An Overview of the Science of Alloying

Definition
An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance usually has different properties (sometimes substantially different) from those of its components.

Alloys are usually prepared to improve on the properties of their components. For instance, steel is stronger than iron, its primary component. The physical properties of an alloy, such as density, reactivity and electrical and thermal conductivity may not differ greatly from the alloy's elements, but engineering properties, such as tensile strength, shear strength and Young's modulus (measure of stiffness), can be substantially different from those of the constituent materials. This is sometimes due to the differing sizes of the atoms in the alloy—larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the alloy resist deformation, unlike a pure metal where the atoms move more freely.

Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. However, for most pairs of elements, there is a particular ratio which has a single melting point; this is called the eutectic mixture.

Alloys can be classified by the number of their constituents. An alloy with two components is called a binary alloy; one with three is a ternary alloy, and so forth. Alloys can be further classified as either substitution alloys or interstitial alloys, depending on their method of formation. In substitution alloys, the atoms of the components are approximately the same size and the various atoms are simply substituted for one another in the crystal structure. An example of a (binary) substitution alloy is brass, made up of copper and zinc. Interstitial alloys occur when the atoms of one component are substantially smaller than the other and the smaller atoms fit into the spaces (interstices) between the larger atoms.

In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminum used as a structural building material are also alloys.

The term "alloy" is sometime used in everyday speech as a synonym for a particular alloy. For example, automobile wheels made of "aluminum" are commonly referred to as simply "alloy wheels". The usage is obviously indefinite, since steels and most other metals in practical use are also alloys.

Effect of copper:silver ratio on color

Pure (24 karat) gold is a deep yellow color (an orange shade of yellow) and is soft and very malleable. The colored karat gold alloys range in gold content from 8 to 22 carats (33.3% - 91.6% gold) and can be obtained in a range of color shades: green (actually a green shade of yellow), pale yellow, yellow, deep yellow, pink/rose and red. There is also white gold and even unusual colored gold such as 'purple gold'. They all have different mechanical properties such as strength, hardness and malleability (ductility) and some alloys can be heat treated to maximize strength and hardness. There are gold alloys that are optimized for different manufacturing routes such as lost wax (investment) casting and stamping.
How can color be varied and why do different gold alloys have different mechanical and other properties? To answer these questions in depth requires a good technical knowledge of metallurgy. However, it is possible to give some simplified answers.

Colored Karat Gold

Almost all conventional, colored karat gold is based on gold-silver-copper alloys, often with minor alloying additions. All three metals have the same crystal structure (face centered cubic, FCC) and so are compatible with each other over a large range of compositions. Typical minor additions include deoxidizers such as zinc and silicon, grain refiners such as iridium and cobalt and possibly metals such as nickel to strengthen the alloy. Larger zinc additions (about 1-2%) can improve melt fluidity and hence 'castability' in lost wax casting, as can silicon, resulting in better filling of the mold and better reproduction of surface detail. Even larger zinc additions (up to 10%) can improve malleability of certain karats of gold, particularly 14 karat and lower, used for making jewelry by stamping from sheet. Additions of low melting point metals such as zinc, tin, cadmium and indium lower melting ranges and hence are used to make karat gold solders.

Color

Gold is yellow and copper is red, the only two colored pure metals. All other metals are white or grey in color. The addition of a red color to yellow, as every school child knows, makes the yellow pinker and eventually red. The addition of a white makes the yellow color paler and eventually white. This principle of mixing colors is the same in karat gold. Adding copper to gold makes it redder and adding silver, zinc and any other metal makes gold paler. Thus, we can understand that in lower karat gold, because we can add more alloying metals, there can be a wider range of colors than the higher karat gold.
Thus at 22 karat (91.6% gold), we can only add a maximum of 8.4% of alloying metals and hence can only obtain yellow to pink/rose shades. At 18 karat (75.0% gold) and lower, we can add 25% or more alloying metals and hence get colors ranging from green through yellow to red, depending on the copper: silver plus zinc ratio. Thus for any given karat we can vary the color by varying the copper: silver plus zinc ratio. This can be demonstrated in the following table:

Effect of copper:silver ratio on color

Properties

Alloying additions affect other physical properties as seen in the next table:

As karat level reduces, the melting range and alloy density are lowered. But at any given karat (gold content), the actual values vary according to the relative silver and copper contents.
As well as affecting physical properties, alloying additions to gold generally increase the strength and hardness, with some reduction in malleability / ductility. The silver atom is slightly larger than that of gold, so alloying gold with silver gives a moderate improvement in strength and hardness. The copper atom is significantly smaller than that of gold and so it has a greater effect on strengthening gold than silver, as it distorts the gold crystal lattice more. Thus reducing the karat from 24 karats through 22 kt and 21 kt down to 18 karat gold results in stronger and harder alloys, as can be seen in Table 3. Beyond 18 kt down to 10, 9 and 8 karats does not have much further effect.

