BAUSCHINGER EFFECT AND DISLOCATION STRUCTURE IN STAINLESS STEEL DURING CYCLIC DEFORMATION

Cyclic deformation in symmetrical push-pull mode was carried out at room temperature in airusing a Schenck hydropuls machine.The total strain amplitude which was kept constant dur-ing the test ranged from ±0.004 to±0.012.The 0.2% offset yield stress σ_(0.2f) in tensionand σ_(0.2r) in compression and peak stress σ were measured from the stress-strain hysteresisloop at various cycles.The Bauschinger strenghth differential factor(BSDF)was then calcu-lated from σ_(0.2f) and σ_(0.2r).The energy loss △E of each cycle was determined fromhysteresis loop areas.These parameters,BSDF,σ and △E,appear to have two distinctivelydifferent stages.The dislocation structures were observed using TEM in specimens deformed cyclically,forvarious cycles.The observation shows that the dislocations pile-up mainly against grainboundaries and there exist large amount of deformation twins.The addition of 0.25 wt-% ni-trogen reduced the stacking fault energy of the alloy significantly.Cross-slip and climb aretherefore rather difficult to occur during the cyclic deformation at room temperature,andwell-defined dislocation cells and walls can only be seen at the final stage of fatigue.

BAUSCHINGER EFFECT AND DISLOCATION STRUCTURE IN STAINLESS STEEL DURING CYCLIC DEFORMATION

Cyclic deformation in symmetrical push-pull mode was carried out at room temperature in air using a Schenck hydropuls machine.The total strain amplitude which was kept constant dur- ing the test ranged from ±0.004 to±0.012.The 0.2% offset yield stress σ_(0.2f) in tension and σ_(0.2r) in compression and peak stress σ were measured from the stress-strain hysteresis loop at various cycles.The Bauschinger strenghth differential factor(BSDF)was then calcu- lated from σ_(0.2f) and σ_(0.2r).The energy loss △E of each cycle was determined from hysteresis loop areas.These parameters,BSDF,σ and △E,appear to have two distinctively different stages. The dislocation structures were observed using TEM in specimens deformed cyclically,for various cycles.The observation shows that the dislocations pile-up mainly against grain boundaries and there exist large amount of deformation twins.The addition of 0.25 wt-% ni- trogen reduced the stacking fault energy of the alloy significantly.Cross-slip and climb are therefore rather difficult to occur during the cyclic deformation at room temperature,and well-defined dislocation cells and walls can only be seen at the final stage of fatigue.