Observation of ultrathin metastable fcc Ti films on Al(110) surfaces

Abstract

High-energy ion-scattering spectroscopy, x-ray-photoemission spectroscopy, and low-energy electron diffraction (LEED) were used to study the growth of very thin Ti films on Al(110) surfaces at room temperature. The Al surface peak areas in the backscattering spectra of 0.96-MeV He+ ions, incident along the [1 10] direction of AI(110), decreased sharply during the deposition of the first five monolayers of Ti.This suggests a growth model in which a pseudomorphic Ti film with a fcc structure forms, shadowing the Al surface atoms. This model is supported by LEED measurements where sharp diffraction spots persisted with the rectangular symmetry of the substrate lattice structure. Throughout this coverage regime, the attenuation of the Al photopeak intensities as a function of Ti coverage agrees with the attenuation rate calculated using a laminar growth mode. With additional Ti deposition the Al surface peak intensities increase while the Al photopeak intensities remain unchanged. A discussion of possible growth models for the high-coverage regime is presented.

Thickness-dependent fcc–hcp phase transformation in polycrystalline titanium thin films

“…The relative integrated intensity of the fcc Ti phase is found to reduce almost exponentially with increasing film thickness, suggesting that the fraction of the fcc Ti phase decreases with increasing film thickness. Observation of metastable fcc Ti phase has also been reported in bulk Ti powders during mechanical milling of Ti [18][19][20] and also ultrathin ($2-50 nm thick) epitaxial films [12][13][14][15][16][17] on various substrates. The strain-free cubic lattice parameter of 0.41638 nm obtained for fcc Ti in the case of 5 min deposited film is in close agreement with the cubic lattice parameters reported earlier in mechanically milled polycrystalline Ti powder (0.4356 nm [19] and 0.4237 nm [20]), in Ti epitaxial films (0.433 nm [13] and 0.415 nm [16]) and also from ab initio calculations (0.411 nm [9]).…”

“…Electronic structure calculations have also predicted the stability of the x and c phases in Ti [9] at high pressures. Apart from these four types of phases mentioned above, Ti has also been reported to exhibit a cubic closepacked (ccp) structure in very thin epitaxial films [12][13][14][15][16][17] and in high-energy milled powder [18][19][20]. The formation of this phase has never been anticipated in the equilibrium phase diagram of Ti [2].…”

“…However, based on the elastic stability criterion, the possibility of a locally stable ccp structure has been proposed for Ti [9]. Historically, the fcc Ti phase was first observed in epitaxial Ti films on NaCl substrate below certain critical film thickness [13], and subsequently on various other substrates (both metallic and ceramic) [12,[14][15][16][17] and also in Ti/Al epitaxial multilayer films [21,22]. However, no reports exist to date concerning the observation of the fcc Ti phase and its polymorphic transformation to the hcp phase in polycrystalline Ti thin films.…”

An evaluation of the hexagonal close-packed to face-centered cubic phase transformation in a Ti-6Al-4V alloy during high-pressure torsion

“…In addition, an fcc phase was reported in an investment cast Ti-6Al-4V alloy during different heat treatment processes [2] and very recently in a surface layer after high energy shot peening [3]. Basically, the formation of the fcc phase is not anticipated from the equilibrium phase diagram of Ti [4], but there are experiments showing an fcc structure in very thin epitaxial films [5,6] and in high-energy milled powders [7,8]. Very recently, an hcp to fcc phase transformation was reported in a bulk commercial purity α-Ti during cryogenic channel die compression [9] and cold rolling [10].…”

Effects of anharmonic strain on the phase stability of epitaxial films and superlattices: Applications to noble metals

“…Furthermore, elemental metals and alloys have been found to form epitaxially in structures which are unstable in bulk form. [18][19][20][21][22] Recently, the topic of surface alloy formation in bulk immiscible systems has attracted considerable attention. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] These systems are usually strained due to film/substrate lattice mismatch.…”