A Study On The Substructural Characteristics And Evolutional Mechanisms Of Materials Imposed By High Strain Rate Loading

Nanosecond duration impulse laser can produce GPa order of stress and 10~6-10~7/s order of strain rate. The characteristics and mechanisms of grains ultra-refinement and modification of substructures for austenitic stainless steels imposed by laser shock processing(LSP) were investigated by transmission electron microscopy(TEM),scanning electron microscopy(SEM) and x-ray diffraction(XRD) techniques.The effects of ultra-refinement were examined by comparison with different models and parameters. It showed that the magnitude of peak pressure and stress profile induced by LSP were the key elements for a satisfactory refinement top layer. A smooth and no trace of ablation 2Cr17Mn15Ni2N austenitic stainless steel refinement surface, with mean 0.5μm grain in size, was synthesized by the process of black paint as absorbent coating, peak pressure imposed by LSP being about twice of the dynamic yield strength of target and single laser loading. It indicated that thermoplastic destabilization had happened in heavily localized regions imposed by LSP. Strip-like subgrains formed in the direction perpendicular to Gaussian type stress profile, in order to accommodate plastic strain, the subgrains experienced necking, breaking up, rotating and refining. This process is so-called dynamic rotational recystallization.The substructures in the treated zones of austenitic stainless steel subjected to LSP, such as deformation twins, microbands, slip bands and dislocations were characterized and analyzed. It showed that deformation twins had formed in 2Cr17Mn15Ni2N and 304L targets, despite the peak pressures induced by LSP were no more than one third and half of the critical stress for twinning of corresponding targets. Different types of microbands and dislocations configurations were observed in the two targets. The features of dislocations slip deformation induced by ultrahigh strain rate followed FCC metallic crystallology. No deformation martensite phase was discovered. It suggested that, when submitted to ultrahigh strain rate loading, thecritical stress for twinning was not only a constitutive function of stacking fault energy, stress geometry and other factors, but also a function of strain rate. Only local movement of partial dislocations is needed during the formation of deformation twins. In terms of present austenitic stainless steels, the critical stress for twinning decreases with increasing strain rate. Stacking fault energy plays a crucial role to determine microbands and dislocations configurations. The materials having high stacking fault energy are more convenient for the formation and evolution of dislocations and microbands. Ultra-short deformation duration may be responsible for the lack of deformation martensites, as well the black slip planes contrast due to numerous dislocations left in them.