18. MICROSTRUCTURE Namelist

Truchas is able to capture and analyze data about a solidification process as it evolves within each mesh cell, and output the results for later analysis. This data is generally used by analytic 0-dimensional models to predict the local microstructural characteristics and defects of the solidified material. This capability is described in terms of the usual liquid-solid transformation but can be used for other transformation processes (e.g., solid-solid). Use the MICROSTRUCTURE namelist to enable this feature and set the parameters that control it.

Note

Required/Optional:: Optional
Single/Multiple Instances:: Multiple

18.1. Common Namelist Variables

The following variables are common to all analysis modules.

low_temp_phase

The name of the low-temperature phase (e.g., solid) of the transformation for which data will be collected. This is optional. When defined, the fraction of low temperature phase will be made available to the analysis module for use.

cell_set_ids

A list of cell set IDs that specify the part of the domain where data will be collected. Cell-based data is output for the entire mesh, using dummy values for cells that are not included in the analysis and included cells without valid data. If not defined, all cell sets are assumed.

18.2. Basic GL Analysis Module

By default, the thermal gradient \(G=\nabla T\) and cooling rate \(L=-\partial T/\partial t\) are recorded at the onset of solidification, and the local solidification time (time spent in the mushy zone) is recorded at the completion of solidification. These are written to the output file as the cell-based fields “ustruc-G”, “ustruc-L”, and “ustruc-t_sol”, respectively. A dummy value of 0 is written for cells not included in the analysis and cells without valid GL or solidification time data.

Tip

Many, if not most, analytic models for microstructure and defect prediction can be defined in terms of the fundamental G, L, and solidification time data produced by this basic analysis module. These models can be easily computed and visualized within Paraview using its calculator tool. For example, the (vector) solidification front velocity is \(V = (L/\Vert{G}\Vert^2)G\), and the solidification front speed is \(\Vert{V}\Vert = L / \Vert{G}\Vert\).

There are two methods of specifying the parameters that control how the data is collected, which are pictured below: one based on the fraction of the low-temperature phase and the other based on temperature. They are mutually exclusive. The phase fraction method requires low_temp_phase to be defined. The collection procedure is designed to only report data for those cells that have ultimately passed monotically from liquid to solid (or are in the process of doing so). Thus some cells included in the analysis may report invalid data even after all solidification is complete.

Namelist Variables for the Temperature Based Method

begin_temp

Solidification is regarded as having started when the temperature first falls below this threshold; it need not be the actual liquidus temperature. This is a required variable.

end_temp

Solidification is regarded as complete when the temperature first drops below this threshold; it need not be the actual solidus temperature. This is a required variable.

gl_temp

When the temperature first falls below this threshold, G and L are set to the current value of the thermal gradient and cooling rate. The value must belong to the interval [begin_temp, end_temp). This is an optional variable; its value defaults to the value of begin_temp.

begin_temp_reset

If the temperature rises back above this threshold while solidifying, it is regarded as having returned to a liquid state and any G and L data that may have been computed is erased. This is an optional variable; its value defaults to the value of begin_temp, and if specified it must be greater than or equal to that value.

end_temp_reset

If the temperature rises back above this threshold after becoming solid, it is regarded as having (partially) re-melted, and the previously computed G, L, and solidification time are erased. This is an optional variable; its value defaults to the value of end_temp, and if specified it must be greater than or equal to that value.

Tip

This option can be used to capture temperature GL data for any cooling process, and not just those associated with an explicitly modeled phase transformation.

Namelist Variables for the Solid Fraction Based Method

begin_frac

Solidification is regarded as having started when the solid fraction first rises above this threshold. This is a required variable; its value must belong to the interval (0,1).

end_frac

Solidification is regarded as complete when the solid fraction first rises above this threshold. This is a required variable; its value must belong to the interval (begin_frac, 1).

gl_frac

When the solid fraction first rises above this threshold, G and L are set to the current value of the thermal gradient and cooling rate. The value must belong to the interval [begin_frac, end_frac). This is an optional variable; its value defaults to the value of begin_frac.

begin_frac_reset

If the solid fraction falls back below this threshold while solidifying it is regarded as having returned to a liquid state and any G and L data that may have been computed is erased. This is an optional variable; its value defaults to the value of begin_frac, and if specified it must belong to the interval (0, begin_frac].

end_frac_reset

If the solid fraction falls back below this threshold after becoming solid, it is regarded as having (partially) re-melted, and the previously computed G, L, and solidification time are erased. This is an optional variable; its value defaults to the value of end_frac, and if specified it must belong to the interval [begin_frac, end_frac].

18.3. Custom Microstructure Analysis Modules

The Truchas microstructure analysis framework is designed to readily accept additional user-written analysis modules. Contact the Truchas developers for details and refer to the template file located in the src/truchas/ustruc directory.