Mechanical Properties of Typical Gold Alloys

Table 3.2: Mechanical Properties of 18 Karat Gold

c.w. = cold worked

However, copper-containing karat golds in the range of 8-18 karats can be hardened even further because of their metallurgy. Hard second phases can be precipitated out in the solid state as they cool below about 400°C, making the karat gold less ductile. Because of this, such alloys must be quenched in water after annealing to retain the single phase, ductile state if further working is required. This can be seen in the next table, Table 4.1

Effect of Cooling Rate on 18 Karat Gold after Annealing at 650°C

Special low temperature (aging) heat treatments (typically 3-4 hours at 280 -300°C) can later be employed to give substantial hardening to such annealed and quenched alloys. This is known as age-hardening. In 18 kt red gold, the hardness can be doubled, as shown in Table 4.2.

Effect of Heat Treatment on 18 Karat Alloys

As all goldsmiths know, working a metal makes it harder and stronger, as we can see in the previous tables, but if it is overworked, it will eventually fracture. So, they know that worked karat gold must be annealed to restore the soft ductile condition. Typical annealing temperatures for karat gold are given in the following table:

Typical Annealing Temperatures

White gold

Apart from copper, all other alloying metals to gold will tend to whiten the color and so it is possible to make karat gold that is white in color. White gold for jewelry was developed in the 1920's as a substitute for platinum.
Additions of any white metal to gold will tend to bleach it's color. In practice, nickel and palladium (and platinum) are strong 'bleachers ' of gold ; silver and zinc are moderate bleachers and all others are moderate to weak in effect.
This has given rise to 2 basic classes of white gold - the Nickel whites and the Palladium whites. At the 9 carat (37.5% gold) level, a gold-silver alloy is quite white, ductile although soft and is used for jewelry purposes. White gold is available up to 21 karat.
There is no legal definition of what constitutes a 'white' color in gold and hence trade description of white gold may not mean 'detergent white'. Many commercial white gold  mixtures are not a good white color.

Nickel white gold

Nickel alloying additions form hard and strong white gold up to 18 karat. They are difficult to work and suffer from so called 'firecracking'. Most commercial alloys are based on gold-nickel-silver-zinc alloys with copper often added to improve malleability. This copper addition, of course, affects color, and so such white gold alloys are not a good white color - more a slight yellow/ brown tint, particularly if nickel content is also low. As a consequence, such white gold jewelry is normally electroplated with rhodium (a platinum metal) which is tarnish resistant and imparts a good white color.

Unfortunately, many people, the female population especially, are allergic to nickel in contact with the skin and this gives rise to a red skin rash or irritation. The European Union countries have enacted legislation valid from the 20th January 2000 that limits nickel release from jewelry. Thus, in Europe, nickel white gold is being phased out and being replaced by palladium white gold. The USA is taking a more relaxed approach, requiring jewelry to be labeled as nickel-containing, and much jewelry in the West is now advertised as 'non-allergenic' or 'nickel-free'. Some typical nickel white gold compositions are shown in Table 6.

Typical Nickel White Gold

Palladium white gold

Additions of about 10 -12% palladium to gold impart a good white color. But palladium is an expensive metal, dearer than gold and it is also a heavy metal. Thus jewelry in such palladium white gold will be more expensive than identical pieces in nickel whites for 2 reasons: firstly, the cost of the palladium and secondly, the impact of density - palladium white gold is denser and so such jewelry will be heavier and also contain more gold. It is also more difficult to process as the melting temperatures are substantially higher.
Many commercial palladium white gold alloys only contain about 6-8% palladium plus silver, zinc and copper. Some may even contain some nickel [so a palladium white gold is not necessarily nickel-free]. These may also have less than a good white color and so may also be rhodium plated.
Palladium white gold tends to be softer and more ductile compared to nickel whites and so will not wear as well. They are available in all karatages up to 21 karat. It is not possible to have a 22 kt white gold, for example. Some typical compositions are given in Table 7.

Typical Palladium Alloys

Pd- palladium; Ag- silver; Cu - copper; Zn - zinc, Ni - nickel. [In wt %]