Low-Temperature Fracture of Ultrafine-Grained Iron

Abstract

Metals with a body-centered cubic structure such as iron exhibit a ductile-brittle transition which results in a brittleness below a particular temperature. This temperature depends on many factors such as strain rate, size and geometry of investigated samples or the purity of the material. Another important parameter influencing fracture mechanism is grain size. It is known that grain refinement can be an efficient way to change fracture mechanism.The goal of this study was to investigate fracture mechanisms of ultrafine-grained iron processed by hydrostatic extrusion (HE). Materials subjected to various total strain levels were tested. The average grain size of the HE-processed iron was below 350 nm. The mechanical tests were carried out at various temperatures, ranging from room temperature to liquid nitrogen temperature. It was found that the fracture mechanism depends on a density of dislocation and the loading direction. It was found that materials with the dislocation density above a certain critical value break in a ductile manner even at impact tensile test in liquid nitrogen. However, bending tests of miniature beams have shown that ductile fracture occurs only when the crack propagates along the radial direction of the extruded material, whereas, on the direction parallel to the axial direction, cleavage fracture was observed. A theoretical model explaining this phenomenon was proposed. This model is based on the Rice model and it considers the competition between two phenomena - dislocation slip in the stress field of the crack front and Griffith cleavage.