Atomic Simulation About Effect Of Grain Boundary On Phase Transformation In Iron Under Shock

Due to that iron is an important part of the earth’s core and plays an important role in human industry and society,the α → ε martensitic transformation of iron have attracted a great interest.Previous studies have shown that defects in crystals such as impurities,voids,dislocations and grain boundaries have a significant effect on the shock response of materials,but the understanding of their details is almost blank.Grain boundary is one of the most common defects in iron,but the relationship between grain boundary and phase transition is very complex.Different researchers even proposed totally different views on it.it is still unclear about the role of GBs during the phase transition.Therefore,it is of great significance to study the effect of grain boundaries on Martensitic Transformation under shock loading,especially the importance of grain boundaries under shock loading has been emphasized in recent experiments.Moreover,previous studies in our group have found that grain boundaries can lead to new transformation mechanisms by changing the coupling process between transformation and plasticity.The details of that grain boundaries change transformation mechanisms are one of the important contents of this study.Our findings have an important significance for experimental studies and macroscopic and mesoscopic scale simulations regarding iron martensitic transformation.Firstly,the effects of ∑5 [001] twist grain boundary,∑3 [110] twist grain boundary and ∑3 [110] tilt grain boundary on the nucleation of martensitic transformation under shock-loading are studied.The results show that the three kinds of grain boundaries can reduce the phase transition threshold of region near the grain boundary,but only the ∑3 [110] to grain boundary can significantly reduce it,because its local structure is composed of coherent twins,which correspondes elastic wave precursor can directly trigger the phase transition near the grain boundary.In addition,it is also found that the impact response near the grain boundary is not only related to the type of grain boundary,but also closely re lated to the shocked direction,and the slip planes on both sides of the grain boundary will also affect the selection of martensitic variants.Moreover,this paper also explains why some grain boundaries seem to be less favorable for the transformation in the experiment.Then,the interaction between plasticity and phase transformation in α-Fe bicrystals under shock is studied.The results show that the grain boundary can trigger the strain induced transformation mechanism because the local structure of the grain boundary can changes into HCP structure with a lower potential barrier.This is also consistent with the results of experiment,that is,the pressure threshold corresponding to the strain induced transformation mechanism is much lower than that of stress assisted transformation mechanism.In addition,we also reveal the knetics of dislocation assist transformation mechanism,and confirm that dislocation activity can can provide a special compression mechanism and the slip trend of lattice,so dislocation activity can provide help for phase transition nucleation.Particularly,it is found for the first time that dislocations can be emitted directly fr om grain boundaries under shock,which provides a new understanding of the relationship between dislocations and grain boundaries.At last,this study investigated the shock-induced migration of Σ3 [110] asymmetric tilt grain boundary in α iron bicrystal.Without the interference of phase transformation,the migration of the asymmetric tilting grain boundary of Σ3 [110] will not change the shape and local structure of the grain boundary,but if phase transformation occurs when grain boundary migrates,the phase transformation near the grain boundary will change the shape and migration mechanism of the grain boundary.On the other hand,grain boundary migration can also promote phase transformation,but will not significantly reduce the pressure threshold of phase transformation.Another important finding is that the difference of shear stress on both sides of grain boundary caused by shock wave sweeping through grain boundary is the key factor leading to grain boundary migration,which can explain why grain boundaries migration occurs mainly in asymmetric grain boundaries.The most innovative points can be summarized as follows: 1.The reason why two different transformation mechanisms are excited by grain boundaries is explained.2.An important puzzle about martensitic transformation is explained,that is,why the transformation thresholds near different grain boundaries are very different under shock.3.It is revealed why grain boundaries migration occurs mainly in asymmetric grain boundaries.The studies successfully show the effect of grain boundary(or defects)on the phase transformation,which is of great significance in the fields of grain boundary engineering and material science.