奥氏体析出相激发形核的原位TEM研究

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.