Kirkendall Void Formation During Selective Oxidation

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e N2 + 5 pct-10 pct H2 gas atmosphere of largescale continuous annealing furnaces used for the recrystallization annealing of steel products reduces the iron oxides. It also causes the selective oxidation of the Mn, Si, and Al alloying additions. The presence of filmforming surface oxides, especially amorphous oxides, leads to the deterioration of the surface properties of steel, in particular during continuous hot dip galvanizing.[1,2] The selective oxidation processes at the surface and in the subsurface of advanced high-strength steels, such as dual-phase steel and transformation-induced plasticity steel, is well documented.[3–7] The selective oxidation processes that occur during the annealing of high-Mn austenitic twinning-induced plasticity steel[8,9] have not yet been observed, and the high resolution transmission electron microscopy (HR-TEM) of focused ion beam (FIB) cross-sectional samples was, therefore, used to carry out an analysis of the composition and microstructure of external and internal oxides formed at the surface and in the subsurface of an 18 pct Mn twinning-induced plasticity steel containing 1 pct Al. The sample surface of the twinning-induced plasticity steel, with composition 0.5 mass pct C, 18 mass pct Mn, 1 mass pct Al, and 0.2 mass pct Si, was mirror polished with a 0.5-lm diamond suspension prior to annealing to avoid all effects related to the roughness of the strip surface. The heat treatment was carried out in a N2 + 10 pct H2 gas atmosphere with a controlled dew YONG FENG GONG, Post-Doctoral Researcher, and BRUNO C. DE COOMAN, Professor, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail: [email protected] Manuscript submitted January 18, 2010. Article published online July 1, 2010 2180—VOLUME 41A, SEPTEMBER 2010

point of 256 K ( 17 C) and an oxygen partial pressure of 9.05 9 10 23 atm. The annealing was carried out at 1073 K (800 C) and the soaking time was 3 minutes. In these conditions, the atmosphere is known to favor external oxidation of the alloying elements in low carbon steels. Cross-sectional samples were prepared by FIB milling. The samples were investigated in a JEOL JEM-2100F field emission (FE)-transmission electron microscope (TEM) (JEOL Ltd., Tokyo, Japan) operated at 200 keV. Oxide composition analysis was conducted by energy dispersive spectroscopy (EDS). A typical cross-sectional view of the steel surface after annealing is shown in Figure 1. The surface was covered by a continuous 400–600-nm thick MnO oxide layer, and a Mn-depleted ferrite layer with a thickness of approximately 600 nm was formed in the adjacent steel matrix. Between the surface MnO layer and the steel matrix, a transition region existed that consisted of three layers: crystalline c-xMnO.SiO2 (1 £ x £ 2), amorphous a-xMnO.SiO2 (x