ADIABATIC SHEAR BAND FORMATION DURING DYNAMIC TORSIONAL DEFORMATION OF AN HY-100 STEEL

Dynamic torsion experiments have been conducted on thin-walled tubular specimens of a tempered martensitic HY-100 steel, causing adiabatic shear bands to form. The strain rates imposed were approximately 10(3) s-1 and local temperature increases up to 600-degrees-C within the shear bands were measured. The shear band microstructure was examined by transmission electron microscopy (TEM), revealing two distinct microstructures. In some regions, highly elongated narrow subgrains extended in the shear direction, while in other regions, fine equiaxed cells were characteristic. The proportions of the two microstructures varied for different specimens, and the observations were interpreted to indicate that a process of dynamic recovery accompanying large deformation and a high temperature rise occurred within the shear band. Although thermal effects were apparent, there was no evidence to support a phase transformation to austenite followed by martensite formation. On the basis of present findings, it appears that the thermodynamic stability of the original microstructure can influence the tendency toward shear localization under dynamic loading conditions.