Hcp/fcc nucleation in bcc iron under different anisotropic compressions at high strain rate: Molecular dynamics study

Jian-Li Shao,An-Min He,Pei Wang,Feng-Guo Zhang

doi:10.1038/s41598-018-25758-1

Abstract

Previous researches have revealed the importance of shear and the orientation dependence in the structural transition of iron. In this work, we introduce a series of shear deformations by adjusting the strain ratio between the longitudinal ([001]) and transversal ([010] and [100]) directions, and then investigate this structural transition under different anisotropic compressions with molecular dynamics simulations. It is found that the shear deformation can lower the transition pressure notably, and even change the nucleation structure and morphology. Under 1D-dominated compression (along (001) direction), there only appears hcp nucleation with a few fcc stacking faults. For other cases, more equivalent planes will be activated and fcc structure begins to nucleate. Under 2D-dominated compression (along (010) and (001) directions), the fcc mass fraction is already over the hcp phase. At last, we compare the variations of shear stress and potential energy for different phases, and present the sliding mechanism under typical anisotropic compressions.

Highlights

Most metal crystals may experience a polymorphic transition under shock compressions, which belongs to the first order phase transition from a thermodynamic point of view
The bcc to hcp transition in iron under shock compression was firstly simulated by Kadau et al.[20], and the slip mechanism suggested by molecular dynamics (MD) simulations was confirmed by Kalantar et al via ultra-fast in situ x-ray diffraction studies of iron[14]
The structural transition is shown by the radial distribution functions (RDFs), and the hcp/fcc close-packed atoms are identified by their respective coordination numbers (CNs), where the cut-off radius are determined by the first and second peaks of RDF

Summary

Computational Details

The interaction between atoms is described by the embedded-atom-method (EAM) potential developed by Voter-Chen[35], for which can well reproduce the bcc to hcp/fcc structural transition of iron[20,21,22,25]. After a sufficient relaxation of the sample at the temperature of 60 K in the canonical ensemble, we perform the simulations on the anisotropic compression. A0 is the initial lattice constant (2.8725 Å), and ax,y,z are the variables along the three directions during compression. The structural transition is shown by the radial distribution functions (RDFs), and the hcp/fcc close-packed atoms are identified by their respective coordination numbers (CNs), where the cut-off radius are determined by the first and second peaks of RDF. The hcp or fcc structures are distinguished from each other by the centro-symmetry parameter (CSP)[38]

Results and Discussion

Conclusion

Additional Information