Date of Award

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

En Ma

Abstract

A new route has been developed to produce full-density, bulk, two-phase nanocomposites. Nanocrystalline, single-phase, fcc or bcc Cu$\sb{\rm 100-x}$Fe$\sb{\rm x}$ (x = 0 to 100) solid solution precursors were obtained by mechanical alloying of Cu and Fe at room temperature or liquid nitrogen temperature. These supersaturated solid solutions were decomposed on nanoscale upon hot consolidation, forming Cu-Fe two-phase nanocomposites in situ. Fully dense composite specimens have been obtained using either unconstrained or constrained sinter forging for the entire composition range (x = 0 to 100). The microstructures of the consolidated nanocomposites at representative second-phase volume fractions (x = 60, 85, 100) were characterized using transmission electron microscopy. The Cu and Fe phase domains and their distributions were analyzed using energy dispersive X-ray spectroscopy with a focused electron beam. The average domain/grain sizes of Cu and Fe observed were well below 100 nm, confirming the formation of nanocomposites. Alloying on the atomic scale to ensure uniform mixing of the two elements in the precursor was found to be important for obtaining homogeneous microstructure and nanophase grain/domain size in the consolidated product. The full density nanocomposites exhibited microhardness well above the rule-of-mixtures estimates obtained using nanophase Cu and Fe as constituent phases. It is concluded that microstructures, rather than the phase volume fractions alone, determine the mechanical behavior of the composite. A modified rule of mixtures is used to explain the microhardness observed in terms of the geometric arrangements of the two phases and the effect of interphase boundaries as efficient dislocation barriers. Other possible contributions due to solid solution hardening, precipitation hardening, and dispersion hardening are also discussed.

ISBN

9780591997804

Pages

117

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