Influence of the Crystal Surface on the Austenitic and Martensitic Phase Transition in Pure Iron

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.

Dislocations Help Initiate the α–γ Phase Transformation in Iron—An Atomistic Study

“…The occurrence of the transformation depends in particular on the surface orientation chosen and the thin-film thickness. In previous work [39], we showed that the martensitic transition temperature of ideal thin films decreases with the thin-film thickness, such that the transformation does not occur at all for films that are too thick; this is the case in the present study.…”

“…We find these Kurdjumov-Sachs orientation relationships obeyed in most material nucleated in the vicinity of the dislocations; occasionally, the Nishiyama-Wassermann orientation relationships [46,47] While the local nuclei lead to the observation of Kurdjumov-Sachs and Nishiyama-Wassermann orientation relationships, the final structure of the new phase is governed by a different principle based on the proposition that the free surface of the thin film must be conserved over the transformation [39]. We call, for simplicity, the final evolving structure the 'global phase', as it will eventually cover all of the thin film and was encountered in all cases studied here irrespective of the initial dislocation density.…”

“…After the martensitic transformation, the bcc-phase is characterized by a homogeneous phase consisting of only few twinned grains separated by twin boundaries; the austenitic phase, on the other hand, is single-crystalline, containing planar defects such as stacking-fault planes and plates of hcp material. This simple microstructure is the consequence of the free surfaces of the thin film, which tend to form conserved planes under the transformation [39]. As a consequence, the final orientation relationships of the transformed sample are characterized by the Bain and the Pitsch pathway.…”

Shear Deformation Helps Phase Transition in Pure Iron Thin Films with “Inactive” Surfaces: A Molecular Dynamics Study

“…Urbassek and his coworkers [3,4] studied the strain induced solid-solid phase transition in bcc-Fe thin films and reported a rich variety of phenomena such as back transition to the bcc phase and grain reorientation at high strains. Recently, Meiser and Urbassek [19] investigated thermally induced phase transition in Fe thin slabs with different orientated surfaces using the Meyer-Entel potential [20], which is widely used in studies of the phase transition, both in bulk-dominant and surface-dominant systems. The authors discussed the results from the aspects of the surface energy and transition pathway.…”

“…In the present work, the phase transition in Fe thin films with three inactive free surfaces under shear deformation will be studied based on the results of [19]. We aim to check whether the applied shear deformation will induce the austenitic/martensitic phase transition.…”