Strain-induced elastic moduli softening and associated fcc↔bcc transition in iron

Using molecular dynamics calculations we demonstrate that with decreasing the thickness of ultrathin body-centered-cubic (bcc) α-Fe film with (001) surfaces, the biaxial strain results in first bcc(001)→face-centered-cubic (fcc) (001) transition along the inverse Bain path due to softening of C33, and then fcc(001)→bcc(011) because of shear modulus vanishing along fcc ⟨110⟩ directions. For the bulk fcc γ-Fe, the tensile biaxial strain along the Bain path transforms fcc (001) into bcc (001) with fcc⟨110⟩‖bcc⟨100⟩, while compressive strain results in shear instability, in agreement with recent ab initio calculations.