I am trying to understand the concept of iso-activity lines and their significance in ternary phase diagrams
An iso-activity 'line' (or surface in a quaternary system) of C in a certain solution can be seen as the collection of compositions where C has the same tendency to undergo a reaction.
The easiest example of this is the line for an activity equal to 1. On this line, the solution would be in equilibrium with pure graphite. Any activity higher than 1 would mean the solution is unstable, as at equilibrium graphite would precipitate out of the system.
Activities lower than 1 might also result in the precipitation of other compounds, for example Fe3C, although the point at which these compounds precipitate would depend both on the activity of C and the activity of (in this example) Fe. (This isn't a perfect example as Fe3C is actually a metastable compound)
I am not myself familiar with ThermoCalc. I know that FactSage has the option to calculate iso-activity lines on ternary diagrams.
Hello,
In Thermocalc, an iso-activity line can be calculated by setting a condition in the calculation :
1/ the value of the activity of a species in a phase acr(specie,phase)
For example, for C in BCC_A2 with an activity of 0.2 add : acr(C,BCC_A2)=0.2
2/ the value of the activity of a component in the system
For example, for C with an activity of 0.2 in the system add : acr(C)=0.2
Do not forget to add a reference state for C (generally Graphite) : set-ref C graphite * *
where the to stars represents the temperature and pressure of the calculation.
Regards
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Metals
- Alloys
- Steel