Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Vinci, Richard P.

Other advisers/committee members

Chan, Helen; Darling, Kris A.

Abstract

Nanocrystalline materials have been shown to exhibit superior properties compared to coarse-grained materials. Ni-W in particular shows potential in the electronics, microfabrication technology, automobile, and aerospace industries. Research in the nanocrystalline field has in part been driven by an increasing demand for materials to withstand high temperatures without compromising performance. However, studies on heat-treated Ni-W are limited. Furthermore, additional fracture data is needed if these materials are to be used for structural applications. Ni-W films were electrodeposited in a highly controlled sulfate-citrate bath. Direct current and reverse pulse plating yielded alloys with wide ranging composition, i.e. Ni-3 to 21 at.% W. Films were heat treated in a controlled reducing atmosphere at 437°C, 728°C, and 1019°C for 24 hours. Isothermal heat treatments at 728°C were conducted for 1, 4, 12, and 24 hours.The microstructure was characterized using TEM and STEM. Films exhibited a variety of different microstructures with grain sizes ranging from ~9 μm in diameter down to the nanocrystalline and amorphous limit. Typically, the grain size decreased with increasing W concentration. With the exception of Ni-21 at.%, the microstructure was a single-phase Ni(W) solid solution of FCC structure. Ni-21 at.% W was nominally amorphous in the as-deposited condition and slightly more crystalline at 437°C. At 728°C, Ni6W6C, elemental W, and Ni4W second phases precipitated, and at 1019°C, the microstructure was a mixture of Ni(W) and elemental W. The origin of the carbides was traced to carbon contamination. Because the processing was so meticulously controlled, however, evidence of carbides suggests carbon contamination may be hard to avoid, thus the Ni-W-C ternary phase diagram may be better suited, at least under practical circumstances. The fracture behavior was examined with in-situ microcantilever deflection testing. Linear elastic fracture mechanics was used, but non-negligible plasticity prompted elastic-plastic fracture mechanics as well. Fracture toughness quantification of micro-scale elastic-plastic metals is a developing field. Thus, many complexities were encountered while developing a framework for testing and analysis. Periodic partial unloading was implemented along with J-integral analysis. As-deposited Ni-21 at.% W exhibited the most plastic yielding and thus highest fracture toughness. The fracture toughness decreased significantly at 437°C and was comparable at 728°C. The hardness, however, increased significantly at 437°C but decreased at 728°C. Thus, there appears to be no benefit to annealing to such high temperatures. For microstructural optimization purposes, one may want to consider the trade-off between the increased hardness and deteriorating toughness at 437°C. The biggest strengthening contributions to the hardness were the grain size and second phases, with solid solution strengthening being negligible. This study aimed to investigate the microstructure and fracture behavior of Ni-W alloys and to establish structure-property relationships necessary for the successful exploitation of these materials. A novel fracture analysis was conducted, progressing the field of micro-mechanical testing of elastic-plastic materials.

Available for download on Friday, June 01, 2018

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