Fu, Zezhong (1993) The mechanical alloying of aluminum and zirconium. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-08272007-103954
Over the last several years, mechanical alloying/milling (MA/MM) process has been applied to produce different types of metastable or non-equilibrium materials such as metallic glasses, quasi-crystalline materials, nanocrystalline metals, compounds and supersaturated solid solutions. It appears that MA offers greater latitude in controlling the microstructure than other non-equilibrium processing methods such as rapid solidification. Despite a considerable number of experiments using MA/MM, many questions regarding the mechanisms of phase transformations induced by MA, especially the mechanism of amorphization, remain to be answered. In this thesis, the sequence of phase transformations induced by the mechanical alloying of aluminum and zirconium has been studied. The structural analysis indicated that none of the thermodynamically stable intermetallic compounds found in the phase diagram are formed during MA of the Al-Zr system. Instead, the nanocrystalline supersaturated [alpha]-Zr solid solution and amorphous phase are synthesized depending on the initial composition of the powder mixture. The thermodynamic and structural properties of these ball-milled materials have been characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).
As we know, one of the crucial aspects of nanophase and amorphous phase materials is the stability against grain growth and crystallization. In Chapter 4, the metastability of nanocrystalline materials based on the thermodynamics and grain boundary segregation arguments proposed by Johnson has been discussed. The thermal stabilities of the nanostructured [alpha]-Zr solid solutions and amorphous materials have also been studied. The experiments found that the grain size of nanocrystalline supersaturated solid solution is stable under heat treatment until reaching a temperature where the crystallization of an equilibrium compound phase occurs.
In Chapter 5, the temperature effects on the mechanical alloying of Al and Zr have been studied by milling at different ambient temperatures. The experiments reveal that the ultimate grain size of nanostructured materials prepared by ball milling is determined by two main factors. One is densities of structural defects which depends on the competition between the severe plastic deformation induced by MA and the recovery behavior of the materials. Another is chemical effects which are related to the composition of the sample and the interactions among the components. Obviously, the recovery or relaxation behavior is temperature dependent, so the milling temperature could influence the average grain size. The chemical effects on grain size may also change with the milling temperature. The experimental results suggest that the steady state obtained by ball milling at higher temperature is much closer to a chemical equilibrium state compared with the state formed by milling at room temperature. A faster alloying rate is obtained at higher temperature milling. This is consistent with the solid state reaction mechanism by MA.
The mechanisms of amorphizations by MA of aluminum and zirconium under different experimental conditions have been discussed in Chapter 6. The emphasis is to argue the possibility of polymorphic amorphization.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Materials Science|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||14 October 1992|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||29 Aug 2007|
|Last Modified:||26 Dec 2012 02:58|
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