14Al-W alloy films with various W contents up to ~12 at% were prepared by 15 electrodeposition using 1-ethyl-3-methylimidazolium chloride (EMIC)-AlCl3 ionic 16 liquids with different concentrations of W precursor, W6Cl12. The hardness (H) and 17 Young's modulus (E) of the films were examined by nano-indentation. The films were 18 composed of a single-phase fcc Al super-saturated solid solution, an amorphous phase, or 19 both, depending on the W content and the deposition conditions. The H value increased 20 with increasing W content up to 9.8 at% and then decreased slightly with further increases 21 in the W content up to 12.4 at%. A similar trend was observed in the E value with 22 increasing W content, but the decrease in E value at 12.4 at% W was more significant than 23 that in H value. The changes in the H and E values are discussed from the viewpoints of 24 the grain size and the constituent phases. The 9.8-12.4 at% W films, which had relatively 25 high H values and H/E ratios, are expected to have a higher resistance to mechanical 26 damage than Al films.27 28 Keywords 29 Electroplating, Ionic liquid, Nano-indentation, Amorphous alloy, Aluminum, Tungsten 30 31 2 1 1. Introduction 2 Al metal shows high oxidation and corrosion resistance because of the thin passive film 3 on its surface. Thus Al metal films have attracted much research attention as corrosion-4 protective coatings on reactive materials such as a Mg alloy [1,2]. However, if exposed 5 to an environment containing halide anions such as Cl − , the passive film on Al metal is 6 destroyed locally, followed by pitting corrosion [3]. The resistance of Al metal to pitting 7 corrosion is enhanced by alloying with other transition metals [3]. Among Al alloys, Al-8 W alloys are known to exhibit significant resistance to pitting corrosion [3,4]. Therefore, 9 the formation of Al-W alloy films was intensively studied. 10 The formation of Al-W alloy films by sputtering [4-15], ion implantation [16], laser 11 surface alloying [17], and electrodeposition [18-22] was reported. Of these methods, 12 electrodeposition has particular advantages in that a thick film can be formed over a large 13 area relatively quickly by means of simple equipment. The electrodeposition of Al-W 14 alloys with high W contents was achieved using a 1-ethyl-3-methylimidazolium chloride 15 (EMIC)-AlCl3 ionic liquid containing potassium tungsten(III) chloride, K3W2Cl9 [18-16 20]. However, alloys with more than 5 at% W obtained from this bath showed a powdery 17 morphology. The morphology was improved by changing the W precursor from K3W2Cl9 18 to tungsten(II) chloride, W6Cl12 [22]. We reported that dense Al-W alloy films with up to 19 ~12 at% W can be electrodeposited from an EMIC-AlCl3 ionic liquid containing W6Cl12
20[22]. The Al-W alloy films obtained from this bath were composed of a super-saturated 21 fcc Al solid solution with W contents lower than ~9 at%, and with an amorphous phase 22
