# Getting Started with the Diffusion Module (DICTRA)

## Welcome to the Getting Started Guide for the Diffusion Module (DICTRA)

This guide is for those who are new to the Diffusion Module (DICTRA) of Thermo-Calc and want a quick guide to get started with your calculations. You will learn how to set up a simple calculation in Graphical Mode.

The calculation in this guide can be run by anyone with a license for Thermo-Calc, including the free Educational Package, as it uses demo databases and only three elements.

Use the side menu to the right to navigate through the different parts of the guide, or read the entire guide by just scrolling down on this page.

Note: In this guide, we assume that you know all the basics from the Getting Started Guide for Thermo-Calc. If you have not already read it, we recommend you do that before you begin this guide.

We hope you will find this guide helpful!

Before you begin, make sure that you have the Thermo-Calc software installed on your computer, either the full package or the free Educational Package. If you need help with the installation, read our installation guide or watch our installation videos.

This guide will cover the general procedure of how to set up a basic DICTRA calculation by guiding you through the set-up of a calculation example of a Fe-Cr-Ni diffusion couple with different nickel contents.

The diffusion couple consists of two foils of different materials in the same phase, Austenite (FCC), but with different compositions. The two foils are joined together and kept at an elevated temperature of 1200 K.

The simulation is set up as a single region calculation with a step in the composition profile where the different materials meet. The material on the left side is Fe-18Cr-8Ni and the material on the right side is Fe-18Cr-26Ni. The thickness of each of the two foils is 50 micrometers. The image shows an illustration of the two foils where the Ni content is indicated with a blue dotted line.

A schematic illustration of the diffusion couple.

## General Procedure

The Diffusion Module (DICTRA) is, as the name implies, used for diffusion calculations.

The general procedure of setting up your DICTRA calculation includes the following steps:

1. Add the necessary nodes and define initial system
• Choose databases and elements
• Select phases
2. Set conditions for the diffusion calculation
• Select units
• Select geometry
• Define region(s)
• Set boundary conditions
• Set thermal profile
3. Run the calculation and visualize the result

This guide will walk you through each step.

## Add the Necessary Nodes and Define the Initial System

On the home screen of Thermo-Calc, click the Diffusion template to add the necessary activity nodes.

### Select Databases and Elements

In the System Definer node, select the suitable databases and elements for your simulation. Since you are going to perform a diffusion simulation, you need both a thermodynamic and mobility database. Under the databases package dropdown menu, click the plus sign to add a second database or choose a database package.

For this example:

• Use the FEDEMO and MFEDEMO databases (Demo: Steels and Fe-alloys (FEDEMO, MFEDEMO database package))
• Select the elements Fe, Cr, and Ni from the periodic table.

### Select Phases

Diffusion simulations can sometimes be quite demanding and take a long time to run. It is therefore recommended to deselect the phases that are not necessary for the specific calculation. In the System Definer, click on the Phases and Phase Constitution tab and deselect all phases that are not necessary for your calculation.

In this example, we will calculate at a temperature of 1200 K, and the material will be fully austenitic (only FCC phase) at that temperature. Therefore:

• Deselect all phases except FCC_A1

Note: To know which phases are present in your material at a specific temperature, you need to perform an equilibrium calculation in Thermo-Calc, for example, a phase diagram. On the home page of the software, there is a Phase Diagram template that is useful for this kind of calculation. You can also watch our video demonstrating how to calculate a binary phase diagram.

## Set Conditions for the Diffusion Calculation

The system definer is now set and the next step is to set the conditions for the diffusion calculation. This is done in the Diffusion Calculator node.

### Select Units

In the Diffusion Calculator, you can set which length and composition units you want to use for your calculation. The default units are mass percent (wt%) and meter (m).

For this example:

• Change the composition unit to mass percent, and the length unit to micrometer.

### Select Geometry

The Diffusion Module handles diffusion problems where composition varies along one spatial coordinate. You can choose planar, cylindrical, or spherical geometry.

For this example:

• Since we want to calculate the diffusion in two foils, which have planar geometry, select Planar geometry.

### Planar

This geometry corresponds to an infinitely wide plate of a certain thickness. If the system has a planar geometry then the lower boundary (the zero coordinate) is at the left side of the system. The upper boundary (the coordinate with the highest value) is on the right side of the system.

