Phase equilibria in steelsPhase equilibria in steels

 

 

MTDATA – Phase Diagram Software from the National Physical Laboratory

 

 

 

Home 

News 

Products 


Training and Support 

FAQs 

Links 

Agents 

Other Services 

 

Services 

 

 

COMPUTE

Function:

initiates the computation of all currently specified equilibria. Output is sent both to the screen and to a file.

Parameters:

initial_point_option

stages_required

print_level

renormalisation

Defaults:

initial = auto

stages=1_only

print=default

Examples:

compute !

compute stages = both_small initial = last !


Parameter values:

Most parameters within the COMPUTE command deal with the management of the mathematical methods used in the calculation. The default values set by the program will usually suffice. However, four settings are of particular interest:

INITIAL

takes the values AUTOMATIC/PREVIOUS_SOLUTION/LOWER_BOUNDS/LAST_ACTIVE_SET

AUTO the program chooses an initial set of starting values.

LOWER the amounts of all constituents are initially set at the lower bounds, i.e. effectively zero.

LAST the equilibrium substance amounts produced by the previous calculation are used as the starting point for the current calculation. In addition, the intermediate variables used to produce the previous STAGE_1 result are also available, so that the iteration does not have to start "cold". INITIAL = LAST is particularly useful, for example, in generating output from which graphs are to be plotted, where only small changes to the T, P or amounts are made from one calculation to the next.

PREVIOUS the equilibrium substance amounts produced by the previous calculation are used as the starting point for the current calculation.

STAGE

takes the values: 1_ONLY/2_ONLY/BOTH_LARGE/BOTH_SMALL/STAGE_1_PLUS/ROBUST_STAGE_2

1_ONLY specifies use of the STAGE_1 Gibbs energy minimisation algorithm, which gives equilibrium compositions to six decimal places in amounts. STAGE_1 is the only method available for certain models, including the multiple sublattice model. It must not be used when the amounts at equilibrium of more than two substances are constrained. It is not suitable for calculations of amounts less than 10-6 mol.

2_ONLY specifies the STAGE_2 algorithm, which is not as reliable as STAGE_1, and can fail unless preceded by a STAGE_1 calculation or a previous calculation for similar conditions and the appropriate setting of the initial point. The accuracy of the results is increased to six decimal places in the logarithm of the composition. See 1_ONLY description for limitations on models.

BOTH_LARGE both stages of the calculation are carried out automatically and the constraints on equilibrium are applied in both stages. BOTH_LARGE is appropriate if the constraints on equilibrium amounts are of the order of 1.0E-5 or above and there are at most two constraints on the equilibrium, otherwise BOTH_SMALL should be used. See 1_ONLY description for limitations on models.

BOTH_SMALL both stages of the calculation are carried out automatically but the constraints on equilibrium are applied only in STAGE_2. BOTH_SMALL is appropriate if the constraints on equilibrium amounts are of the order of 1.0E-5 or below or if there are more than two constraints. If the guessed FREE component values are reasonably near the true values, BOTH_SMALL may be quicker but, where it is applicable, BOTH_LARGE may be more reliable. See 1_ONLY for limitations on models.

STAGE_1_PLUS This provides an intermediate between STAGE_1 (highly robust for all phase types) and calculations using the STAGE_2 algorithm (giving extra precision for a slightly restricted set of phase models). The STAGE_1_PLUS option effectively carries out a STAGE 2 calculation following a STAGE_1 but only for certain phase types. This produces a refinement of the composition in phases such as a gas.

ROBUST_STAGE_2 This is an "intelligent" combination of STAGE_1 followed by STAGE_2. If the STAGE_2 equilibrium is successful then a STAGE_2 is repeated with STAGE_1 only being used if difficulties arise. This option is therefore efficient at rapidly calculating STEPped equilibria when more than STAGE_1 precision is required in systems with large numbers of species. WARNING: The ROBUST_STAGE_2 option should not be used if the number of solutions phases at equilibrium changes within a series of calculations.

