Fracture of Fe–3 wt.% Si single crystals

Abstract

The ductile to brittle transition in α-iron was studied on four oriented single crystals of a Fe–3 wt.% Si alloy using tensile tests of flat-notched specimens, scanning and transmission electron microscopy. The experimental results are compared with molecular dynamic simulations. Single-edge notched specimens were loaded in tension at room temperature, the crack propagated in a (0 0 1) plane and in the [ 1 ¯ 1 0 ] direction. The crosshead speed was changed in the range from 0.1 to 5.0 mm/min. Under the lowest loading rate, a plastic zone was formed at the notch tip, faster loading lead to brittle fracture. Fractographic analysis of one specimen ruptured at 1 mm/min loading showed flat cleavage facets and tongues formed by the interaction of the principal crack with deformation twins. Besides the tongues, the fracture surface of the second sample ruptured at the same loading rate exhibited signs of plastic deformation. In the first specimen, transmission electron microscopy in the vicinity of the fracture surface confirmed deformation twinning and a very low dislocation density. In the second specimen, deformation twinning was assisted by slip of dislocations in the 〈1 1 1〉 {1 1 2} slip system. Molecular dynamics simulations confirmed that the crack growth has a more brittle character with increasing loading rates. At a slower loading rate, the crack growth is more difficult since it is impeded by emission of shielding dislocations from the crack tip in the 〈1 1 1〉 {1 1 2} slip system. Twin formation at the crack front was detected in simulations with edge cracks.