With the thickening of V layers, V gradually transforms from the metastable fcc structure to a stable bcc structure due to the difference of strain-free bulk energy . The amorphization can be the transition state between the fcc structure and bcc structure. From the XRD results, V layers transform from the transient amorphous state into a stable bcc structure when the V layer thickness increases to 3.0 nm. Therefore, when the V layer thickness is in the range
of 2.0 ~ 3.0 nm, V layers present the amorphous state between fcc structure and bcc structure. We also observed the amorphization of yttrium (Y) layers between fcc structure and hcp structure with the increase of Y layer thickness in FeNi/Y nanomultilayered films, which will be discussed in another paper. It must 3-MA cost be pointed out that amorphous-featured diffraction corona is not observed in the SAED pattern, which can be attributed to the facts that the diffraction inclick here formation is only gathered from the circular region with the diameter of about 20 nm and in such small area, the low amount V with the thickness of 1.5 nm cannot produce enough strong diffraction signal. The microstructural evolution of V layers in FeNi/V nanomultilayered films can be explained by a thermodynamic model. The total energy of the V layer, PS-341 in vivo E T, is composed
of strain-free bulk energy, strain energy, and interfacial energy, which can be written as (1) where E bulk and E str, respectively, are the strain-free bulk energy and strain energy per unit of V layer, in which E str takes a larger value with a small t V and decreases with the increase of t V, and E int is the interfacial energy between FeNi and V layers. During the initial increase of t V (less than 1.5 nm), since t V is small, E int is the main component of E T. Formation of a coherent interface between FeNi and V layers can lower E int. Therefore, V layers can transform
into a fcc structure Baf-A1 molecular weight and grow epitaxially with FeNi layers. When t V rises to 2.0 nm, the strain-free bulk energy and strain energy increase, which occupy a larger proportion in E T than in E int. E T cannot be reduced by forming the coherent interface. Therefore, the V layers cannot maintain the fcc structure and epitaxial growth with FeNi layers. In addition, since E str takes a larger value when t V is comparatively small, E T is dominated by the strain energy relative to the strain-free bulk energy. In this situation, formation of a bcc structure of V layers within the FeNi/V nanomultilayered film can lead to the increase of the strain energy. Consequently, amorphization, as a transition state between fcc and bcc structures, has been formed to lower the strain energy and thus E T, as additionally shown in Figure 4. Figure 4 Amorphization of V layers within the FeNi/V nanomultilayered film with a V layer thickness of 2.0 nm. (a) Low magnification. (b) High magnification.