Diffusion, Interdiffusion coefficients, Phase boundaries
A new method to extract composition dependent ternary interdiffusion coefficients from a single diffusion couple experiment is presented. The calculations involve direct determination of interdiffusion fluxes from experimental concentration profiles and local integration and differentiation of Onsager's formalism. This new technique was applied to concentration profiles obtained from selected semi-infinite, single-phase diffusion couple experiments in the Cu-Ni-Zn, Fe-Ni-Al, and Ni-Cr-Al systems. These couples exhibit features such as uphill diffusion and zero flux planes. The interdiffusion coefficients from the new technique along with coefficients reported from other methods are graphed as functions of composition. The coefficients calculated from the new technique are consistent with those determined from Boltzmann-Matano analysis and an alternate analysis based on the concept of average ternary interdiffusion coefficients. The concentration profiles generated from the error function solutions using the calculated interdiffusion coefficients are in good agreement with the experimental profiles including those exhibiting uphill diffusion. The new technique is checked for accuracy and consistency by back-calculating known interdiffusion coefficients; in this exercise, the new method accurately predicts constant diffusivity. After rigorous verification, the new technique is applied to previously unexamined couples in the Ni-Pt-Al system. With Ni as the dependent component, the main coefficients are shown to be relatively constant and the cross coefficients are negative. The interdiffusion coefficient representing the contribution of the concentration gradient of Pt to the interdiffusion flux of Al is relatively large for couples whose Al content is low, indicating that Pt has a significant effect on Al when Al concentration is low. Another important aspect of analyzing diffusional interactions is the movement of single and multi-phase boundaries within a diffusion couple. Phase boundaries for an n-component system are newly classified and boundary movement is analyzed in terms of degrees of freedom. Experimental evidence of a category 2:1 boundary is presented with a solid-to-solid semi-infinite diffusion couple in the Fe-Ni-Al system with two single-phase terminal alloys. The diffusion path for this couple surprisingly passes through the vertex of the equilibrium tie triangle on the phase diagram to exhibit three phase equilibria in a ternary system. Here is shown for the first time experimental verification of this phenomenon.
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Master of Science (M.S.)
College of Engineering and Computer Science
Mechanical, Materials and Aerospace Engineering;
Materials Science and Engineering
Length of Campus-only Access
Masters Thesis (Open Access)
Diffusion; Dissertations, Academic -- Engineering and Computer Science; Engineering and Computer Science -- Dissertations, Academic
Elliott, Abby Lee, "Phenomenology And Experimental Observations In High Temperature Ternary Interdiffusion" (2004). Electronic Theses and Dissertations. 89.