Threshold and structure of HCP/FCC nucleation in BCC iron under arbitrary triaxial compression: Atomistic simulations

Abstract

Shear deformation is one of the primary factors determining the threshold of structural transition (ST). It is important to quantitatively express the relationship between shear deformation and the ST threshold for the in-depth development of ST dynamics models. This work used classical molecular dynamics methods to study the effect of shear deformation on the ST in iron by controlling the triaxial ratio of compression. Based on the simulation results, there are significant differences in the microstructure for different loading paths. HCP lamellar twins, HCP-FCC thin twins, and crossed HCP twins embedded with parallel hexahedral FCC grains were all observed, as uniaxial loading transitions to equiaxial loading. The changes in pressure and the softening range of shear stress under different triaxial compressions were revealed. By introducing the strain triaxiality (i.e., the ratio of shear strain to volumetric strain), a general expression of the pressure threshold of ST under high strain rate compression-shear was proposed. In addition, whether the close-packed plane is easily activated under different strain environments was explained based on the results of stacking fault energy.