| Shock induced phase transition has always been the hot spot in the research of the shock wave physics.The mechanical,thermal,acoustic,electrical and chemical properties of the materials will change significantly during the phase transition.Therefore,the research of the shock induced phase transition occupies an important position in the field of condensed matter physics,material science,earth physics and weapon design.Iron is the most important and the most widely used element in human industrial production,and it is also the constituent element of the earth's core.Therefore,the research on the phase transformation of iron under high pressure,especially under impact pressure,is a hot topic in the fields of weapon physics and earth science.Since 1955,many efforts have been made to study the phase transition of iron under dynamic loading,but they mainly focus on the study of a forward phase transition.There are few studies on the reverse phase transition,even no records about the phase transition with loading-unloading-reloading path,which is much closer to the applications.Obviously,the transition with complex loading path is of high value in science research and industry application.In our work,an innovative composite flyer structure was used to realize the loadunload-reload phase transformation path.And we combine the technology of realizing different unloading time control in one single shot,shockwave profile measurement technology with high time resolution and the technology of shock recovery to investigate the characterization of shock loading phase transition.We measure the interface particle velocity history of shock loading iron with load-unloadreload path with hydrogen-oxygen gas gun for loading,front-surface impact technology and DPS laser displacement interferometry velocimetry technology.The result shows that the unloading period has a three-part structure: elastic unload,plastic unload and unload reverse transition.The onset pressure of the reverse phase transition is 11.3±0.5GPa,the complete pressure of the inverse transition is 9.0?9.6 GPa.And the reload phase transition onset pressure decreases with the increase of unloading time.In order to have a clearer understanding of the reloading segment,the Boettger-Wallace model was used to calculate the reloading segment and the results were compared with the experimental results.The result shows that the calculate profile with smaller phase transition characteristic time has better consistency with measured profile.That means the reload phase transition occurs faster than the one time loading phase transition.The pressure range of reload phase transition is 11±1GPa,lower than the first phase transition about 2GPa.And as the unloading time becomes longer and the remained ? phase becomes lesser,the reload onset phase transition pressure becomes lower.We infer that the reason causes the results is that,after load segment,there are lots of twins or grain boundaries are produced,which would make the reload phase transition occur easier and faster.This conclusion has been proved by our shock recovery experiment.This work use the method of combining the shock profile measurement and the phase transition kinetics model calculation to analyze the characterization of phase transition.We gain the in-situ pressure from interface velocity profile back calculation for the first time.And with the help of calculation,we gain the onset pressure of reload phase transition and the characterization of kinetics.Based on these phenomena we gave our conclusion,and the conclusion is proved by our shock recovery analysis.The method of our work can be used for more kinds of materials of similar research. |
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