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

RK Barik and S Biswal and KK Bhandari and A Ghosh and D Chakrabarti, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 885, 145616 (2023).

DOI: 10.1016/j.msea.2023.145616

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 per-formed 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 suf-ficiently large < 110 > straight crack front propagates along a 001 cleavage crack plane.

Return to Publications page