Effect of thermomechanical treatment of 0.45 wt.% C steel in the superplastic mode on plastic zone parameters after impact fracture in the ductile-brittle transition temperature range

Transmission electron microscopy was used to study the structure of plastic zones (PZ) resulting from crack propagation during impact bending tests of a steel containing 0.45 wt.% C at +20, −40, −110 °C in the as-received state (dg ≈ 45 μm) and after super-plastic thermomechanical treatment at intercritical temperatures (dg ≈ 2–5 μm). Scanning electron microscopy was used to study the sample fracture surfaces. Superplastic deformation of steel was found to be responsible for the 2–6-fold increase in dislocation density in PZ induced by low-temperature plastic deformation and also the crack branching and secondary cracking as compared with the as-received state. The cold formability of the steel subjected to superplastic thermomechanical treatment can be expected to improve.