Determination of energetic parameters controlling cleavage fracture in a Ti-V microalloyed ferrite-pearlite steel

Abstract

In BCC metals the process of brittle fracture has been attributed to the nucleation of a microcrack followed by its propagation into the surrounding matrix. This process implies three different steps that must take place dynamically. In the first step a microcrack nucleates from the rupture of some microstructural feature (carbide, non-metallic inclusion, ...). In the second and third steps, this microcrack propagates across the particle-matrix (p-m) and matrix-matrix (m-m) boundaries, respectively. The critical cleavage fracture stress required to achieve this three step brittle process has been experimentally determined for a wide range of microstructures. This value and the microstructural and energetic parameters ({gamma}{sub pm} and {gamma}{sub mm} effective surface energies for the second and third steps, respectively) are currently being used in the models developed to predict brittle fracture processes. One of the problems which arises in the modelling process is the definition of {gamma}{sub pm} and {gamma}{sub mm} effective surface energy values. Concerning the {gamma}{sub pm} energy, extensive studies have been carried out with different steels and microstructures, usually proposing values in the range from 7 to 14 J/m{sup 2}. In contrast, {gamma}{sub mm} experimental measured data are very scarce. In this work {gamma}{sub pm} and {gamma}{sub mm}more » effective surface energies have been experimentally determined in the case of a Ti-V microalloyed steel with ferrite-pearlite microstructure. In this steel, the nucleation of cleavage fracture takes place through the rupture of coarse TiN particles (>0.5 {micro}m).« less