Microstructures of deformation and fracture of cementite in pearlitic carbon steels strained at various temperatures

In order to make clear the effect of temperature on deformation and fracture of cementite in steels, observations by transmission electron microscopy were made on cementite in carbon steels strained in tension at various temperatures ranging from -196 to 700°C. Hardly any plastic deformation of cementite was detected at-196 and-78°C. It was confirmed that above room temperature cementite in steel can deform due to dislocation slip and that the deformation becomes easier as the temperature increases. Slip in cementite at room temperature and at 300°C seems to be confined to only (100) or (001). Dislocations observed at room temperature and at 300°C were mostly isolated and straight. Above 400°C (100), (010), (001), and some {110} planes are all operative slip planes. Dislocation loops, dipoles, cusps, and networks were frequently found. These observations indicate that double slipping and the interaction of dislocations can occur and that the deformability of cementite above 400°C is very large. An appreciable degree of dynamic recovery was detected above 500°C. The fracture of cementite at -196 and-78°C occurred in a cleavage manner along some crystallographic planes such as (110), (100) or (210). Above room temperature fracture occurred along an activated slip plane and was preceded by some amount of slip on that plane. Above 400°C cementite fracture, caused by joining of voids, formed along an activated slip plane was frequently observed.