Mechanics of microvoid nucleation and growth in high-strength metastable austenitic steels

Recent experiments on high-strength metastable austenitic steels have demonstrated the possibility of achieving record toughness levels when the fracture process is associated with martensitic transformation in the strain-induced regime.

For this class of materials, strain localization, which ultimately leads to fracture, is driven by void softening controlled by nucleation at second-phase particles. In contrast to lower strength, lower purity materials, where significant porosity evolves prior to ultimate localization and fracture, in high strength steels voids formed at the relatively more "tenacious" submicron refining particles do not seem to evolve in such a gradual manner, and the fracture process appears to be more directly related to the void nucleation event which occurs through decohesion of the particle-matrix interface.

In an effort to improve the current understanding of the failure processes in metastable austenitic steels, and, in particular, to identify local effects of strain-induced martensitic transformation on microvoid nucleation and growth, we have developed a numerical model to simulate the nucleation of voids by interface decohesion around hard second-phase equiaxed particles in transforming and non-transforming matrices.

The introduction of a cohesive zone model for the particle-matrix interface allows us to conduct parametric studies of the void nucleation and growth process from initial debonding through complete decohesion, for varying interface characteristics and loading conditions, while the incorporation of a constitutive model for the transforming austenitic steel allows us to explore the effects of different material parameters.

The results of these studies allow a quantitative assessment, at the microscale level, of the effects of martensitic transformation on void nucleation and growth, and yield considerable insight toward a more general understanding of the conditions under which void nucleation can occur for this class of materials. Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.