About Ternary Phase Diagrams

There are many types of ternary phase diagrams but perhaps the most common is the isothermal section. In this case, the temperature is constant, which means that the tie-lines are in the plane of the calculation. Isothermal sections and monovariant lines on the liquidus surface can be calculated in traditional way. They can also be calculated in the TERN module, but this requires a special database (such as PTERN).

In order to set up calculation of a ternary isothermal section in the system Al-Mg-Si, do the following (after retrieving the data from the database (TDB) module):

SET_CONDITION T=823,P=1E5,N=1,X(MG)=.01,X(SI)=.01

C_E

Note that there is no condition on the amount of Al as that is the "rest". N=1 specifies that the system is closed. This calculation gives one point inside the system. Now define the axis:

S-A-V 1 X(MG) 0 1 0.01

S-A-V 2 X(SI) 0 1 0.01

Of course, you might not be interested in the whole section, and in such cases specify different lower and upper limits for the axes. Then give a SAVE command before the MAP command:

SAVE almgsi-823

MAP

The result can be plotted in the POST module. Ternary isopleths are equally simple to calculate. Use the same conditions as above if you are interested in a section of Al-Si at 1 mole percent of Mg, just define a different set of axis:

S-A-V 1 X(SI) 0 1 .01

S-A-V 2 T 500 2000 25

SAVE almgsi-1mg

MAP

In the resulting diagrams, the tie-lines are not in the plane of the diagram, thus the lever rule etc. cannot be applied. But you can calculate more elaborate sections. An interesting diagram is the section from pure Al to Mg2Si, which is almost a quasi-binary section. In order to calculate this, set up the conditions differently:

SET-CONDITION T=823, P=1e5, N=1, X(MG)-2*X(SI)=0, W(MG)=.01

C-E

Note that there is no condition on the amount of Si as this is given by the relation X(MG)-2*X(SI)=0. Also note that you can mix conditions in X and in W. Then set the axis, save the workspace and map.

S-A-V 1 W(MG) 0 .1 .01

S-A-V 2 T 500 1500 10

SAVE al-mg2si

MAP

A special type of ternary phase diagrams is liquidus surfaces. This is still not simple to calculate as a separate start point is needed for each separate line at each temperature.

However, you can easily calculate the monovariant lines where the liquid is stable together with two condensed phases:

SET-CONDITION T=800, P=1e5, N=1, X(MG)=0.01, X(SI)=.01

C-E

C-S P LIQ=FIX 0.3

S-C T=NONE

C-E

Now there should be an equilibrium where liquid is just stable. Set the axis to be the compositions.

S-A-V 1 X(MG) 0 1 .01

S-A-V 2 X(SI) 0 1 .01

SAVE almgsi-luni

MAP