Physical mechanism of the intermittent plastic flow at extremely low temperatures

Abstract

The physical mechanism of the intermittent plastic flow (IPF) of austenitic steels at extremely low temperatures is explained in the context of microstructure evolution and 3D deformation arising in the area of the shear band. The microstructure change is identified by X-ray diffraction with the use of synchrotron radiation as well as electron backscatter diffraction (EBSD). This complete, global and local, approach reveals the intensive formation of twin boundaries which perform two functions. They become a part of the macroscopic shear band, but also constitute barriers piling up dislocations. Thus, their overcoming results in a stress drop as in the case of breaking the Lomer-Cottrell locks. The investigations show that the rapid displacement is blocked by the intensively produced martensite α’. The main features of the uncovered microstructural evolution are captured in the 3D reconstruction of the shear band obtained with the use of profilometer. Moreover, the effect of the thermodynamic instability is taken into account when assessing the conditions of the IPF. Finally, new kinetics of the IPF is developed. It comprises two functions: B reflecting the surface density of the dislocation pile-ups on the internal lattice barriers, and G corresponding to the surface density of twin boundaries. The breakthrough of the work consists in the discovery of multi-scale mechanism of the IPF, including participation of twin boundaries and the Lomer-Cottrell barriers.