Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2 - Microstructural study

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α′ martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α′ martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α′ martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α′ martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α′ martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σ-σ_y) and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α′ martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.