How To Read A Phase Diagram

Brazing Education

Phase diagrams are a great tool for the Brazing Engineer. While its is certainly true that most brazing applications involve systems more complicated than a binary alloy represented by the common phase diagram, nevertheless, the binary phase diagram is an invaluable tool both for answering questions about why a particular braze alloy and substrate interact the way they do and it can also help to predict what to expect from a novel application. While they are extremely useful, like any power tool, they can be difficult to use and must be fully understood to be of most use.

So how do you read a phase diagram? This phase diagram Phase Diagram shows a typical binary system that happens to contain a eutectic. This is a fairly common characteristic of bimetallic alloys, the copper-silver system for instance has a phase diagram very similar to the one in the link. First lets consider the information that is displayed.

The horizontal axis displays the range of possible compositions of the alloy. The far left hand side indicates pure element A and as you move to the right element A decreases and element B increases until you reach the far right which is a composition of pure element B. So the horizontal position indicates the A-B composition in percent. Phase diagrams can be expressed in either atomic or weight percentage. The two ways of expressing the chart are equivalent and you can convert between the two by using the atomic mass of each element to convert.

The vertical axis represents temperature. It’s very straight forward. The higher on the chart the hotter. So if you pick a point on the chart. You read along to the x-axis and read off the composition. You read along the y-axis to read off the temperature. Therefore any single point on the phase diagram represents a specific alloy composition at a specific temperature. So I like to think of the phase diagram as depicting a composition-temperature space. Great, but how does that relate to the phases?

Metallurgical phase diagrams typically only present liquid and solid phases. Note that it is possible for there to be more than one liquid phase present, think about water and oil for instance. If you mix them you get two distinct liquid phases forming that are immiscible, meaning they don’t mix. One is mostly oil and the other is mostly water. Since they don’t mix and are separately identifiable they are considered separate phases. immiscible liquid phases are not common in metal alloys, but immiscible solid phases are very common and that is what has caused the eutectic in the linked phase diagram. Most typically these separate solid phases are the result of two different crystal structures and are favored by the particular element or compound.
The various phases are depicted using the curved lines on phase diagrams, actually to be more precise the lines represent phase boundaries whereas the space between the lines depicts the areas where a particular phase or phases are present in the composition-temperature space. Note that these areas can contain either a single phase or two phases, but never more than two. In the case of the example eutectic diagram, the several phase regions are labeled: alpha, beta, alpha+beta, L (= liquid), liquid + alpha, and liquid + beta. Once you understand the logic of phase diagrams it’s possible to deduce what phases must be present in what regions.

The alpha and beta phases are phases that represent a solid solution of elements A and B. The alpha phase will have a crystal structure like pure A and will contain some B substituting in the crystal lattice. Likewise, beta phase will have an element B crystal structure with A substituting for some B atoms. In both cases if the substituting element exceeds some percentage limit, the limit is a function of the thermodynamics of the particular elements in question, then further substitution becomes energetically unfavorable and the “extra” substituting element will start to form a phase made up of its own crystal structure (also with substitutions of the other element). That is what is occurring in the alpha + beta region. In that region there will be two solid phases present, one will be the alpha phase and the other will be the beta phase. Both phases will be “saturated” with the substituting element and the relative volumes of each phase will be determined by the relative richness of the A and B elements at that composition.

The relative stability of the substituting elements in the foreign crystal lattice is temperature dependent, recall that stability is a matter of specific thermodynamics. That is why the phase boundaries curve. Also the melting point of the phase is dependent upon the percentage of substitution causing the curve in the liquidus lines.
In the case where a eutectic forms it just so happens that substitution lowers the melting points of both the alpha and beta phases. That means in those cases a pure crystal lattice is more stable than a crystal lattice with substitutions. That is true for both the A and B elements in this case and also it just so happens that there is a composition where both the alpha and beta phases have the same melting point. That point is known as the eutectic. Its the point at the bottom of the “V” formed by the liquidus lines.
Solidus and Liquidus are terms that name the phase boundaries between a solid phase and a phase that contains both a solid and a liquid (solidus) or between that solid+liquid phase and a liquid phase (liquidus). Eutectic is a term that names the point of the “V” described above.

There is a lot more information that you can glean about the characteristics of the alloy formed between the two elements from this phase diagram and also information about how this alloy might perform in relationship to a braze application but that will be the subject of a future post or two.