Surface phenomena and fracture in Fe–(3 wt. %Si) single crystals with edge cracks (001)[110] on macro and atomic level

Abstract

Fracture tests on single crystals with edge cracks (001)[110] (crack plane/crack front) of different lengths were performed at room temperature in tension mode I in a test machine with a cross head speed of 3 mm/min. Time development of the fracture process was monitored in situ on a free specimen surface (110) via optical microscopy. Post-fracture analysis was performed using 3D optical microscopy. The crystal with the shorter crack was more ductile. Here, the formation of a distinctive plastic valley was observed initially at the crack front and later a widespread plastic zone creating a slope (Z-profile) on the observed surface (110), leading to higher fracture toughness. In the specimen with the longer crack, just a narrow plastic valley was created and fracture was more brittle with lower fracture toughness. Three-dimensional (3D) atomic simulations via molecular dynamics (MD) were performed under an equivalent load rate dP/dt in mode I at a temperature of ∼295 K in bcc iron cracked crystals of the same orientation and similar geometry. The ratio of the energies and of the stress needed for fracture in the crystals with the short and longer crack from experiment complies with those obtained from MD. Activation of oblique slip systems (with Burgers vector ⟨1¯1¯1⟩ a0/2) explains the newly detected surface phenomena, such as a 3D plastic valley and the oblique and parallel slip traces with respect to the crack plane. Widespread oblique twinning on (112) planes explains the plastic zone with the Z-profile perpendicular to the observed surface (110). Plastic processes start from free surfaces (110) due to the higher shear stress at the crack tip near the free surface.