Chemical and morphological evolution of decomposing copper-nickel-iron alloy particles dimensionally confined in sapphire

The competition between maximizing the thermodynamic driving force and minimizing the transport distance in systems undergoing a variety of phase transformations produces a microstructure with a tunable characteristic length scale that maximizes the transformation rate. Spinodal decomposition is one special diffusional phase transformation in which unstable compositional variations grow in amplitude to produce a microstructure with a periodic compositional variation. During spinodal decomposition, the thermodynamic and kinetic effects, which favor long- and short-wavelength compositional fluctuations respectively, result in a microstructure with regions of alternating composition and some characteristic spacing that offers the optimum compromise. In a volumetrically constrained system, the compositional wave can then be "directed" to propagate only along the longer dimensions of the confining medium. CuNiFe thin-films were grown using pulsed laser deposition techniques and confined in a chemically inert sapphire matrix using conventional UV micro-lithography and diffusion bonding techniques. These CuNiFe particles were then converted to well-oriented single crystals using a nucleation-controlled liquid-phase epitaxial solidification process. The resulting single-crystal ceramic-single-crystal metal systems facilitate model studies of spinodal decomposition in constrained single-crystal alloys. When subjected to aging treatments that promote spinodal decomposition into a paramagnetic Cu-rich and ferromagnetic NiFe-rich phase along the elastically soft <100> directions, the compositional modulations formed multi-domain structures that were observed using energy-filtered imaging. During coarsening, the behavior eventually deviated from bulk behavior to stabilize a characteristic length scale that depended on the dimensions of confinement. Empirical evidence that volumetric confinement is a feasible route for microstructural patterning at various length scales will be shown. This technique provides opportunity to study phase transformations and/or morphological transformations with the ability to modify the dominant energetic terms.