The Deformation Twinning And Strengthening Effect In Mild Steel Q235 Under The High Strain Rate Loading

Q235 mild steel is a kind of structural steel and it is widely used in transportation,architecture and machinery.Slip and deformation twinning are two main deformation mechanisms in mild steel.Deformation twins can only nucleate under high strain rate loading at room temperature.Deformation twinning in a mild steel is investigated under quasi-isentropic compression(IC)and shock compression(SC)to about 12 GPa,as regards the effects of pulse duration(plateau width)and strain rate(rising edge slope),and associated twin texture and twin strengthening effect.The pulse duration ranges from 80 ns to 790 ns;and two rise times are explored,approximately 30 ns for SC and 300 ns for IC.Free-surface velocity histories are measured by Doppler pins system and obtain strain and strain rate.The postmortem samples are characterized with electron backscatter diffraction,and the yield strengths of the samples pre-deformed by impact are examined with a materials testing system.Results and conclusions are as follows:For SC,twin density and size increase with pulse duration up to 580 ns.At longer pulse durations,the increase in twin density is stagnated as a result of deviatoric stress relaxation,while twin size continues growing.For IC(410 ns),twin density and size are much larger than the SC counterpart,as a result of shallower rising edge.Increased deformation time can compensate the effects of reduced strain rate or applied stress for deformation twinning.In previous study,quasi-static compression loading before shock loading will stagnate twin nucleation in previous study.Experimental results shows that pre-shock loading will not decrease the twin density for shock loading.This result can be explained that twin and dislocation nucleate during rising edge for pre-shock loading,but do not nucleate during quasi-static compression.Twin activation is accompanied by slip and twinning itself.Stress concentration required for twinning is induced by dislocation pileup and impedance of twins at GBs,and the buildup increases over time,which can compensate the effects of reduced strain rate or applied stress.Under long pulse duration of shock loading,grains close to <101>-<111> orientation tend to nucleate deformation twins near <001> orientation.These results are conform to the Schmid law and the principle of minimum shear strain for twin nucleation.Twin strengthening effect is strong in postmortem samples,depends on twin density instead of area fraction,and follows the empirical Hall–Petch relationship.Generating high density twins by controlling impact parameters may provide an effective means to obtain high strength bcc metals.