The mechanism of brittle fracture in a microalloyed steel: Part II. Mechanistic modeling

In Part I of this study, cuboidally shaped inclusions were found to be responsible for cleavage initiation in a low-carbon, microalloyed steel. In Part II, electron microdiffraction was used to identify these inclusions as the fee phase (NaCl prototype) in the titanium-nitrogen system. A model for cleavage as induced by these inclusions is proposed. A microcrack begins on one side of the TiN inclusion, propagates to the other side, and then transfers into the matrix. Initiation at a particular location in the particle is believed to be caused by dislocation pileup impingement and stress concentrations such as crystal defects and surface irregularities within the TiN. Dislocations in the TiN inclusions were imaged by transmission electron microscopy (TEM), After the TiN microcrack transfers into the matrix, propagation spreads radially. From the area of crack transfer, two simultaneous propagation paths reverse directions and rotate around the particle. The particle separates these cracks for a short distance, they travel on different cleavage planes, and upon rejoining, a ridge of torn matrix is created. The location of this ridge can be used to infer where cleavage began in the TiN and where the microcrack transferred into the matrix. Tessellated residual stresses arising from differential thermal contraction between the TiN and the matrix are suggested to increase the cleavage-initiating potency of TiN inclusions.