Transmission Electron Microscopy Study on the Grain Boundary Precipitation of an Fe-Ni-Mn Maraging Steel
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INTRODUCTION
SUBSTANTIAL age hardening has been found in iron-nickel-manganese martensitic steels.[1] Formation of body-centered cubic (bcc) b-NiMn zones has been suggested for early stages of aging.[2,3] However, facecentered cubic (fcc) Ni3Mn,[3] bcc NiMn,[4] and facecentered tetragonal (fct) h-NiMn[5–11] precipitates have been identified at later stages of aging. The Fe-Ni-Mn maraging steels suffer from premature intergranular fracture along prior austenite grain boundaries after aging. Hereafter, the prior austenite grain boundary will be referred to as grain boundary. Analogous to the deleterious effect of manganese segregation in grain boundary failure of temper-embrittled low alloy steels, early studies suggested that segregation of manganese at grain boundaries during isothermal aging causes grain boundary embrittlement of Fe-Ni-Mn maraging steels.[11–13] However, it has recently been found that pronounced grain boundary precipitation causes intergranular brittleness in these steels. For instance, Mun et al.[14] reported precipitation of fcc austenite particles at grain boundaries in an Fe-8Mn-7Ni (wt pct) maraging steel. Lee et al.[15] then reported precipitation of the fct h-NiMn phase at grain boundaries of an Fe-10Ni5Mn (wt pct) steel. Nevertheless, Wilson[16] disputed S. HOSSEIN NEDJAD, Assistant Professor, is with the Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Contact e-mail: [email protected] M. NILI AHMADABADI, Professor, is with the School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran. T. FURUHARA, Professor, is with Institute for Materials Research, Tohoku University, Sendai 9808577, Japan. Manuscript submitted July 31, 2007. Article published online November 20, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
that manganese segregates at grain boundaries as an initial step in the formation of grain boundary precipitates and acts as a major embrittling element in the early stages of aging. Therefore, grain boundary precipitation of Fe-Ni-Mn maraging steels has remained controversial, and more research is needed to clarify the grain boundary precipitation behavior and corresponding interrelation with brittle fracture of these steels. Intergranular embrittlement and age hardening behavior of an Fe-10Ni-7Mn (wt pct) maraging steel during isothermal aging at 753 K have been reported.[10,17–19] The aim of this article is to study grain boundary precipitation of the Fe-10Ni-7Mn (wt pct) maraging steel.
II.
EXPERIMENTAL PROCEDURE
An Fe-Ni-Mn steel weighing 6 kg was prepared in a vacuum-induction melting furnace under 10-2 mbar using electrolytic iron, electrolytic manganese, and pure nickel shots. Bars weighing 200 g were cut from the ingot and remelted under argon gas in the water-cooled copper mold of a vacuum-arc melting furnace. Chemical composition of the remelted steel is given in Table I. Remelted bars were encapsulated in quartz tubes under argon gas after evacuation to 10-5 mbar. Homogenizing treatment was performed at 1473 K fo
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