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The production and crystallization of amorphous Fe-C alloys

University of British Columbia

The production and crystallization of amorphous iron-carbon alloys has been investigated experimentally. A physical vapour deposition (PVD) technique has been developed at UBC to create films with a variety of carbon contents. A system using controlled flow of reactive gas has been developed to allow the variation of carbon content, and films have been shown to have a reproducible amorphous structure and consistent chemistry. Characterization of the chemistry and structure of the as-sputtered alloys has been performed. Amorphous films were annealed to assess the kinetics of crystallization. For films containing less than 25 at.% carbon, a two-stage crystallization involving the formation of ferrite followed by cementite was observed at low temperatures. The structure and chemistry of these crystallization products were characterized by x-ray diffraction, electron microscopy and atom probe tomography. In-situ annealing was also per- formed in transmission electron microscopy, allowing for direct observation of the nucleation and growth of the product phases. This annealing study showed a significant decrease in the nucleation and growth rate of ferrite within the amorphous matrix. Simple models of diffusion-controlled and interface-controlled growth were not able to capture this slowing of the transformation. In addition to thermodynamic factors, it is proposed that the ferrite growth rate is strongly affected by a decrease in diffusivity arising from aging of the amorphous matrix and its enrichment in carbon during crystallization. Alloy films containing more than 25 at.% carbon were also found to crystallize in a two-stage process during annealing. This crystallization involved the initial formation of ferrite and cementite, with a secondary formation of cementite to fully consume the original structure. This two-step process has not been previously reported in the related literature. The secondary crystallization led to large grains of cementite that exhibited a systematic lattice compression in the [010] direction. The final cementite structure was found to have a super-stoichiometric carbon concentration that can only be possible via a process of ‘chemical twinning’. While faulting on the correct lattice plans was observed, no diffraction evidence for such ‘chemical twinning’ could be identified. This is proposed as an area for future research.

Text

2017-08-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/62904

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/