Mechanism of the formation of lamellar M 23 C 6 at and near twin boundaries in austenitic stainless steels
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I. INTRODUCTION
WHILE studying transformations on aging of an austenitic stainless steel that had been slowly cooled from 1050 8C, Hatwell and Berghezan[1] did not find precipitation of carbide at coherent twin boundaries, while the precipitation easily occurred at incoherent twin boundaries. They, however, observed that only 5 pct deformation, prior to aging, induced precipitation at coherent twin interfaces as triangular particles. Both optical and electron microscopic studies have revealed carbide precipitation at incoherent as well as coherent twin boundaries during aging of the specimens that had been quenched earlier to room temperature from high annealing temperatures.[2–12] Nucleation of M23C6 occurs at incoherent twin boundaries at approximately the same time as that on grain boundaries.[8] Partial dislocations at incoherent twin interfaces act as preferential sites for precipitation in different alloy systems,[13,14] and in austenitic stainless steels, twinning dislocations of Burgers vector a/6 ^112& nucleate M23C6, which grows along the interface.[10] M23C6 precipitated at this interface has the same orientation as that of the matrix on one side of the interface[8] and has been reported to form ribbons of connected trapezoids.[4] At coherent twin boundaries, M23C6 has been reported to have different morphologies such as triangular,[9,12] needleshaped,[12] and platelike[8,9] particles growing on one side of the boundary and coalescing to long continuous layers. Extrinsic dislocations that might have been transferred to these interfaces during quenching have been suggested to nucleate these precipitates.[8] The growth of these particles has also been suggested to be aided by a flux of vacancies supplied by the boundary.[9] B. SASMAL, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur721302, India. Manuscript submitted September 14, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
M23C6 has also been reported to precipitate preferentially in austenite matrix very close to an incoherent twin boundary and to grow as plates on either side of the boundary.[2,6–8,10,11] These plates in the form of lamellae have been reported to grow along ^110& directions parallel to the twin plane with their {111} flat faces lying on {111} matrix planes parallel to the twin plane.[8] According to Beckitt and Clark,[10] plates of M23C6 form by repeated nucleation on migrating twinning dislocations thrown out of an incoherent twin boundary after precipitation at the boundary. Singhal and Martin[11] observed intrinsic stacking faults emanating from an incoherent twin boundary after precipitation of carbide at the boundary in a 25Cr24Ni-0.4C steel with comparatively lower stacking fault energy. They have suggested that the M23C6 plates close to incoherent twin boundaries form by nucleation on the partial dislocations bounding these stacking faults and by growth across the faults. In the present article, a detailed study on the precipitation of M23C6 a
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