Lattice orientation relationship between the M 2 C carbide and the ferrite matrix in the M50NiL bearing steel
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INTRODUCTION
THE M2C carbide is known to be responsible for the secondary hardening of high-strength alloy steels.[1,2] The carbide is present in steels that are alloyed with refractory metals, such as tungsten, vanadium, and molybdenum. Because of their low diffusivity in the ferrite matrix and a higher affinity to carbon than iron, the alloying elements precipitate slowly with carbon, forming the stable M2C carbide during tempering in part at the expense of Fe3C (cementite) phase. A significant secondary strengthening effect is commonly observed as a result of this precipitation. The degree of the strengthening depends on the particle size and morphology, as well as the volume fraction of the carbide. The lattice elastic strain, resulting from phase transformation, and interfacial energy of the carbide precipitates both play important roles in controlling the particle morphology and growth kinetics.[3] These two terms are closely associated with the lattice misfit between the precipitates and the matrix. Therefore, the lattice orientation relationship (OR) between the carbide and the matrix is a critical structural parameter needed to understand the precipitation kinetics and strengthening behavior of M2C carbides in alloy steels. Dyson and co-workers first reported an OR, (011)␣ // (0001)C and [100]␣ // [1120]C , between M2C carbide and the ferrite matrix in their study of an Fe-1.85 pct Mo-0.117 pct C model system.[4] This OR has been widely referenced in studies of M2C carbide precipitation in high-strength alloy steels. The OR is similar to that previously reported for the -carbide, a transitional phase in the martensitic matrix[5] and the wellknown Pitsch-Schrader OR.[6] Like M2C carbide, -carbide has a hexagonal close-packed (hcp) structure, which perhaps led Dyson and co-workers to believe that a similar OR might exist between M2C and the ferrite matrix. Based on limited transmission electron microscopy (TEM) results and diffraction analysis, they derived the aforementioned OR. One of the main concerns regarding Dyson’s OR is the presence of a large lattice misfit on the matching planes, S.G. SONG, Staff Scientist, is with Materials and Processing, United Technologies Research Center, East Hartford, CT 06108. Contact e-mail: [email protected] H. DU, Staff Engineer, is with the Mechanical Systems Product and Module Center, Pratt & Whitney, East Hartford, CT 06108. E.Y. SUN, Staff Engineer, is with the United Technologies Research Center, East Hartford, CT 06108. Manuscript submitted March 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
namely, the (011)␣ and (0001)C planes of the two phases (see Figure 1). The misfit ␦ is approximately 0.27 along the [011]␣ or [0110]C direction. Such a large misfit between matching planes is questionable from an interfacial energy point of view.[7] The present study is part of an investigation aimed at understanding the precipitation and strengthening behavior in an M50NiL bearing steel with a fixed carbon content of 0.4 pct.[8] A detailed study of the OR between t
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