Formation mechanism of face-centered cubic phase in impact process of single crystal iron along [101] direction

Abstract

The impact phase transformation of body-centered-cubic iron is one of the hotspots in current research. Many studies demonstrated that when iron is impacted along the [100] direction, body-centered-cubic phase will transform into hexagonal close-packed phase; while when it is impacted along the [101] direction, a certain amount of face-centered-cubic phase will also appear besides hexagonal close-packed phase. The transformation from body-centered-cubic to hexagonal close-packed phase has been clarified, however, the transformation from body-centered-cubic to face-centered-cubic phase still needs further exploring. In the present work, molecular dynamics simulation is used to study the phase transformation of body-centered-cubic iron impacted along the [101] direction. The results show that the body-centered-cubic phase will transform into a close-packed structure including hexagonal close-packed phase and face-centered-cubic phase). The formation mechanism of face-centered-cubic phase is as follow. In the loading process, single crystal iron suddenly contracts along the [101] and <inline-formula><tex-math id="Z-20200630101515-1">\begin{document}$ [\bar101] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-1.png"/></alternatives></inline-formula> directions, and expands along the [010] direction, leading to the transformation from body-centered-cubic phase to face-centered-cubic phase. The formation mechanism of hexagonal close-packed phase can be divided into two stages: first, (101) plane is compressed into close-packed plane, then hexagonal close-packed phase is obtained by the relative sliding of adjacent close-packed planes. To further investigate the formation mechanism of the close packed structure, the effect of stress state on the phase transformation of body-centered-cubic iron is further studied. Under one-dimensional (along the [101] direction) or two-dimensional loading (along [101] and <inline-formula><tex-math id="Z-20200630101515-2">\begin{document}$ [\bar101] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-2.png"/></alternatives></inline-formula> directions), the body-centered-cubic iron transforms into face-centered-cubic iron. In the loading process the lattice constants along the three dimensions are monitored. When the transformation from body-centered-cubic phase to face-centered-cubic phase finishes, the ratio of lattice constants along three directions is 1∶1∶1.31 under one-dimensional loading; while the ratio of lattice constants is 1∶1∶1 under two-dimensional loading. Obviously, the body-centered-cubic phase transforms into distorted face-centered-cubic phase under one-dimensional loading. Under two-dimensional (along the [101] and [010] direction) and three-dimensional loading (along the [101], [010] and <inline-formula><tex-math id="Z-20200630101616-1">\begin{document}$ [\bar101] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101616-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101616-1.png"/></alternatives></inline-formula> direction), the body-centered-cubic phase transforms into hexagonal close-packed phase. Gibbs free energy value for each of BCC, HCP and FCC phase is calculated. The calculation results show that the BCC phase is stable under low pressure, while the HCP and FCC phase are stable under high pressure. Finally, based on Gibbs free energy and the effect of stress state on the phase transformation, the phase transformation mechanism of body-centered-cubic iron under [101] impaction is investigated, and a reasonable explanation for the phase transformation is obtained.