| Understanding the coupling mechanism of plasticity and phase transition in materials under shock loading is significant for physical properties and the constitutive equation at highpressure.However,the response behavior of materials under shock loading is very complex,which is affected by the geometrical shape of shock wave front,alloying elements,defects,and so on.At present,the microstructure response of single crystal iron under planar shock has been studied extensively and made great progress.Compared with planar shock,cylindrically shock produces different stress states and strain constraints,which can cause significant changes in the relative motion and local deformation between the internal elements of the material.Secondly,the introduction of alloying elements will change the interaction between atoms,which will affect the microstructure of materials.The change of these internal factors will have a significant effect on the shocking behaviors.Limited by experimental loading and testing techniques,the laws of plasticity and phase transition under cylindrically shock loading are rarely understood at present.Especially,the interaction mechanism of alloying elements in the process of plasticity and phase transition is still unclear.This thesis takes single crystal iron(Fe)and its alloy(Fe-Li)as the research object and uses non-equilibrium molecular dynamics method to simulate the shocking behaviors of Fe and Fe-Li alloys under planar and cylindrically shock loading.By tracking the local structure,the distribution of stress and strain,energy and lattice evolution,the coupling mechanism of plasticity and phase transition under different shock loading was studied,and the effect mechanism of alloying elements on the shocking behaviors was explored.The main research contents and results are as follows:The response of iron under cylindrically divergent shock loading was investigated.under continuous loading,the BCC-HCP-BCC-HCP phase transition occurs in iron single crystal,and form HCP twins with abnormal c-axis orientation.It has been shown that the 45°(135°)redirection of the BCC lattice driven by the shear stress is the key factor for the formation of HCP twins.In addition,the coupling process of plasticity and phase transition is that slip of(112)[ 111] and(112)[111] trigger the atomic rearrangement and compression along the [100]direction on the(1 10)plane at one time.The lattice instability caused by the first step compression provides the driving force for the phase transition.However,under the planar shock loading,the compression of [001] direction is obtained by a small angle rotation of(110)plane caused by plastic slip.The response of iron under cylindrically convergent shock loading was investigated.The results are obviously different from that of under cylindrically divergent shock loading.The HCP twins under cylindrically convergent shock loading is the same as that of under planar shock,which are formed by two HCP variants with mirror-symmetric c-axis touching each other,but the HCP phases with different c-axis orientations grow alternately due to the local fluctuation of stress distribution under cylindrically convergent shock.In addition,the 1/2<111>vacancy dislocation loops were found in the single crystal iron.Although the coupling between plasticity and phase transformation induced by two kinds of cylinder loading are driven by atomic rearrangement and compression in the direction [001] caused by dislocation slip,the atomic rearrangement and lattice compression are conducted in multiple times under cylindrically convergent shock,which is different from that of under divergent shock.The shock loading mode is an important factor for affecting the coupling mechanism of plasticity and phase transition.The effect of Li element on the planar-shocking behaviors of Fe-Li alloy was investigated.The results show that adding Li element can effectively reduce the pressure threshold of plastic deformation and phase transition because of the reduce of pressure-volume work during lattice evolution.With the increase of Li content,the accumulation of internal stress in Li-enriched regions becomes more significant,which promotes the nucleation and multiplication of dislocation loops.However,dislocation multiplication inhibits the nucleation of phase transition and increases the resistance to dislocation movement,resulting in obvious plastic hardening for shock along [111] direction.With the increase of Li element,the diversity of phase transition variants for shock along [110] direction was inhibited due to the destruction of initial stress and potential energy uniformity.The effect of Li element on the cylindrically shocking behaviors of Fe-Li alloy is studied.The results show that the geometrical dispersion of waveforms in both single crystal Fe and FeLi alloys is severe under cylindrically divergent shock loading,which accelerates the attenuation of wave amplitude and shock pressure.Besides,the weakening effect of Li element on the shock pressure leads to a significant decrease of HCP phase and dislocation density in Fe-Li alloys compared with that of pure iron.By contrast,the cylindrically convergent shock loading causes solid solution segregation near the Li atom and speeds up the plastic hardening process of Fe-Li alloy.Due to the different stress-strain state and energy distribution inside the Fe-Li alloy,the orientation of plastic hardening and the concentration of Li element are obviously different from that of planar shock loading.In addition,compared with planar shock loading,the energy and stress under cylindrical impact loading are extremely uneven,Li element has no significant effect on HCP variant diversity of Fe-Li alloy under cylindrically shock loading.In this thesis,the difference of shocking behaviors of single crystal Fe and Fe-Li alloys under of planar and cylindrically shock loading are studied in detail.The effect of stress and strain states on plasticity and phase transition is revealed,and the mechanism of Li element effect on the lattice evolution of iron matrix is clarified.These results provide a theoretical basis for understanding the high-pressure constitutive relation and optimizing the mechanical properties of iron and Fe-Li alloys under complex stress-strain conditions. |
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