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You know how to calculate a single equilibrium, that is for one temperature and one composition. Here you learn how to step a variable and obtain the results you are interested in.

Let's consider a simplified type 304: Fe-18Cr-12Ni-0.06C in which we want to calculate the amount of M23C6 carbide as a function of temperature.

## Defining the system:

multiphase

define source plus sub_sgte !

define system 'Fe,Cr,Ni,C' !

You can see some error messages, knowing what they mean exactly helps to know if you have to worry.. Then mt-data tells you he found 38 phases. How many did you have in mind ?

## Getting rid of useless phases:

In the first example, we used all the phases found by MTDATA in the calculation. In general, this is safer if you do not know very well what phases are expected. However, once you are sure about which phases will never appear, you can 'remove them', that is prevent them to enter the calculation and therefore speed it up.
This is very important for complex systems as, often, more than 100 phases are found and give very long calculation times if all present.

There are two ways to deal with the problem:

• you know more or less exactly which phases can form (do this if you follow the example):

class absent phase(*)

will classify all the phases as absent

class normal p(fcc_a1,bcc_a2,m23c6,7,9-13) !

will classify as normal Austenite, ferrite, and some carbides, note the different ways to refer to a phase.
• you know almost nothing of your system:

class normal p(*) !

class absent p(...)

you should remove phases like diamond_a4,c60, pure elements, etc and do a first calculation to get an idea. Since it will involve a lot of phases, even a simple calculation will be quite long.

## Setting the composition:

set w 100 !

set w(2) 18 w(3) 12 w(4) 0.06 !

This is equivalent to :

set w( ) undef 18 12 0.06 !

## Stepping the temperature:

step temp 873 1173 10 !

tells MTDATA that temperature has to be stepped by 10C between 600-900C.

## Starting the calculation:

The instruction to start the calculation is slightly different here: if you use

compute print brief print mole !

(or weight !)

MTDATA will give you 30 tables as the one you get in a single calculation. A better way to do it is to type:

compute print graphic !

All the results are then in a file called defxxx.gph, located in /home4/users/guest, xxx being the larger number you find.  It is a good idea to remove it as soon as possible from this space and place it in your home directory, to avoid saturation of MTDATA space.

However, to use the results, you should generate another file as follows:

 ordinate n phase(m23c6) ! you choose the ordinate, here the amount of m23c6. abscissa temperature system ! you choose the abscissa, note that the default is the stepped variable, so this line is useless. plot goor will give you a graph on the screen... plot tabulate spreadsheet ! generate a .gtb file in the home directory, this is a two columns ascii file which you can use in spyplot.

## A bit more :

There si no reason, if your system has more than one phase, why you wouldn't want to know how their equilibrium amounts vary with temperature.  To have a look, simply type:

ordinate n phase !

Since you don't say which phase, mt-data will put all of them in ordinate. Now type:

plot go

and you are not happy: the amount of carbides is so small that any variation is invisible due to the large scale choosen for austenite and ferrite. You have two solutions: either you give up the variations of these two phases by typing

ordinate limits 0 30 !

(you first have to leave the graphical interface with a+return)

plot go

and you almost only see the variations of M23C6, or you choose a logarithmic scale:

ordinate limit auto log yes !

 PT-group 2003 Thomas Sourmail, Cambridge. Please email feedback !