) profiles show the level of structure and property control available using this method. The specimen in (a) comprises eight layers of monotonically decreasing W content, while that in (b) contains alternating layers of low and high W content.

Published Articles:

Mechanics of indentation of plastically graded materials—II: Experiments on nanocrystalline alloys with grain size gradients
Choi IS, Detor AJ, Schwaiger R, Dao M, Schuh CA, Suresh S; JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 56 (1): 172–183 JAN 2008 (PDF)

Tailoring and patterning the grain size of nanocrystalline alloys
Detor AJ, Schuh CA; ACTA MATERIALIA 55 (1): 371-379 JAN 2007 (PDF)