Microstructure design of mechanically alloyed materials

Abstract

Nanocrystalline metals have exceptional mechanical properties that make them attractive for structural applications. However, these materials' properties tend to degrade due to grain growth when they are exposed to high temperatures; this makes producing bulk, nanocrystalline components particularly difficult as the most promising synthesis methods involve high temperature densification of powders or foils. Several alloy design strategies have been developed to overcome these thermal stability issues, but their efficacy depends on the spatial distribution of the stabilizing element in the feedstock materials, which are typically prepared using extensive plastic deformation or mechanical alloying. There is thus a need to predict the chemical mixity of mechanically alloyed materials, and this thesis seeks to address this need. To this end, phase strength effects are incorporated into a kinetic Monte Carlo simulation of a mechanically-driven, binary alloy, which can provide quantitative insight into the combination of processing and material parameters that dictate the steady state chemical mixity. Using such simulations, dynamical phase diagrams are generated that predict temperatures and compositions at which a couple with a given phase strength mismatch should chemically homogenize during mechanical alloying. Several of these dynamical phase diagrams are validated using mechanical alloying experiments, in which tungsten-transition metal couples with various phase strength mismatches are mechanically alloyed in a high energy ball mill. This thesis also describes an alloy design case study in which the insights from these simulations and experiments are used to develop a nanocrystalline W-based (W-7Cr-9Fe, at%) alloy powder that can be rapidly compacted to high relative densities while maintaining ultrafine grain sizes. Two-phase compacts made from the alloy exhibit microhardnesses of 13 GPa and dynamic compressive strengths in excess of 4 GPa. Furthermore, postmortem images of compressed micropillars machined out of these compacts suggest that this alloy deforms by shear localization. The penetration performance of this alloy is explored in sub-scale ballistic tests into concrete targets, and is found to be at least as good as current state-of-the-art penetrator materials.