Combinatorial exploration of color in gold-based alloys
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Combinatorial approaches comprised of combinatorial magnetron co-sputtering deposition and fast screening methods are introduced to study color as a function of composition in Au-based alloys. The microstructures of the thin films and bulk alloys are identified by X-ray diffraction, and their colors of the alloys are characterized by optical reflectivity. The results reveal that when comparing microstructures and reflectivity, thin films are similar to bulk alloys. In Au-Ag-Cu solid solutions, the color of the ternary alloy follows the rule of mixture. For colors resulting from AuAl2 intermetallic, the color of an alloy scales with the percentage of the intermetallic phase and the deviation from its ideal binary composition. In the Au-Al-Cu library, we found a ∼90 % AuAl2 area fraction compositional window where copper addition can be tuned to improve mechanical properties while keeping purple color, even though Al and CuAl2 phases exist. Moreover, when comparing the color in Au-Cu-Si-Ag amorphous and crystalline state solid solution for the same composition, the colors are essentially identical.
KeywordsGold alloys Microstructure Amorphous alloys Intermetallics Solid solution Decorative thin film coating
Color is ubiquitous and plays an important role in our daily life . Although there are various mechanisms underlying the generation of color, for gold(Au) and copper(Cu), it is their electron configurations that are responsible for the significant variation in absorption and reflection of light over the visible part of the spectrum. This leads to their yellow and red appearance . Because of their unique colors compared with other metals, gold and its alloys have been used dating back to early human civilizations for decoration, jewelry, dentistry, and a wide variety of tools . Since gold is soft, other elements, such as copper, silver, and nickel, are added to improve mechanical properties. However, the addition of alloying elements into gold also results in variation of color, depending on how the electron configuration changes with composition. Dramatic changes in color as a function of composition exist in systems which form intermetallic phases, such as gold(Au)-aluminum(Al). In order to improve their workability, variable of additions into Au–Al binary system are developed, such as cobalt, nickel, palladium, and copper . For instance, the Au-Al-Cu system exhibits a wide range of colors, including reddish, yellow, apricot, silver, and pink, among which the pink color mainly originates from the formation of purple AuAl2 phase [4, 5]. AuAl2 is the most thermally stable phase in this system and mechanically brittle [2, 6]. This unique color of AuAl2 is due to the interband transitions between the valence and the conduction bands, leading to a dip in the reflectivity spectrum at ∼2.2 eV, corresponding to a wavelength of 563 nm [7, 8, 9]. Because of this green light absorption, the combination of reflected blue and red light of higher energy results in the intense purple hue exhibited by AuAl2. In addition, the low energy bulk plasma frequency may also contribute to the purple color of AuAl2 . Au-based alloys also exist in an amorphous form [11, 12, 13, 14]. These metallic glasses have been discovered in Au-Si and extended to more practical Au-Ag-Pd-Cu-Si alloys . For Au-Ag-Pd-Cu-Si alloys, the color and variation of color with composition have been studied by measuring yellowness index after tarnishing . Such tarnishing mechanism lies in the internal oxidation. Therefore, whether color change happens upon crystallization without oxidation is not known.
Despite the wide use of some colored Au alloys, a systematical investigation on the change of color with composition and thus phase formation is essential to gain a fundamental understanding of the color-microstructure relationship and therefore better control over these properties. Conventionally, the investigation of colored gold alloys relies on the sequential casting technique [4, 5], which is extremely slow and also discontinuous.
In the present study, we use magnetron co-sputtering to create compositional libraries that represent large fractions of the phase space with a continuous compositional gradient [12, 16, 17]. The material libraries can be categorized into three different groups based on phases formed: solid solution, intermetallics, and amorphous state. We explore how color varies with composition and phase formation by using systematic compositional, structural, and optical mappings, which reveal the dominant factors controlling the color of the alloys.
The composition of the alloys in the library was identified by EDX mapping using an Oxford EDS system with their X-Max 80 mm2 detector and INCA software. The grid spacing is 4 mm for EDS measurement. Structural characterization was conducted by using scanning XRD with a Cu Kα radiation source. The spacing between XRD scanning measurements was 2 mm. Morphologies were observed using a Zeiss SIGMA variable pressure scanning electron microscope (SEM).
To quantify the color of the libraries, reflectivity with a wavelength range from 345 to 790 nm was measured using a microreflectance setup equipped with a commercial silver reflector for calibration. Since color perception is highly subjective, CIE 1931 XYZ color space was used to quantitatively define the color differences of the alloys . The reflectivity over visible light range (380 to 780 nm) was chosen to calculate the coordinates in the chromaticity diagram.
