Structures and stabilities of nanocrystalline materials synthesized by mechanical alloying and modeled as driven alloys

Citation

Hong, Liubo (1996) Structures and stabilities of nanocrystalline materials synthesized by mechanical alloying and modeled as driven alloys. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/shrg-rd86. https://resolver.caltech.edu/CaltechETD:etd-12172007-114124

Abstract

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Nanocrystalline materials (NCM) are single-phase or multi-phase polycrystalline materials with crystal sizes in the nanometer range (2-20 nm). Owing to their very small grain size, NCM have enhanced or novel physical and mechanical properties compared to conventional materials, making NCM attractive for various technical applications. Mechanical alloying (MA) is now one of the most commonly used methods to synthesize NCM, among other far-from-equilibrium materials. During mechanical alloying, nanocrystalline materials are sustained in nonequilibrium states by continuous external driving (e.g., collisions, shearing, fracturing, welding, etc.), and can be studied as driven alloys -- a simplified model that grasps the essentials of external driving in a general way. In this thesis study, the structures and thermal stabilities of nanocrystalline materials synthesized by mechanical alloying were studied by experimental techniques, and were modeled as driven systems by Monte Carlo simulations.

A general introduction to nanocrystalline materials is given in Chapter 1 and discusses their structures, synthesis methods, characterization techniques, properties and technical applications. In parallel, introductions on phase transformations, driven alloys, and Monte Carlo simulations are given in Chapter 2. Our theoretical and simulational work on critical temperature of ordering transformations of driven square alloys is presented in Chapter 3, while the study on the ordering kinetics and low temperature phase diagrams of driven bcc alloys is presented in Chapter 4. We found that the ballistic (random) atom movements, caused by external driving, suppressed the critical temperature of ordering on square lattices. The decrease in critical temperature was linear only at small driving intensity. The ballistic atom movements also changed both the transient states and the steady states of ordering of bcc alloys. The stability of B32 order was increased with respect to the stabilities of B2 order and an unmixed state because of the smaller defect enthalpy sustained in B32 phase. In steady state, the B32 phase region encroached into adjacent B2 or unmixed phase regions, and regions of two-phase coexistence were formed. The changes were significant and nonintuitive, and provided useful guidance to the experimental studies presented in Chapter 5, 6, and 7.

In the experimental studies described in Chapters 5, 6, and 7, we studied how the microstructures of NCM prepared by MA depended on milling intensity, temperature, and composition (I, T, c). We found: (1) I -- The intensity of MA had little effect on the average grain size or strain, but changed the phase boundaries of Fe-Ni nanocrystalline materials drastically and in nonintuitive ways. Similar to what was found in Monte Carlo simulations, a region of bcc plus fcc two-phase coexistence occurred from MA, and shifted asymmetrically to the bcc side with increased milling intensity. This was attributed to the larger heterogeneities in free energy density in the alloy at higher MA intensity. (2) T -- Mechanical alloying of Ni3Fe and Fe3X (X=Si, Zn, Sn) at temperatures from 23 [...] to 300 [...] showed that the effect of milling temperature was little different from the role of temperature itself on the microstructure of NCM. (3) c -- The same Hume-Rothery rules for 5% solubility in equilibrium alloys translated to a 25% solubility in nanocrystalline materials Fe3X (X=Al, As, Ge, In, Sb, Si, Sn, Zn) prepared by MA. Also observed was the transient DO3 ordering of NCM Fe3Ge prepared by MA upon annealing at high temperature.

Thesis Files