Abstract
Using classical molecular dynamics simulations, we studied the influence that free surfaces exert on the austenitic and martensitic phase transition in iron. For several single-indexed surfaces—such as ( 100 ) bcc and ( 110 ) bcc as well as ( 100 ) fcc and ( 110 ) fcc surfaces—appropriate pathways exist that allow for the transformation of the surface structure. These are the Bain, Mao, Pitsch, and Kurdjumov–Sachs pathways, respectively. Tilted surfaces follow the pathway of the neighboring single-indexed plane. The austenitic transformation temperature follows the dependence of the specific surface energy of the native bcc phase; here, the new phase nucleates at the surface. In contrast, the martensitic transformation temperature steadily decreases when tilting the surface from the (100) fcc to the (110) fcc orientation. This dependence is caused by the strong out-of-plane deformation that (110) fcc facets experience under the transformation; here, the new phase also nucleates in the bulk rather than at the surface.
Highlights
Solid–solid phase transformations, such as the well-known α–γ transition occurring in pure iron and in steels, play an important role in the behavior of these materials and in technological applications [1,2]
We used molecular dynamics simulation to investigate the influence that the surface exerts on the α-γ transition in pure iron; both the α → γ transition—denoted for convenience as the “austenitic”
In contrast to the austenitic transformation, where transition temperatures were dominated by the surface energy, the transformation path played a decisive role in the martensitic transformation
Summary
Solid–solid phase transformations, such as the well-known α–γ transition occurring in pure iron and in steels, play an important role in the behavior of these materials and in technological applications [1,2]. Phase boundaries [11,12,13,14] and heterogeneous interfaces, such as with carbide phases [15], affect the transformation behavior; in these cases, the crystal orientation at the interface is decisive. Surfaces provide another source of planar defect that may influence the transformation. We used molecular dynamics simulation to investigate the influence that the surface exerts on the α-γ transition in pure iron; both the α → γ transition—denoted for convenience as the “austenitic”. Our study allowed identifying the influence of the specific surface energy and the surface orientation on the transformation pathway and the transformation temperature
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