Deformation and Fracture Behaviours of Cementite

Abstract

In order to clarify the deformation and fracture behaviours of cementite, observations by an optical microscope and a scanning electron microscope were made on the slip lines and fracture surfaces which took place around Vickers hardness indenters pressed on the surface of large cementite grain in white pig iron. Although slip lines in cementite were generally straight, wavy slip lines and cross slip lines were occasionally observed under specified conditions of temperature and crystal orientation. Large dependences on deformation temperature and crystal orientation were recognized for the slipping behaviour and hardness of cementite crystal. The slipping region increased and the hardness decreased with increase in deformation temperature. These temperature effects were remarkably large for cementites having the surface orientation around the center or near the [001]_θ–[\bar111]_θ boundary of a stereographic triangle. The gliding of cementite having these orientations was very easy in comparison with those of the cementites having the orientation around [001]_θ or near the [001]_θ–[110]_θ boundary of the stereographic triangle. It was further made clear that the operation of the slip system of cementite crystal was closely related with the Schmid factor in spite of a strong anisotropy in the crystalline structure. The fracture of cementite at room temperature was of a cleavage type, judging from the topographic features of fracture surfaces. The indeces of these fracture surfaces were determined to be {110}_θ, {100}_θ and {210}_θ.