Formation of intermetallic compounds and their effect on mechanical properties of aluminum–titanium alloy films

Abstract

Formation of intermetallic compounds and their effect on mechanical properties of aluminum-titanium alloy filmsAl-Ti alloy films with various Ti contents were prepared through magnetron co-sputtering with Al and Ti targets and treated with vacuum annealing at 400 8C. Energy dispersive spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and nanoindentation were used to determine the composition, microstructure and hardness of the films to reveal the existence of intermetallic compounds and their effects on the microstructure and mechanical properties. The results showed that Al-Ti films with lower Ti contents formed nanocrystalline structures of highly supersaturated solid solution. With the increase of Ti content, various types of Al-Ti intermetallic compound bonds formed. The existence and increasing concentration of these compound bonds gradually transformed the films into amorphous structures and supported the continual increase of the film hardness, reaching a high value of 8.8 GPa at 36.3 at.% Ti.Keywords: Highly supersaturated solid solution; Intermetallic compound bond; Al-based alloy film; Mechanical properties IntroductionAlloy films prepared by physical vapor deposition (PVD) exhibit features of highly supersaturated solid solution (i. e., solute content significantly higher than that of the equilibrium solid solubility). This type of structure remarkably improves film performance [1,2]. For instance, existing studies [3 -8] have demonstrated that, when some sputtered Al alloy films form highly supersaturated solid solution, their hardness can reach up to about 8 GPa, equivalent to the hardness level of high-speed steel. This technique therefore became an important approach to produce films with desired properties. However, controversy still exists regarding the underlying driver of the hardness enhancement, especially when the solute content far exceeds its equilibrium solubility limit: whether it is due to structure changes (e. g., highly supersaturated solid solution, or nanocrystal/amorphous structure) or the formation of intermetallic compounds. Some researches believe that in the systems of sputtered Al-based films (e. g., Al-Cr, when the solute content is much greater than the solid solute limit and even the films possess an amorphous structure, no intermetallic compounds would form. Some researches found that in Al-Cu [7,8,10] and 11] alloy films, intermetallic compounds formed at alloy contents great than 1.8 at.% Cu or 50 at.% Fe, respectively. In the case of Al-Ti films however, two different views exist. One maintains that intermetallic compounds cannot form in these films [3,6,9] while the other believes it is possible [12]. These different reports on the existence of the intermetallic compounds demonstrate that there is currently no consensus regarding the underlying cause of the alloy hardness enhancement. This paper reveals conditions and causes of the formation of intermetallic compound bonds in alloy films, through in-