SHEAR-BAND FORMATION IN 4340 STEEL - A TEM STUDY

Experiments have been conducted to investigate the process by which ad iabatic shear bands are formed in an AISI 4340 VAR steel with H(RC) = 44. To produce the shear bands, short, thin-walled tubular specimens w ere deformed in torsion at a nominal shear strain rate of about 10(3) s-1 in a Kolsky bar apparatus (split-Hopkinson bar). Strains of a few hundred percent and local strain rates of 10(5) s-1 were reached in th e region of localization. The temperature rise in the band, recorded u sing an infra-red detector, was approximately 460-degrees-C. Microscop ic observations of deformed samples were made by transmission electron microscopy (TEM). The TEM observations revealed that the material wit hin the shear band was characterized by highly elongated subgrains and dislocation cells. The subgrain boundaries were generally well-define d and the subgrains varied in dislocation density. To elucidate the so ftening mechanism, the misorientations between subgrains and dislocati on cells were measured. There was no indication of a phase transformat ion within the shear band. The probable metallurgical processes involv ed in shear band formation are presented in light of these experimenta l results.