In situ investigation of synchronized dislocation array nucleation and phase transformation at mode I-II cracks of single-crystalline Mo

The deformation activities near crack tips of submicron-sized single-crystalline Mo were investigated using in situ transmission electron microscopy (TEM) with mixed mode I-II loading. Results show that dislocations in multiple slip systems were activated in front of crack tips. These dislocations glided on the uncommon slip planes of {123}, forming dislocation arrays. These dislocations moved at velocities of 3–5 nm/s with spacing of ∼10–34 nm in the zone of ∼50–300 nm away from crack tips. Dislocation velocity and spacing were influenced by the force from elastic crack stress field. Additionally, phase transformation from body-centered cubic to face-centered cubic was also activated in front of crack tips, and high densities of interface dislocations were observed at the semicoherent phase interfaces. Two kinds of phase transformation mechanisms were uncovered. One is the Pitsch mechanism, which is rarely accessed, while the other is the Nishiyama-Wasserman/Kurdjumov-Sachs mechanism.