On Constituents of the Periodic Layered Microstructure Developed in the Solid-State Reaction Between Zinc and Co 2 Si
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INTRODUCTION
THE present study originates in the quest for better understanding of the operational mechanism governing periodic layer formation in displacement solid-state reactions. This special class of reaction zone morphology, which has been found in several metal/ceramic (i.e., SiC/Ni, SiC/Pt, and SiO2/Mg) as well as in a number of metallic (e.g., Fe3Si/Zn, Co2Si/Zn, Ni3Si2/Zn, and Ni50Co20Fe30/Mg) systems, is now understood as a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state. The latter induces a shift of in-situ inert inclusions (reaction product) toward the faster diffusing component. In a more general way, the mechanism is operative when the components have widely different mobilities in adjacent reaction product layers. More details concerning this peculiar reaction phenomenon can be found in our recent compilation,[1] and here, we will concentrate only on the diffusion microstructure developed in the Co2Si/Zn system. The selection of this system was dictated by three reasons: (1)
this is the finest periodic layered structure (with spacing of ~1 lm) found until now in metal silicide systems,[2,3] (2) the apparent influence of crystallographic orientation of the Co2Si-substrate grains on the band frequency (spacing) within the product periodic morphology (Figure 1(a)), and (3) the striking pattern (appearance of ‘‘loops’’ composed of CoSi particles, resembling the dislocation loops emerging from a Bardeen–Herring source[4]) observed in a section of the periodic layered reaction zone of the Co2Si/Zn couple perpendicular to the diffusion direction (Figure 1(b)). This peculiar pattern is also reminiscent of the spiral wave activity in the thin-layer homogeneous Belousov–Zhabotincky reaction.[5] (Note: Before embarking upon the present investigation (for the sake of comparison), we have reproduced (repeated) some diffusion couple experiments reported in Reference 2). The purpose of this communication is twofold: (1)
(2) J. WOJEWODA-BUDKA, A. WIERZBICKA-MIERNIK, L. LITYNSKA-DOBRZYNSKA, and A. KORNEVA are with the Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta St., 30-059 Krako´w, Poland. Contact email: [email protected] A. KODENTSOV is with Mat-Tech BV, Development & Testing, Ekkersrijt 4605, 5692 DR Son, The Netherlands. Manuscript submitted November 20, 2019. Article published online May 1, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
to present structural information about constituents of the product layered morphology developed in the annealed Co2Si/Zn diffusion couples; and to draw attention to some apparent ambiguities in correlating available data on binary Co-Zn intermetallics and crystallographic information obtained from the examination of the diffusion zone in the Co2Si/Zn reaction couples.
VOLUME 51A, JULY 2020—3497
Fig. 1—BEIs of the reaction zone morphology developed in the Co2Si/Zn diffusion couple annealed at 390 C for 28 h in argon showing (a) the general view (at point A, the substrate grain boundary me
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