Development of shear deformation in lath martensite of medium alloy steels under tension

Evolution of shear deformation in steel with the structure of tempered martensite was studied under active tension. Purpose of the work was to identify the patterns of deformation development at the scale-structural levels: package, plate, fragment of a package and a lath. The authors investigated the deformation relief formed at different stages of plastic deformation by optical, transmission and scanning electron microscopy. Quantitative characteristics of the deformation relief were measured: shear strength (P), distance (X) between the shear traces and their length. Statistical processing was carried out, the average values and relationship with the degree of plastic deformation were determined. It was established that development of shear deformation in the lath component of martensite occurs with the formation of two subsystems of shear traces: thin and coarse. Subsystems of thin traces are formed from the very beginning of plastic deformation. Appearance and evolution of the subsystem of coarse traces correlates with formation of the first (long) neck in the sample, and it is the main mechanism leading to the localization of plastic deformation on the sample scale. The places of localization of rough shift are the border areas of the laths and fragments of the package. Connection between localization of subsystems of coarse shear traces and formation of a fragmented dislocation structure were revealed. The values of the average shear power in thin and coarse traces do not depend on the degree of local plastic deformation of the sample in the entire range of deformation degrees and remain constant until destruction ( = 0.1 μm and = 0.3 μm).