Ternary Phase Diagrams
What is a ternary phase diagram?
A ternary phase diagram has three components. The three components are usually
compositions of elements, but may include temperature or pressure also. This type of
diagram is three-dimensional but is illustrated in two-dimensions for ease of drawing and
reading. Instead of being a rectangular plot, it is a triangle. Ternary phase diagrams exist
for many metallic alloys, but are also widely used in ceramics. Stainless steel (Fe-Ni-Cr)
is a perfect example of a metal alloy that is represented by a ternary phase diagram.
Stainless steel is a very common metal alloy. Almost everyone knows of an everyday
object that is made with stainles steel. In the figure below stainless steel has been used to
construct a portable hoist. You can find out more about other products made from
stainless steel via the link provided in the Fig. 1 caption.
Figure 1: Stainless steel hoist constructed by Halliday Products
Ternary phase diagrams are needed so that three components can be compared at once.
For example, stainless steel has iron, nickel, and chromium compositions. To view all
three compositions at the same time, a triangular plot is set up with an element at each of
the vertexes with the temperature and pressure stated. In ceramic systems, sometimes
compounds are located at the vertexes instead of elements. The derivation of the ternary
plot is too complicated to go into, but the analytical derivation of a binary system is
available along with the experimental method of determining the phase diagram
How do you read a ternary phase diagram?
Figure 2: Stainless steel phase diagram at 900 degrees Celsius (ASM 1-27)
Reading the compositions of iron, chromium and nickel at any point on the stainless
steel ternary phase diagram in Fig. 2 is simple. Instead of drawing one tie-line, as in a
binary phase diagram , three lines are drawn, each parallel to a side of the triangle and
going through the point in question. Extend the lines so they pass through an axes. To
find the iron composition, the line drawn parallel to the axis opposite the Fe vertex is the
one needed. The percent iron is then read off the axis.
For example, to determine the compositions of the 18-8 circle point near the lower left
corner of Fig. 2, draw these lines: 1. draw the first line to be parallel with the axis
opposite the Fe vertex, we find that the composition of iron is 74% , 2. next draw a line
parallel with axis opposite the Ni vertex and read the composition of nickel to be 8% , 3.
and finally draw a line parallel to the axis opposite the Cr vertex and we see that there is
18% of chromium at that point. The point described is then called 18-8 stainless steel,
naming only the percentages of the chromium and the nickel; the iron content being
dependent on the other two. This 'recipe' for stainless steel is the most common.
Figure 3: Stainless steel solidus projections over a range of temperatures (ASM 3-44)
Figure 3 represents the entire ternary phase diagram solidus projections of stainless steel
over a range of temperatures under constant pressure. This picture illustrates how
temperature affects solubility.
What about the pressure?
Before now, the pressure was kept constant (1 atm), which is acceptable for most
applications but in extreme pressure environments, a scientist must consider the pressure
dependence of a system. The variance of pressure brings about a whole new dimension -the fourth dimension. The pressure is usually not taken into consideration unless a gas
phase is liable to be present.
For information on the materials class that this subject is studied in, visit the MSE 3424
syllabus, Phase Equilibria and Crystal Chemistry.
Jamie Yeakle 4/28/96
http://www.eng.vt.edu/eng/materials/classes/MSE2094_NoteBook/
96ClassProj/experimental/ternary2.html
Ternary diagram Plot - Download Software
Ternary Diagram Plot - Introduction
A Ternary Diagram Plot are in chemistry used for depicting chemical compositions phase diagrams. These diagrams are three-dimensional but is illustrated in twodimensions for ease of drawing and interpretation. In a ternary diagram the relative
percentage (normally weight %) of three components are represented by A, B and C. The
only requirement is that the three components have to sum to 100%. If they don't, you
have to normalize them to 100%.
Fig. Ternary Diagram Plotter
Data Entry - Ternary phase diagram plotter