RENORMALISATION

takes the value <sum of component moles>/NONE

Renormalisation is used in order to make the total of component amounts add up to a particular value (normally 1 or perhaps 100) when one of the components has been classified as free and a related constraint has been imposed. All the components are adjusted in response to changes in the calculated amount of the free component(s) in such a way as to make the total equal to the value given to the RENORMALISATION parameter. The default value is NONE.

PRINT

takes the values DEFAULT/EACH_STAGE_RESULTS/GRAPHICS_OUTPUT/BRIEF/PHASE_TABLES/MOLE_FRACTION_TABLE/ WEIGHT_FRACTION_TABLE

DEFAULT gives typically tabular output.

EACH_STAGE_RESULTS When stage 1 and stage 2 calculations are requested this option allows the intermediate stage 1 results to be seen.

GRAPHICS_OUTPUT If a variable has been stepped, specification of PRINT GRAPHICS_OUTPUT causes output to be written to a file for subsequent use with the MULTIPHASE PLOT command.

BRIEF This option is of limited use on its own and is more likely to be used in conjunction a PRINT option to select one of the summary tables (PHASE, MOLES or WEIGHT). For example:

compute print brief print phase !

PHASE_TABLE This gives a summary table showing the distribution of components between the phases found at equilibrium. This along with MOLE and WEIGHT tables provide an addition or alternative to the DEFAULT speciation results table. PHASE tables show only the phases that were found to be stable. For each component the distribution between these phases is given.

MOLE_FRACTION This gives a summary table showing the composition of each phase found to be stable given in terms of component mole fraction. This along with PHASE and WEIGHT tables provide an addition or alternative to the DEFAULT speciation results table.

WEIGHT_FRACTION This gives a summary table showing the composition of each phase found to be stable given in terms of component weight fraction. This along with PHASE and MOLE tables provide an addition or alternative to the DEFAULT speciation results table.

SPECIATION_AND_BASIC   This provides an addition to the BRIEF results table. For example:

compute pri spec_a_basic !

SPECIATION_AND_BASIC_TABLE show only the species in phases that were found to be stable together with the component chemical potential and activity table. For gas and aqueous phases with stage 1 results only nonzero species are reported to the screen.

SPECIATION_AND_PHASE  This provides an addition to the BRIEF results table. For example:

compute pri spec_a_pha ! 

SPECIATION_AND_PHASE_TABLE show only the species in phases that were found to be stable together with the table in which for each component the distribution between these phase is given. For gas and aqueous phases with stage 1 results only nonzero species are reported to the screen.

SPECIATION_AND_MOLE  This provides an addition to the BRIEF results table. For example:

compute pri spec_a_mol !

SPECIATION_AND_MOLE_FRACTION_TABLE show only the species in phases that were found to be stable together with the table in which for each phase the composition in terms of component mole fraction is given. For gas and aqueous  phases with stage 1 results only nonzero species are reported to the screen.

SPECIATION_AND_WEIGHT  This provides an addition to the BRIEF speciation results table. For example:

compute pri spec_a_wei !

SPECIATION_AND_WEIGHT_FRACTION_TABLE show only the species in phases that were found to be stable together with the table in which for each phase the composition in terms of component weight fraction is given. For gas and  aqueous phases with stage 1 results only nonzero species are reported to the screen.

SPECIATION_AND_NONE This provides an addition to the BRIEF results table. For example:

compute pri spec_a_none ! 

 SPECIATION_AND_NONE show only the species in phases that were found to be stable. For gas and aqueous phases  with stage 1 results only nonzero species are reported to the screen.

General comments:

For calculation of phase equilibria in condensed systems or in systems involving gases where only major species are important compute ! is quite adequate. If many adjacent temperatures have been SET so that the output can be plotted, then the speed of the calculation is improved by using: compute initial = last !, which is also useful in educating the MULTIPHASE to find miscibility gaps.

 

Updated 15 March 2011