Some phase-transformations are such that only some elements can partition, while the others remain in identical relative amounts in the different phases.

During the formation of bainite for example, the interstitial element C partitions, but all substitutional elements (Fe,Cr,Mn..) do not.

MTDATA can be forced to compute an equilibrium with these modified constraints. By classifying 'para' some elements, you can force their relative amount to be constant.

In the following we calculate two pseudo-binary diagrams for Fe-Mn-C under normal and paraequilibrium conditions. By classifying para Fe and Mn, we force their relative amount to be identical in FCC_A1 (austenite) and BCC_A2 (ferrite).

• 'Normal' equilibrium:

  WHICH MODULE ? app
  APPLICATION OPTION ? def sy 'Fe,Mn,C' sou plu !
  APPLICATION OPTION ? cl ab p(*) no p(FCC_A1,BCC_A2,CEMENTITE) !
  APPLICATION OPTION ? set w 1 !
  APPLICATION OPTION ? range t 873 1273 0.1 !
  APPLICATION OPTION ? set 'start' 0.98 0.02 0.0 !
  APPLICATION OPTION ? set 'end' 0.9795 0.02 0.0005 !
  APPLICATION OPTION ? range 'start' 'end' 30 !
  APPLICATION OPTION ? '/opt/mtdata/bin/isopleth.mac'
  APPLICATION OPTION ? compute !

  

  To leave the graphical window, enter a [Return] twice.

• Para-equilibrium, after the above:

  APPLICATION OPTION ? class para comp(Fe,Mn) !
  APPLICATION OPTION ? compute !

  

General information about the graphical display are here.

In version <= 4.73, there is a small problem with the above example and MTDATA in default configuration: the BCC/BCC+FCC boundary has an elbow at around 920 K, which is not possible. To correct this, the user must set the configuration variable PHASE_PRESENT_REL_TOL to 0.00001. For this, enter:
[PHASE_PRESENT_REL_TOL=1E-5
at the MTDATA prompt. The default is currently 0.0001 which should be corrected in future releases (at time of writing, 4.73).
Thanks to John Gisby and Hugh Davies from NPL for spotting and solving this problem.