Micro-analytical studies of discontinuous precipitation in Fe-13.5 at.% Zn alloy
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(2020) 20:76
ORIGINAL ARTICLE
Micro‑analytical studies of discontinuous precipitation in Fe‑13.5 at.% Zn alloy Paweł Zięba1 · Mateusz Chronowski1,2 · Jerzy Morgiel1 Received: 14 April 2020 / Revised: 13 May 2020 / Accepted: 10 June 2020 © The Author(s) 2020
Abstract For the first time, the analytical electron microscopy has been used to determine the solute concentration profiles left behind the moving reaction front (RF) of the discontinuous precipitation (DP) reaction in a Fe-13.5 at.% Zn alloy. These profiles have been converted into grain boundary diffusivity (sδDb) values, using Cahn’s diffusion equation in its original form and the data of the growth rate of the discontinuous precipitates obtained from independent measurements. This approach has essentially removed existing difference in comparison to sδDb values obtained from Cahn′s simplified and Petermann–Hornbogen models relevant for the global approach to the DP. Simultaneously, the local values of sδDb have been up to 8–10 orders of magnitude higher than the data for volume diffusion coefficients and much greater than for diffusion at the stationary grain boundaries of Zn in pure Fe. This is clear indication that the rate controlling factor for DP reaction in the Fe-13 at.% Zn alloy is diffusion at the moving RF.
1 Introduction Discontinuous precipitation (DP) belongs to a group of solid state diffusive phase transformations occurring at the moving interface called the reaction front (RF). During DP reaction, a supersaturated solid solution α° decomposes into colonies of alternate lamellae of a new solute-rich β phase and solute-depleted α phase. The DP is relevant example of nano-phase transformations as the solute redistribution leading to creation and growth of the lamellar product takes place at the moving RF which is ca. 0.5 nm in width and maximal diffusion distance is equal to half width of the α lamella. Moreover, the thickness of α phase lamellae is usually in order of 100–500 nm and the thickness of β lamellae even 7–10 times smaller. Fe–Zn system is very suitable for the examination of mechanism and kinetics of DP reaction as it shows a large range of solid solubility (up to 42 at.% Zn) and DP occurs in a wide range of ageing temperatures and solute concentrations. According to Predel and Frebel [1–3], only at * Mateusz Chronowski [email protected] 1
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta St. 25, 30‑059 Kraków, Poland
AGH-University of Science and Technology, Kraków, Poland
2
relatively high temperatures located ca. 20–50 °C below solvus line, continuous precipitation (CP) is the only one way of precipitation. On the other hand, a competition between CP and DP occurred at relatively low temperatures for the Zn content more than 25 at.%. Therefore, iron alloys with different content of Zn were the subject of extensive studies in the past. One of the first information, showing the typical DP morphology, was delivered by Schramm and Mohrnheim [4] and Hornbogen [5]. A few years later, Spe
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