Microscopic Deformation Of Medium Carbon Steel Under Shock Loading

With the development of automotive,aerospace,and defense industry,people favor the metal material which possess the excellent dynamic mechanical property.The dynamic property of material depends on their microstructure and corresponding deformation mode.Among them,shock compression is typical high strain rate loading and used to investigate the material deformation during the crash of transportation facility.In this work,we choose an important engineering material,a medium carbon steel,to explore the individual effects of loading pressure and microstructure on deformation under impact loading.The free surface velocity is measured to obtain the yield stress and phase transition stress.The shocked samples are characterized by metallographic microanalysis,X-ray diffraction(XRD)and electron backscatter diffraction(EBSD).The free surface velocity curve shows the martensite phase transformation wave is observed in the sample at pressure above 15.4 GPa,and not observed at pressure below 11.6 GPa,consistent with the ?-? phase transition pressure of 13 GPa.XRD results of the samples with phase transition wave did not show the formation of new phase,this is because the ? phase reversed to the ? phase with pressure unloading.The dislocation density can be measured by the EBSD and XRD technique,and we find that the residual dislocation is strongly related with shock pressure.This is because that the equivalent strain is directly proportional to the shock pressure.However,the shocked sample show weak texture and low value in GROD map of EBSD characterization.It means that there is almost no grain rotation during shock compression,different from the regular uniaxial compression text.By further analysis,there are two types of twinning in the sample: one is the {112}<111> twinning which is common in body-centered cubic structures,and another unusual {332}<113> twinning is usually existed in metastable ?-Titanium alloy,but rarely in iron and its alloy.{332}<113> twinning is found above phase transition pressure but absent below phase transition pressure.The {332}<113> twinning appears not only as a primary twinning but also as a secondary twinning of the {112}<111> twinning.The statistics of twin fraction shows that these two type twinning both increase with increasing shock pressure,and increase slope of {112}<111> twinning is much higher than that of {332}<113> twinning.Two type twinning also strongly depend on the microstructure.The cementite lamella of medium carbon steel act as the interface and facilitate the nucleation but suppress the growth of {112} twinning.Twin can grow along but can't go across the cementite lamella,resulting that the {112} twinning of the pearlite is lower in twin area fraction than that of ferrite.The {332} twinning only act as second twinning in ferrite but also can exist as the primary twinning in pearlite,since the cementite lamella of pearlite can act as the nucleation source of {332} twinning.{112} twinning tend to nucleate at the grains with [101]-[111] direction,twinned grain is almost in the [001] direction.{332} primary twinning is more likely to be activated at the grain with [101] direction.