Discontinuous Dissolution Reaction in a Fe-13.5 at. % Zn Alloy.

Paweł Zięba,Mateusz Chronowski,Jarosław Opara,Olga A Kogtenkova,Boris B Straumal

doi:10.3390/ma14081939

Abstract

The dissolution process of a lamellar structure with α and Γ phases formed during a discontinuous precipitation reaction is investigated here with a Fe-13.5 at. % Zn alloy by means of optical microscopy and scanning and transmission electron microscopy. The α phase is a solute-depleted solid solution and the Γ phase is the intermetallic compound Fe3Zn10. The examination reveals that the dissolution occurs in a discontinuous mode by a receding of the former reaction front of the discontinuous precipitation towards the position of the original grain boundary. A new solid solution in the post-dissolution area is especially inhomogeneous and reflects the former locations of the Γ lamellae (“ghost images”) and the receding reaction front (“ghost lines”). A simulation procedure is applied to determine the Zn concentration profiles left in the post-dissolution region. Their shapes are mostly affected by the Zn content at the positions where the Γ lamellae have just been dissolved, which was also confirmed by the quantitative microchemical analysis.

Highlights

Fe-Zn systems represent a very well described example [1,2,3,4] in which the decomposition of a α Fe-rich supersaturated solid solution occurs via discontinuous precipitation (DP)
Subsequent annealing above a certain temperature results in a reverse process, discontinuous dissolution (DD), which is characterized by a backward migration of the DP front with a simultaneous formation of an inhomogeneous α~ solid solution [3,5]
The most demanding research was performed using transmission electron microscopy (Philips CM 20 Twin and Tecnai G2 FEG Super-Twin, FEI, Hillsboro, OR, USA), where the Tecnai instrument was equipped with an EDAX Phoenix energy-dispersive X-ray spectrometer (EDX) (FEI, Hillsboro, OR, USA), enabling high spatial resolution chemical analysis of the solute content in the DD products

Summary

Introduction

Fe-Zn systems represent a very well described example [1,2,3,4] in which the decomposition of a α Fe-rich supersaturated solid solution occurs via discontinuous precipitation (DP). Subsequent annealing above a certain temperature results in a reverse process, discontinuous dissolution (DD), which is characterized by a backward migration of the DP front (i.e., towards the original position of the grain boundary) with a simultaneous formation of an inhomogeneous α~ solid solution [3,5]. The process started and occurred at the primary grain boundaries, impingements of the colonies and at the primary cells with the contact with untransformed regions. The kinetics of the process were analysed using a Petermann-Hornbogen equation [6] with the conclusion that the dissolution of the cells occurs via the diffusion of Zn along the receding reaction front (RF). They documented the so-called go- and -stop motion of the receding colonies of discontinuous precipitates using scanning electron microscopy

Methods

Results

Conclusion