One of the most important tools, and perhaps one of the most confusing tools, for metallurgists is the binary phase diagram. A binary phase diagram plots the composition, expressed as a percentage of one element against temperature. At any given combination of composition and temperature, the phase of the metal can be determined.

What is a phase?

Middle school science class provides an introduction to phases: solids, liquids and gases, and perhaps an argument to be made for plasma. When thinking of these materials at the molecular level, a gas is energetic enough that it does not stay bonded with other gas molecules for very long. As the temperature cools, the material becomes a liquid, and more bonds are formed between adjacent molecules, and then finally, it becomes a solid.

However, that is not the entire picture. The atoms in a solid can arrange themselves in different crystallographic structures. This means that there can be many solid phases, and each of these solid phases can have different physical properties.

Steel (expressed as a combination of iron and carbon) is a good example of this effect. Solid 1080 steel above about 727° C is called “austenite.” Unlike room temperature steel, this steel is non-magnetic. Room temperature 1080 steel may have a variety of microstructures (most notably pearlite), but most of them are magnetic. Austenitic steel only has about half of the tensile strength of room temperature “pearlite” steel. Both are solid. Both have the same composition (as a percentage of carbon), but they have totally different properties.

Magnets are great for moving scrap steel, but should not be used to move austentic steel, such as steel from a continuous caster to a rolling mill for hot rolling. In the latter case, the steel is non-magnetic. Source: PXhere

Example: Gold and silver

Gold and silver are atomically similar, and so their phase diagram is relatively simple. Because of this, they really only form one solid phase (alpha phase) at room temperature and up to the lower line (solidus line). Above a certain temperature, the alloy is totally liquid. This line is called the liquidus line. Between the solidus line and liquidus line is a pasty region, which is a combination of solid alpha phase and liquid metal.

Gold-silver phase diagram. Source: Wikipedia Commons/modified by Seth Price

An alloy of 50% gold and 50% silver, starting at 1,040° C, would be totally liquid. As it cooled to 1,020° C, bits of solid alpha phase would begin to form. This process would continue until around 1,000° C, when the alloy would be completely solid, alpha phase.

Example: Brass

Gold and silver are simple. However, copper and zinc alloys, better known as brasses or bronzes, are much more complicated. In this phase diagram, there are many solid phases. Alpha phase brass is copper-rich and is used in many “brass” fittings. There is also beta phase, gamma phase, delta phase, epsilon phase and eta phase brass, all with distinct properties.

Copper and zinc phase diagram.

In between the single phase regions are two phase regions. For example, a 60% copper-40% zinc alloy at 600° C is in between the alpha and beta single phase regions; it is a two-phase alpha plus beta region. In some alloys, two phase areas have desirable properties (such as in steel), but in others, they are undesirable.

The author of this paper tried to reduce the thickness of some brass samples through a small set of rollers. The single phase alpha brass could be reduced in thickness easily. The two phase, alpha plus beta brass could be reduced a little, but eventually, the sample began to break at the phase boundaries. A brass bar went in one end of the mill, and something resembling shiny potato sticks came out the other.

Example: Steel

While there is a phase diagram for iron and carbon, the most interesting part of this phase diagram is at low (under about 6.67%) carbon. At around 6.67% carbon, a ceramic (Fe₃C, cementite) is formed instead of a metallic alloy. For the purposes of the phase diagram, the cementite line is the maximum concentration of general interest.

Several things are happening in this phase diagram. First, there are single phase regions (alpha, delta and gamma), and there are several multi-phase regions. There are also several pasty regions (liquid plus something else).

Zoomed-in, an interesting portion of the iron-carbon phase diagram, called the iron-iron carbide diagram, represents where iron and steel alloys are derived. Source: Practical Maintenance

Pearlite is typically found in the combination of alpha plus cementite region of the phase diagram. This structure is stable and often desirable for many applications. Physically, this two-phase region looks like alternating plates of alpha phase and cementite, as shown in the micrograph.

This micrograph of 1045 steel shows alpha phase ferrite (white) and two phase pearlite, or alpha and cementite (striped areas). Source: Seth Price

Why are there both ferrite (alpha phase) and pearlite in the image? For 1045 steel, there is 0.45% carbon. As this alloy cools, it passes through a large, metastable region (gamma phase, austenite). Next, it passes through the little triangle of gamma and ferrite phases. As it turns out, the ferrite is stable and the gamma is not. What has been converted to ferrite is not going to undergo any new transformations. Some of the alloy is permanently ferrite. Below 727° C, the rest of the gamma phase is transformed into pearlite.

Final thoughts

This article discusses “simple” binary phase diagrams. In reality, alloys are made from many elements; each element moves these various phase boundaries around. More than about two or three elements (there are tertiary phase diagrams) and the phase diagram becomes nearly impossible to visualize. Also, phase diagrams really only address equilibrium cooling, and continuous cooling curves or time temperature transition curves are required during most situations. With both of these facts combined, determining the exact microstructure and predicting physical properties by hand is incredibly difficult.

Soon, artificial intelligence (AI) and machine learning (ML) will be able to gather cooling and heat transfer data and incorporate them into adaptive models. As this happens, more precise compositions and customized physical properties will be made possible.