Phase diagrams are graphical representations of the phases present in an alloy being held at a particular temperature. They are used in metallurgical engineering to understand the effects of alloy composition and heat treating on a metal's microstructure. This is important because the properties of a metal component depend on the phases present in an alloy.
Phase diagrams are useful to metallurgists for selection of alloys with a specific composition and design and control of heat treating processes that will produce specific properties. They are used to predict the phase changes that occur in alloys during heating and cooling. This can be during heat treating, casting solidification, joining processes that involve molten metal, and elevated temperature use conditions.
This article explains some of the details of phase diagrams, using the iron-carbon (Fe-C) and aluminum-copper (Al-Cu) systems as examples.
In metallurgy, the term phase is used to refer to a physically homogeneous state of matter, where the phase has a certain chemical composition, and a distinct type of atomic bonding and arrangement of elements. Within an alloy, two or more different phases can be present at the same time. The images below show the phases in aluminum-copper and iron-carbon alloys.
Each phase within an alloy has its own distinct physical, mechanical, electrical, and electrochemical properties. For example, in carbon steel, ferrite is a relatively soft phase and cementite is a hard, brittle phase. When they are present together, the strength of the alloy is much greater than for ferrite and the ductility is much better compared to cementite. Thus, an alloy with more than one phase can be considered to be a composite material.
The phases present in an alloy depend on the alloy composition and the thermal treatment to which the alloy has been exposed. Phase diagrams are graphical representations of the phases present in a particular alloy being held at a particular temperature. Phase diagrams can be used to predict the phase changes that have occurred in an alloy that has been exposed to a particular heat treatment process. This is important because the properties of a metal component depend on the phases present in the metal.
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This article focuses on binary phase diagrams, which are for alloys that consist of two elements. In a binary phase diagram, the alloy composition is shown on the x-axis and the alloy temperature is shown on the y-axis, as shown on the iron-carbon phase diagram below.
A phase diagram is divided into different phase fields, which indicate the phase or phases present for a particular alloy composition and temperature. There are single-phase fields and two-phase fields. For alloy compositions and temperatures that correspond to a single-phase field, the alloy will consist of a single phase. For alloy compositions and temperatures that correspond to a two-phase field, the alloy will consist of two-phases. The boundaries between phase fields are called phase field boundaries. Two-phase fields are always bordered on the left and right side by single-phase fields. A two-phase field contains the phases indicated by the bordering single-phase fields.
Phase diagrams indicate the relationship between the phases present, alloy composition, and temperature under conditions of slow heating or cooling. Slow heating or cooling allows the atoms within a metal to move around so that the alloy is at equilibrium. However, with many heat treatment processes, a metal is exposed to fast heating and cooling. Under these conditions it is possible to have phases missing or present compared to what is indicated by the diagram. Therefore, it is also important to understand the kinetics of phase transformations, i.e. the effects of temperature, time, cooling rate, and heating rate on phase changes within an alloy. This will be a topic of another article.
The iron-carbon phase diagram is used to understand the phases present in steel. The amount of carbon present in an iron-carbon alloy, in weight percent, is plotted on the x-axis and temperature is plotted on the y-axis. Each region, or phase field, within a phase diagram indicates the phase or phases present for a particular alloy composition and temperature. For the iron-carbon phase diagram, the phase fields of interest are the ferrite, cementite, austenite, ferrite + cementite, ferrite + austenite, and austenite + cementite phase fields.
The phase diagram indicates that an iron-carbon alloy with 0.5% carbon held at 900 °C will consist of austenite, and that the same alloy held at 650 °C will consist of ferrite and cementite. Furthermore, the diagram indicates that as an alloy with 0.78% carbon is slow cooled from 900 °C, it will transform to ferrite and cementite at about 727 °C.
Another commonly used example is aluminum-copper phase diagram, which is useful for understanding precipitation strengthening in Al-Cu alloys. The amount of copper present in an alloy is plotted on the x-axis. The phase fields of interest are the Al, θ, and Al+θ phase fields on the left hand side. For precipitation strengthening an Al-Cu alloy, this diagram indicates the minimum temperature to which an alloy must be heated to put all the copper in solution. This is indicated by the solvus line on the phase diagram. The maximum amount of copper that can contribute to precipitation strengthening is indicated by the maximum amount of copper (5.45 %) that can go into solid solution in the aluminum.
You can learn more about how to read and use phase diagrams in a few of our courses. Metallurgy of Steel teaches about the iron-carbon phase diagram. Precipitation Strengthening teaches about the aluminum-copper phase diagram.
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Michael Pfeifer, Ph.D., P.E. is President of Industrial Metallurgists and a metallurgical engineering consultant with more than 30 years of experience in product design, manufacturing, failure analysis, and metallurgy training. He helps clients worldwide solve design and manufacturing problems and is the author of Materials Enabled Designs.