Results and discussions
Color of solid solution
Color of intermetallics
The four selected compositions are marked at their corresponding positions on the thin film library. The color along this compositional line gradually changes from colorless to light purple. From the perspective of the reflectance spectrum, the addition of Au causes a reflectance decrease in the short and mid-wavelength range, resulting in a light purple and pink hue. This results in the shift of the minimum which moves to lower wavelength from 598 to 545 nm, corresponding to 2.3 eV in energy. This value is close to the dip in reflectivity at 2.2 eV in AuAl2. The XRD spectrum (Fig. 4c) reveals that the Al phase disappears with increasing Au.
Color of amorphous alloys
Amorphous metals exhibit superior mechanical properties including high strength, hardness, elastic strain limit, corrosion resistance, and thermal plastic forming capability . Due to this combination of properties and processibility, they are promising materials for applications . However, whether the amorphous metals will undergo a color change upon crystallization is not well understood. Revealing that the possible color difference of amorphous and crystalline phases is important, a possible appearance change might limit their applications originated from the customers’ aesthetic preferences.
These observations are not surprising, because the amorphous state is similar to disordered solid solution. For the current Au-Cu-Si-Ag system, the primary crystalline phases are solid solutions as well. As showed in the preceding section, the color of solid solution follows the rule of mixture, e.g., the color is a mixture of the constitute elements. Therefore, for the Au-Cu-Si-Ag alloy system, the color difference of amorphous state from crystalline state is minor proved by the similar chromaticity diagram coordinates.
Typically, processing and shaping of amorphous alloys are carried out taking advantage of their unique thermoplastic processability [23, 24, 25]. However, in this process, amorphous alloys may transform into crystalline state, especially when processing is conducted by temperature ramping to get maximum deformation . Our finding that the color difference between amorphous and crystalline state is indistinguishable (Fig. 9) is encouraging, because the desired color can be reserved after processing and one is relieved from one constrain from the perspective of material selection.
We used combinatorial methods to study color as a function of composition in Au-based alloys. When comparing microstructures and reflectivity, thin films are similar to bulk alloys. In Au-Ag-Cu solid solutions, the color of this ternary alloy follows the rule of mixture; it is determined by the weighted average of the colors of its elements. For colors resulting from intermetallics, the alloy color scales with the phase percentage of the intermetallic phase. In the Au-Al-Cu library, we found a ∼90 % AuAl2 area fraction compositional window where Cu addition can be tuned to improve mechanical properties while keeping purple color. Moreover, the color in amorphous and crystalline state for same composition is essentially the same.
We are grateful for the support from National Science Foundation (NSF) under grant no. MRSEC DMR 1119826 (CRISP). We also thank Professor Jung Han for generously providing us microreflectance setup and Cheng Zhang for training. We also thank Professor Hui Cao for informative discussion on characterization of optical properties of thin films and for providing spectrophotometer setup.
- 3.Corti CW (2004) Blue, black and purple: the special colours of gold. Proc Santa Fe Symposium Met-Chem Research Inc 121–133Google Scholar
- 8.HSU L-S (1994) Physical properties of AuAl2, AuGa2, AuIn2, and PtGa2. Modern Physics Letters B 08(21n22):1297–1318. doi: 10.1142/S0217984994001278
- 9.Fox M (2010) Optical properties of solids. Oxford master series in condensed matter physics, vol 3, 2nd edn. Oxford University Press, OxfordGoogle Scholar
- 10.Keast VJ, Birt K, Koch CT, Supansomboon S, Cortie MB (2011) The role of plasmons and interband transitions in the color of AuAl2, AuIn2, and AuGa2. Appl Phys Lett 99(11). doi: 10.1063/1.3638061
- 12.Ding SY, Gregoire J, Vlassak JJ, Schroers J (2012) Solidification of Au-Cu-Si alloys investigated by a combinatorial approach. J Appl Phys 111(11). doi: 10.1063/1.4722996
- 13.Schroers J, Lohwongwatana B, Johnson WL, Peker A (2005) Gold based bulk metallic glass. Appl Phys Lett 87(6). doi: 10.1063/1.2008374
- 17.Thienhaus S, Naujoks D, Pfetzing-Micklich J, Konig D, Ludwig A (2014) Rapid Identification of areas of interest in thin film materials libraries by combining electrical, optical, X-ray diffraction, and mechanical high-throughput measurements: a case study for the system Ni-Al. ACS Comb Sci 16(12):686–694. doi: 10.1021/Co5000757 CrossRefGoogle Scholar
- 19.E308-13 A (2013) Standard practice for computing the colors of objects by using the CIE System. American Society for Testing and Materials, West Conshohocken, PA 1–43Google Scholar
- 25.Liu Z, Schroers J (2015) General nanomoulding with bulk metallic glasses. Nanotechnology 26(14). doi: 10.1088/0957-4484/26/14/145301
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