Phase Diagram

 

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.

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in an aluminum matrix.
© DoITPoMS Micrograph Library, Univ. of Cambridge

Ferrite (white) and cementite (dark) in steel.

Ferrite (white) and cementite (dark) in steel.

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.

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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.

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.

Iron-Carbon Phase Diagram
An example of a commonly used phase diagram is the iron-carbon phase diagram, which 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.

Fe-C_PhaseDiagram

Aluminum-Copper Phase Diagram
Another commonly used phase diagram is the 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.

Al-CuPhaseDiagram

Equilibrium Conditions
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 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. This will be a topic of another article.

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You can learn more about how to read and use phase diagrams in a few of our courses. Metallurgy of Steel and Metallurgy of Steel Heat Treating teach about the iron-carbon phase diagram. Metallurgy of Precipitation Strengthening teaches about the aluminum-copper phase diagram.

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