The kinetics of ordering, grain growth and chemical segregation in nonequilibrium Fe3X alloys (X=Al, Si and Ge)

Citation

Gao, Zheng-Quiang (1994) The kinetics of ordering, grain growth and chemical segregation in nonequilibrium Fe3X alloys (X=Al, Si and Ge). Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/d93x-xn03. https://resolver.caltech.edu/CaltechETD:etd-08092005-084814

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Experimental studies on the kinetics of microstructural evolution in highly nonequilibrium Fe3X based alloys (X = Al, Si and Ge) are the topic of this dissertation research. The kinetics of ordering especially, and other processes of microstructural evolution including grain growth and chemical segregation, were studied. Materials far from their states of thermodynamic equilibrium were first prepared by rapid quenching and ball milling methods. Heat treatments at relatively low temperatures were used to allow the alloy microstructure to evolve towards equilibrium. Mossbauer spectroscopy was used heavily for measuring short range ordering, but X-ray diffractometry and transmission electron microscopy were also used extensively for microstructural characterization. The initial states of the Fe3X (X = Al, Si and Ge) binary alloys were chemically disordered bcc polycrystals (nearly random solid solution). Kinetics of disorder -> order transformations were studied by plotting the kinetic paths through the space spanned by two order parameters. The kinetic paths for Fe3Al showed a strong temperature dependence, but no temperature dependence was observed for kinetic paths of Fe3Si and Fe3Ge. A thermodynamically unexpected transient B32 ordered structure was observed in the early stages of ordering in Fe3Al annealed at 300[degrees]C. The kinetics of microstructural evolution, especially grain growth was also studied on nanocrystalline [...] alloys. The Nb atoms were first forced into solid solution in bcc Fe-Si by ball milling. Upon annealing, Nb atoms segregated to grain boundaries, and impeded grain growth. The inter-dependence of grain growth, chemical segregation and chemical ordering was studied. A better understanding of such kinetics, we believe, can be an important step towards the controlled synthesis of new nonequilibrium materials.

Thesis Files