As shown in Figure 5a, without insertion of V layers, the FeNi film exhibits a fcc structure. When the thickness of V inserted layers is less than 1.5 nm, V inserted layers can transform into a fcc structure
under the template effect of FeNi layers and grown epitaxially with FeNi, as indicated in Figure 5b. Since the lattice parameter of V is smaller than that of FeNi, under the coherent growth structure, FeNi layers bear Ganetespib purchase interfacial compressive stress generated from V layers. In reference to the alternating-stress field strengthening theory [23], the maximum shear stress on the interfaces could be calculated as Equation 2: Figure 5 Schematic illustration of the microstructural evolution of FeNi/V nanomultilayered films with different V layer thicknesses. (a) Without insertion of V layers. (b) Less than 1.5 nm. (c) 2.0 nm. (2) where A is the modulation amplifying factor influenced by modulation period, modulation ratio, and roughness and width of interfaces. According to the studies from Mirkarimi [24] and Shinn [25], A takes the value of 0.5 for calculation in this investigation. E WA is the weighted average elastic modulus of the bilayer layers, which is calculated as 195.8 GPa for a FeNi(10 nm)/V(1.5 nm)
nanomultilayered film based on the elastic modulus values for Fe50Ni50 (206 GPa) and V (128 GPa). η is the lattice mismatch between two layers of multilayers. Since V layers transform into a fcc structure, it is difficult to calculate selleck kinase inhibitor the lattice mismatch between two layers. If it is assumed that the lattice mismatch is between 3% and 5%, the maximum shear stress is about 1.20 to 1.99 GPa according to Equation 2. Stress-induced martensitic transformation has been widely observed
and investigated in past decades. Hsu and his collaborators successfully predicted the start temperatures of martensitic transformation (M s ) in Fe-C, Fe-X, and Fe-X-C alloys by the thermodynamics theories and believed that applied stress, as a driving force, could promote martensitic transformation and thus elevate M s [26–29]. Gautier et al. reported Ribociclib a linear enhancement of M s in Fe-Ni alloys with applied stress (σ) with dM S /dσ of 0.07°C/MPa for a cooling rate of 0.5°C/s [30]. According to this result, M s of the FeNi layer in the FeNi/V nanomultilayered film should increase from 84°C to 139.3°C relative to that with no interfacial stress. Therefore, interfacial compressive stress generated in the nanomultilayered film can induce martensitic transformation of the FeNi layer. As the thickness of V layers increases to 2.0 nm, as shown in Figure 5c, V layers can MM-102 hardly keep their fcc structure, and transform into an amorphous state, which destroys the coherent growth structure, leading to the appearance of interfacial compressive stress.