Solid State Reaction Mechanism In Interfaces Of Fe/Ti Nanometer-Scale Multilayers

Solid state reaction in interfaces of Fe/Ti nanometer-scale multilayers fabricated by magnetron sputtering was investigated under the condition of vacuum thermal annealing in order to explore the interface stability of the nanometer-scale multilayers. Using Rutherford Backscattering Spectroscopy (RBS), Differential Scanning Calorimetry (DSC), Small Angle/Wide Angle X-ray Diffraction (SA/WAXRD), High-resolution Electron Microscopy (HRTEM), the concentration profiles, phase state and structure and interface characteristics of Fe/Ti nanometer-scale multilayers were detected, and the kinetics of pure interdiffusion and phase transformation during interface reaction addressed, resepectively. An effective heat of formation model modified by considering the interface energy was proposed to predict intermetallics formation in the interface reaction of metal nanometer-scale multilayers to study the thermodynamic of phase transformation for Fe/Ti nanometer-scale multilayers.Fe/Ti nanometer-scale multilayers with designed modulation periods of 4.0-200.0 nm and the thickness ratio of Fe and Ti sublayer being 1 were deposited onto single-crystal Si (100) substrate at the room temperature using the high-vacuum duplex chamber magnetron sputtering apparatus with a base pressure better than 8×10^-6 Pa and at an Ar pressure of 0.3-0.9 Pa and a sputtering power of 60-80 W. The measured modulation periods of Fe/Ti nanometer-scale multilayers were 4.2-214.7 nm, in consistent with the designed values. Under a working pressure of 0.5 Pa and a sputtering power of 60 W, amorphous-like modulation structure was obtained in Fe/Ti nanometer-scale multilayer with modulation period of 4.2 nm, while for the modulation periods greater than 9.5 nm, the modulation structure of Fe/Ti nanometer-scale multilayers was composed of nanometer-scale crystallines ofα-Fe andα-Ti where the preferred orientations (110) and (002) were found for the Fe and Ti sublayers, respectively. At a working pressure of 0.9 Pa and a sputtering power of 80 W, the preferred orientation (100) of Ti sublayers was detected in the Fe/Ti nanometer-scale multilayers with the modulation period of 19.0 nm. The interfaces of Fe/Ti nanometer-scale multilayers were sharpness at the position adjacent to the Si (100) substrate, and changed to a wavy feature away from the substrate. It is shown that no intermetallics formed in the as-deposited Fe/Ti nanometer-scale multilayers.The thermal vacuum annealing was carried out on Fe/Ti nanometer-scale multilayers with the modulation periods of 9.5-116.4 nm at the temperature ranging from 473 to 873 K for 1-3 h. The interface reaction process of Fe/Ti nanometer-scale multilayers could be divided into three stages: formation of b.c.c, supersaturating solid solution, intermetallics FeTi and Fe₂Ti in sequence with increasing the annealing temperature. For the Fe/Ti multilayers with the modulation period of 15.6 nm and the preferred orientation of Ti (002), the reaction activation energy for the three stages was 1.04 eV, 1.43 eV, and 1.49 eV, respectively; andintermetallics FeTi precipitated at the interfaces between the Fe and Ti sublayers, possessing an orientation relation of FeTi (110)//α-Ti (1010) and FeTi [001]//α-Ti [0001]. The intermetallics formation during the interface reaction of Fe/Ti nanometer-scale multilayers was affected by the modulation periods and the preferred orientation of Ti sublayers. With increasing the modulation period from 9.5 nm to 116.4 nm, the formation temperature of intermetallics FeTi increased from 623 to 873 K when the preferred orientation of Fe and Ti sublayers was (110) and (002), respectively. Intermetallics Fe₂Ti formed at 723 K for the modulation period of 9.5 nm, at 873 K for 37.0 nm, while for 116.4 nm, Fe₂Ti was not detected at 873 K. For Fe/Ti nanometer-scale multilayers with the designed modulation period of 16.0 nm, the formation temperature of FeTi was 623 K with Ti sublayers of the (002) preferred orientation, while 873 K for (100). The formation of intermetallics Fe₂Ti was at 723 K with Ti (002) preferred orientation, while for (100), Fe₂Ti was not detected when annealed at 873 K.Taking into account the contribution of interface energy to the energy state of metal nanometer-scale multilayers systems, the effective heat of formation model was reasonably built to predict the intermetallics formation during the annealing of metal nanometer-scale multilayers, referring to the effective heat of formation model for intermetallics formation prediction during binary thin film reaction. The effective heat of formation of intermetallics was calculated by the heat of formation of the intermetallics multiplying the concentration ratio of the respecitive limiting element of effective concentration and of the intermetallics. The intermetallics with the most negative effective heat of formation was the compound to be formed firstly, followed by the formation of the next intermetallics to form, which at the interfaces between the first formed intermetallics and the reacting element is the next phase richer in unreacted element, has the most negative effective heat of formation. The lowest eutectic of Fe-Ti binary system 29.5 at.% Fe, 70.5 at.% Ti was chosen as the effective concentration. Considering the precision of the thermodynamic data and the nucleation barrier at the solid state interface, the effective heat of formation model modified by considering the interfaceenergy predicted the first formation of intermetallics FeTi, followed by the formation of,intermetallics Fe₂Ti with the increasing of annealing temperature in the solid state reaction of Fe/Ti nanometer-scale multilayers with the modulation ratio of 1. The predicted results are consistent with the experimental ones. The effective heat of formation model for intermetallics formation prediction during solid state reaction of metal nanometer-scale multilayers can be applied to predict the intermetallics formation in the reactions of Nb/Al, Ni/Al, and Ti/Al nanometer-scale multilayers reasonably, verifying the validity of the effective heat of formation model.