Interface structure and chemistry and early-stage spinodal decomposition of dimensionally confined copper nickel iron particles embedded in sapphire

Phase transformations that occur under dimensionally confined conditions offer the possibility of obtaining fine-scale, compositionally modulated structures down to the nanoscale by combining the methods of cop-down" fabrication with "bottom-up" self-assembly processes. As the spatial scale of this constraint is reduced, the effects of interfaces and bounding surfaces assume a more dominant role in the overall energetics of the system. This study examines interface effects on the spinodal decomposition---a diffusional phase transformation---of a CuNiFe alloy embedded in an inert sapphire host matrix, and explores diffusional paths at the alloy-sapphire interface that are not available during bulk-alloy decomposition using imaging and spectroscopic techniques at the manometer scale in the transmission electron microscope. CuNiFe alloys were embedded in (0001)-oriented (basal plane) sapphire and (1120)-oriented (prismatic plane) sapphire. Two major orientation relationships were observed: 1111 10CuNiFe&vbm0; &vbm0;00011 100Sapphire 111 110CuNiFe &vbm0;&vbm0;1120 1100Sa pphire ELNES reveals that the sapphire is aluminum-terminated for both orientations. Results show that interface energy can lead to a surface-directed spinodal decomposition mechanism at the alloy-sapphire interface during the early stages of decomposition as a consequence of an interplay between wetting phenomena and the decomposition process. Interface dislocations provide an enhanced diffusion pathway for the breakup of this surface-directed composition wave. Elastic strain fields due to lattice distortions in the alloy also provide an enhanced diffusion mechanism for spinodal decomposition. Suppression of composition modulations in specific crystallographic directions is observed as a result of the dimensional constraint. Evidence suggests that confinement at smaller spatial scales combined with control and manipulation of interface effects can produce a variety of nanoscale, compositionally modulated structures with a desired scale and properties.