Fracture of metastable materials near absolute zero

Fracture is one of the key issues for integrity of structures made of metastable materials operating at extremely low temperatures. A broad class of these materials, in particular the austenitic stainless steels, behave in a ductile way when strained near absolute zero, showing the evidence of ductile fracture. The stainless steels exhibit at very low temperatures metastable behaviour, consisting in the plastic strain induced fcc-bcc phase transformation, leading to generation of two-phase continuum composed of austenitic matrix and martensitic islands. As the classical models of fracture do not include the phase transformation, there is a clear need to extend the description of fracture to multiphase materials. The present model refers to the Hutchinson solution for the stress and strain fields in front of a macrocrack, obtained in the framework of the Hencky–Ilyushin (H–I) deformation theory. Based on this solution, a model including the plastic strain induced phase transformation is developed, and a closed form analytical solution including the distribution of the secondary phase ahead of the crack tip is presented. Moreover, the analytical solution is cross checked with the experimental data obtained for notched specimens, loaded in liquid helium (4.2$$K) until fracture. Evidence for accumulation of secondary phase along the macrocrack trajectory is shown. The microscopic observations, performed by means of a scanning electron microscope including EBSD, show significant microstructure evolution as well as decreasing martensite content when moving away from the crack tip. A fairly good agreement between the analytical model and experimental data was obtained, indicating the usefulness of the analytical solution.