Phase Diagrams - Industrial Metallurgists

Phase Diagrams

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Phase diagrams are graphical representations of the phases present in a particular alloy being held at a particular temperature.  Phase diagrams 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 is important because the properties of a metal component depend on the phases present in the metal.

Phase diagrams are useful to metallurgists for selection of alloys with a specific composition and design and control of heat treatment procedures that will produce specific properties.  They are also used to troubleshoot quality problems.

Phase diagram features

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.

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.

Equilibrium Conditions

When using phase diagrams, it is important to understand that the relationship between the phases, alloy composition, and temperature is 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 treating and solidification 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 phase 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. Phase diagrams do not provide this information.

Iron-Carbon Phase Diagram

An example of a commonly used phase diagram is 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.

phase diagram

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Aluminum-Copper Phase Diagram

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 phase 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.

phase diagram

Effects of additional alloying elements

The addition of more alloying elements will change the phase-temperature-composition relationships shown on binary diagrams. Ternary phase diagrams show the phase-temperature-composition relationships for alloys that contain three elements. Ternary phase diagrams are more complicated than binary phase diagrams. One reason is that adding more alloying elements often results in the formation of additional phases compared to the corresponding binary phase diagram. For example, in steels, carbon is the significant alloying element added to iron with respect to the primary phases that form. However, the shape and location of the phase fields on the iron-carbon diagram will be altered when alloying elements such as manganese, chromium, and nickel are added. Nevertheless, even for alloys that contain more than three alloying elements, binary phase diagrams are often helpful for a general understanding of the primary phases of interest with respect to the significant alloying element. This is the case for the iron-carbon and aluminum-copper alloys.

Learn more

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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