In a previous study, it was shown that the (111)fcc, (110)fcc and (111)bcc free surfaces do not assist the phase transitions as nucleation sites upon heating/cooling in iron (Fe) thin slabs. In the present work, the three surfaces are denoted as “inactive” free surfaces. The phase transitions in Fe thin films with these “inactive” free surfaces have been studied using a classical molecular dynamics simulation and the Meyer–Entel potential. Our results show that shear deformation helps to activate the free surface as nucleation sites. The transition mechanisms are different in dependence on the surface orientation. In film with the (111)fcc free surface, two body-centered cubic (bcc) phases with different crystalline orientations nucleate at the free surface. In film with the (110)fcc surface, the nucleation sites are the intersections between the surfaces and stacking faults. In film with the (111)bcc surface, both heterogeneous nucleation at the free surface and homogeneous nucleation in the bulk material are observed. In addition, the transition pathways are analyzed. In all cases studied, the unstrained system is stable and no phase transition takes place. This work may be helpful to understand the mechanism of phase transition in nanoscale systems under external deformation.

No abstract