MTDATA – Phase Diagram Software from the National Physical Laboratory

 

 

 

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CLASSIFY

Function:

allows the status of each component, phase and substance to be defined and a reference state to be defined for the thermodynamic properties of each component for the purposes of a subsequent calculation: the number of miscibility gaps (if any) may also be specified. A computational parameter required to deal with FREE components can be fixed in this option. The command is also used to specify special components prior to calculation of para-equilibrium.

Parameters:

absent

normal

reference(*)

free

delta(*)

miscibility(*)

para

Defaults:

normal = <all components, phases, substances (unless flagged as absent in the datafile)>

delta = <program defined>

misc = as flagged in the datafile or database

Examples:

classify absent phase(3) 1 2 7 !

classify normal component(1) !

classify absent "CO2<g>" 12 46 substance(13) !

classify misc(9) = 1 free component(3) !

classify reference(1) 2 !

classify absent all !

classify normal p(*) !

classify absent substance(13,15-17) !

classify para component(1) component(2) !

Parameter values:

ABSENT, NORMAL   take the value [component(*)] [phase(*)] [substance(*)]

[ ] = possible omission or repetition

Something CLASSIFY'd as NORMAL is part of the system, whilst something ABSENT is temporarily excluded until restored. Before a calculation can be performed (see COMPUTE), the amounts of NORMAL components must be specified using the SET command. In addition the equilibrium amounts of substances may be specified also using the SET command.

COMPONENT(*), PHASE(*), SUBSTANCE(*)

where the subscript * is any valid number or list of numbers obtained from the relevant list, given under the LIST command, or * itself. If * is used it refers to all items in the relevant lists.

Substances may also be specified in two other ways:

(a) by quoting the substance number alone.

(b) by giving the substance name, together with its correct phase label, enclosed in quotes; the correct substance name and phase label can be obtained from the relevant list given under the LIST command. This method of specifying the substance is available only if components are elements.

MISCIBILITY(*) takes the values <number>/NONE

* = <phase number>

<number> is the number of miscibility gaps to which the specified phase may possibly give rise. It must be less than or equal to 2. The discovery of miscibility gaps cannot be guaranteed.

NONE is equivalent to setting <number> = 0

FREE takes the value COMPONENT(*)]/ SUBSTANCE(*)/  PHASE(*)/ TEMPERATURE/ PRESSURE

Enough variables must be CLASSIFY'd as FREE if the user has constrained the equilibrium amount of substances that are directly or indirectly related to it - see "General comments" under SET. FREE components are equivalent to NORMAL components but the component amounts should be regarded as initialised rather than fixed. An example is given in the MULTIPHASE Introduction. At least one component must be classified as NORMAL and, if only STAGE_1 is being used or BOTH_LARGE, the maximum number of FREE components is two.

DELTA(*) takes the values <number>/UNDEFINED

* = <component number>

DELTA is effective only when BOTH_LARGE or STAGE_1 is being used in the COMPUTE command. If a component is CLASSIFY'd as FREE, then the calculation requires an incremental component amount to make steps from the user's initial estimate of the FREE component amount. In most cases, it is best to allow the program to select the value of this increment. Only in unusual circumstances would it be necessary for the user to change the DELTA parameter.

REFERENCE(*)   takes the value <phase number>/DEFAULT

* = <component number>

REFERENCE is used to specify a reference state for one or more components. This allows the user to specify how the thermodynamic properties of a component should be referred to eg it might be preferable to specify that the reference state for carbon should be GRAPHITE while that of oxygen should be the gas phase for example in order to tabulate activities of components. The default reference state is that defined within the DATA_INPUT_FILE which will normally be G-Hser.

PARA takes the value [component(*)]

PARA is used to identify specific component prior to a calculation of para-equilibrium. In para-equilibrium it is assumed that during phase transformations some components diffuse very rapidly while others diffuse at a negligibly slow rate. For an example, in the Fe-Cr-C system C may be assumed to diffuse rapidly as the system transforms from ferrite (bcc_a2) to austenite (fcc_a1) while Fe and Cr may not diffuse at all. Fe and Cr in this case are the specific "PARA" components since their sluggishness acts as a constraint on the full thermodynamic equilibrium.

General comments:

The ABSENT/NORMAL classification is in effect a toggle that ignores, or includes phases, components and substances when the calculation is performed. It can be used as many times as is desired on the same entity within the same session.

 

Updated 17 March 2011