Acta Metall Sin  2019, Vol. 55 Issue (4): 511-520    DOI: 10.11900/0412.1961.2018.00166
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In Situ TEM Study on the Sympathetic Nucleation of Austenite Precipitates
Juan DU1,Xiaoxing CHENG2,Tiannan YANG2,Longqing CHEN2,3,Frédéric Mompiou4,Wenzheng ZHANG1()
1. Key Laboratory of Advanced Materials MOE, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2. Department of Materials Science and Engineering, The Pennsylvania State University, University Park,PA 16802, USA
3. State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
4. CEMES-CNRS and Université de Toulouse, 29 rue J. Marvig, 31055 Toulouse, France
Abstract

Duplex stainless steels (DSSs) are widely used for chemical industry, marine construction and power plants, due to the beneficial combination of ferrite and austenite properties: high strength with a desirable toughness and good corrosion resistance. The sympathetic nucleation (SN) of intragranular austenite precipitates has been frequently observed in DSS. This type of nucleation, which occurs in a considerable variety of steels and titanium alloys, has a great effect on the morphological arrangement of precipitates and hence the mechanical properties of metallic materials. Therefore, understanding the SN mechanism of austenite precipitates is essential to knowledge based material design of the microstructure in DSS. Three types of morphological arrangement, i.e., face-to-face, edge-to-edge and edge-to-face SN of austenite precipitates, have been identified in previous investigations on DSS. The adjacent grains of face-to-face and edge-to-edge sympathetically nucleated austenite have approximately the identical orientations, with a small-angle boundary between two austenite crystals. However, as regards to the edge-to-face SN, the lacking of crystallographic features of adjacent austenite precipitates obstructs the understanding of the mechanism for the edge-to-face SN. Moreover, it is usually difficult to distinguish between SN and hard impingement following nucleation at separate sites in conventional experimental observations. Thus, in the present work, the typical morphology of edge-to-face SN of austenite precipitates was directly observed at 725 ℃ in a DSS using in situ TEM. The orientation relationship (OR) between the sympathetically nucleated austenite precipitate and ferrite matrix is determined through analysis of Kikuchi lines. Since the long axes of austenite precipitates parallel to the invariant line are restricted in the thin TEM foil, there are only four types of austenite with different near N-W ORs and cystallographically inequivalent long axes. This work reveals that the ORs of sympathetically nucleated austenite grains belong to different Bain groups with those of the pre-formed austenites. The explanation for the OR selection is provided based on two factors favoring SN, namely the reduction of elastic interaction strain energy and the interfacial energy. The local stress generated by the semi-coherent pre-formed austenite was calculated by Eshelby inclusion method. The local stress field accompanying with the pre-formed austenite assists the subsequent nucleation and growth of sympathetically nucleated austenite. It shows that the elastic interaction energy for the sympathetically nucleated austenite of particular OR is negative. In addition, the pre-formed austenite and the sympathetically nucleated austenite grain are twin related. This indicates that the nucleation barrier associated with SN of austenite with selected OR is comparably lower than other candidates. Hence, the austenite precipitate with a specific OR is preferred during SN.

 ZTFLH: TG113
Fund: National Natural Science Foundation of China(No.51471097)
Corresponding Authors:  Xiaoxing CHENG,Wenzheng ZHANG     E-mail:  zhangwz@tsinghua.edu.cn
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 Fig.1  The snapshots of the initial growth of sympathetically nucleated austenite γ2 on the habit plane of pre-formed austenite γ1 at 725 ℃ using in situ TEM Fig.2  The morphology of sympathetically nucleated austenite γ2(γ3) and pre-formed austenite γ1(γ4) (a) and an enlarged micrograph showing the grain boundaries between γ1 and γ2 (b) Fig.3  Kikuchi diffraction patterns for constructing the orientation matrix(a) diffraction pattern of ferrite (b) diffraction pattern of the austenite along the same direction as in Fig.3a Fig.4  The morphologies of γ1 (a, c) and Kikuchi diffraction patterns (b, d) for determining the long axis of γ1 along the beam direction of [$0.94ˉ0.32ˉ$0.09]b (a, b) and the beam direction of [$1ˉ1ˉ1$]b (c, d) Fig.5  The habit plane (HP) of γ1 viewed in an edge-on condition (a) and Kikuchi pattern of ferrite matrix (b) Fig.6  A stereographic projection around [001]b showing the long axes of austenite γ1~γ4 (Δ) with the foil normal (fn, +), plotted with three possible cones of unextended lines Fig.7  Configuration of austenite and ferrite matrix for the calculation of the strain field created by pre-formed austenite Fig.8  Distributions of mutual elastic strain energy density between sympathetically nucleated austenite and local stress created by pre-formed austenite