Micromechanics of cleavage fracture and the associated tongue formation in ferritic steel

Abstract

The present work investigates the micro-mechanisms of cleavage fracture in a low carbon ferritic steel, subjected to Charpy impact test at low temperature. EBSD crack path analysis reveals {100} crystallographic plane as the predominant cleavage plane in ferrite, with occasional cracking along the secondary {110} cleavage plane. {110} cleavage cracking is particularly feasible when the {100} planes within a given grain exhibit significant twist angles with the preceding crack plane. Moreover, the fracture surface also exhibited numerous “tongue” like features that are intermittently aligned parallel to a common <110> crack front, as evidenced from the point EBSD analysis directly on the fracture surface. To illuminate the origin of such intermittent tongues, we performed molecular dynamics fracture simulation on a pure Fe single crystal, considering a (001) edge crack with a large [110] crack front. It was found that the development of intermittent tongues mainly originates from the intermittent nucleation of symmetric {112}<111> type twins across the entire crack front. Such intermittent twin nucleation results in a sinusoidal crack front, characterized by twin boundary cracking in one region and radial expansion of cleavage cracking in the adjacent regions, which can ultimately lead to the formation of tongues on the fracture surface. Finally, based on the simulation results and experimental observations, we propose a step by step mechanism that potentially explains the formation of intermittent tongues when a sufficiently large <110> straight crack front propagates along a {001} cleavage crack plane.