Transmission Electron Microscope Study on Deformation and Fracture of Cementite in Cold-Rolled Steels

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Abstract
In order to study the deformation behavior of cementite in carbon steels cold-rolled to various degrees of reduction, observations of dislocation structures were made by using a transmission electron microscope. The results obtained are summarized as follows: (1) A plastic deformation of cementite due to dislocation gliding was demonstrated to occur in the carbon steels. The gliding was also found to take place very heterogeneously; it was concentrated only on several slip planes being in parallel and 0.2∼0.3μ apart from each other. (2) Many straight dislocations were observed on slip planes, and the density of such dislocations was greatly increased in proportion to the increase in the deformation of steels. In addition to the dislocation, moiré pattern, cell wall and stacking fault were also observed in the deformed cementite. (3) Slip planes of cementite crystals at room temperature were determined to be (100)θ, (010)θ and (001)θ. The possibility of two slip directions, [001]θ on (010)θ and [010]θ on (001)θ, was suggested from a detailed observation of the image of pile-up dislocations on the slip planes. A slip direction, [010]θ on (100)θ, was also foreseen to be probable. However, it must be noted that the slip direction [001]θ on (010)θ is not consistent with the one expected from the crystalline structure of cementite. The dislocations on (100)θ were straight and their direction coincided with the slipping direction. The dislocations on (001)θ, which exhibited somewhat a curved shape, were also aligned as a whole in the slipping direction. It was confirmed from these experimental results that the majority of dislocations on their slip planes had a screw nature. (4) The fracture of lamellar cementite proceeded in the following way: (a) The gliding of cementite crystals in the heterogeneous manner results in division of the original cementite crystal into several undeformed blocks. (b) The mutual gliding displacement between two adjacent blocks (height of slip steps) increases with a successive deformation of the cementite. (c) Finally, the cementite is completely broken away along the boundaries of the blocks probably due to the stress concentration at the slip steps.

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