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 resp onsible for cleavage initiation in a low-carbon, microalloyed steel. In Par t II, electron microdiffraction was used to identify these inclusions as th e fee phase (NaCl prototype) in the titanium-nitrogen system. A model for c leavage as induced by these inclusions is proposed. A microcrack begins on one side of the TiN inclusion, propagates to the other side, and then trans fers into the matrix. Initiation at a particular location in the particle i s believed to be caused by dislocation pileup impingement and stress concen trations such as crystal defects and surface irregularities within the TiN. Dislocations in the TiN inclusions were imaged by transmission electron mi croscopy (TEM), After the TiN microcrack transfers into the matrix, propaga tion spreads radially. From the area of crack transfer, two simultaneous pr opagation paths reverse directions and rotate around the particle. The part icle 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 Ti N and where the microcrack transferred into the matrix. Tessellated residua l stresses arising from differential thermal contraction between the TiN an d the matrix are suggested to increase the cleavage-initiating potency of T iN inclusions.