Abstract
The peritectoid reactions of FeSi(ε-phase: core)+Fe2Si5 (α-phase: matrix)→3FeSi2 (β-phase: rim) and ε(core)+ Si(matrix)→β(rim) in Mn-doped Fe-66.7at%Si thermoelectric alloy is known to be extremely slow. We have already proposed the following new hypothesis of "Exhaustion of diffusion-contributable vacancy in core/rim structure" for the mechanism of such slow rate of peritectoid reactions: (1) for the progressing of the peritectoid reactions, Si atoms in the matrix of α or, β+Si (Si-source) need to diffuse to the closed interface of ε-core/β-rim (reaction site) via β-rim, and at the same time the atomic-vacancies in ε-core should diffuse in counter-wise direction to the matrix via the closed interface and β-rim, and (2) such mutual counter-wise diffusions strongly suppress vacancies in the matrix to diffuse into ε-core and thus causes "the exhaustion of diffusion-contributable vacancy in ε-core and β-rim", which leads to the extremely slow rate of the peritectoid reactions.
In this study, we succeeded in getting the following one experimental evidence for the above new hypothesis: the β-formation rate was extremely fast for ε-grains contacting with test-piece surface acting as vacancy-source, compared with that for ε-grains in test-piece inside. This conclusion also suggested that general diffusion data obtained by diffusion-couple experiments can not be directly applied to the diffusion-related phenomena which occur at a closed interface and the reactant atoms or ions diffuse from the outside of the closed interface to the interface via the product, i.e., rim, etc..