### Cylindrical

This geometry corresponds to an infinitely long cylinder of a certain radius. The lower boundary (the zero coordinate) is at the center of the cylinder. The upper boundary (the coordinate with the highest value) is at the surface of the cylinder. You can enter a leftmost coordinate that is larger than zero to allow for tube geometry.

### Spherical

This geometry corresponds to a sphere with a certain radius. Similar to cylindrical geometry, the lower boundary (the zero coordinate) is at the center of the sphere. The upper boundary (the coordinate with the highest value) is at the surface of the sphere. You can enter a leftmost coordinate that is larger than zero to allow for hollow sphere geometry.

### Define a Region

For diffusion simulations, at least one region should be defined containing at least one phase. For some calculations, you might want to create several regions, for example moving phase boundary simulations. Naming the regions is optional, but can be useful when having more than one region. To add a new region, click the plus sign next to Region. In this example, we will not add a second region.

For each region, select which phase is present in the region and enter the composition profile. You should also select a grid type for each region. There are three grid types available: linear, geometric, and double-geometric.

For this example:

• Name the region Austenite and set the width to 100 µm (1e-4 m) with 100 points.
• Change the type of grid to Double geometric and set the lower ratio to 0.95 and the upper ratio to 1.05. This is to have a high number of grid points in the middle of the geometry. A preview of the grid is shown in red right above the region, as seen in the image below.

##### Define the Composition Profile of the Region

In the Phase drop-down list, select FCC_A1. The composition profile settings now appear below the Region box, and a visualization of the composition profile is shown in the Visualizations window. Select the dependent component, usually the majority element,  and edit the composition profile for the other components in your system. You can choose either Linear or Step composition profiles, or describe the composition profile with a Function.

In this example, we will simulate how the Ni content varies between the two metal sheets with different Ni concentrations.

• Select Fe as the dependent component, then set the Cr content to be constant at 18 wt%.
• For Component Ni, select Step and set the composition from 8 to 26 wt% at 50 µm (5e-5 m).

This will create a composition profile that simulates two materials with different nickel compositions, i.e. one with 8 wt% nickel and one with 26 wt% nickel. The composition profile is shown in the Visualizations window.

Note that the step in the center is not a phase interface, our setup represents a single phase FCC structure with a concentration jump.

### Set Boundary Conditions

Boundary conditions are conditions that define how matter behaves at the boundaries/surfaces of your system. The simplest case, which is also the default in the Diffusion Calculator,  is the closed system in which all diffusional fluxes on the boundary of the system vanish; we have zero flux. In this case, no material can enter into or leave the system.

In Graphical Mode, you can change the setting for both the left boundary and the right boundary of the system. You can also choose Mixed zero flux and activity or Composition if the system is not closed.

In a material with several components, it is common to have different types of boundary conditions for different elements. For example, the fluxes of substitutional elements such as Fe, Mn, and so on may be zero, while interstitial elements like C and N may have a finite flux. For this boundary condition, select the type Mixed zero flux and activity.

For this calculation:

• Use the default setting, which is Closed system

For more details about boundary conditions, read the Diffusion Tutorial or search for boundary conditions in our Online Help, which is accessible from within the software.

### Set Thermal Profile

Define the Thermal Profile for the calculation at the bottom of the configuration window. Depending on the simulation you want to perform, select isothermal or non-isothermal thermal profile. For isothermal calculations, you set which temperature you will hold the material and for how long (maximum time). For non-isothermal calculations, you set a thermal profile of how the temperature will vary during the heat treatment.

For this example:

• Select isothermal and set the temperature to 1200 K.
• Set the simulation time to 1e10 seconds.

## Run the Calculation and Visualize the Result

The last step before running the calculation is to choose how you want to visualize the result, either with a plot or a table. When using the Diffusion template, the default is that a Plot Renderer is added to the project tree. If you want your results in a table, you can simply add a Table Renderer by right-clicking on the Diffusion Calculator, then click Create New Successor > Table Renderer

The major difference between the Plot and Table Renderers in DICTRA compared to Thermo-Calc is that you need to set the time(s) you want to show, unless you use time as the x-axis unit. In the box right below the axis setting, write the time you want to visualize in the text box. This applies for both the Plot Renderer and the Table Renderer. It is important that this time is shorter or equal to the simulation time you set in the Diffusion Calculator, otherwise, it can not be shown since DICTRA does not have any calculated values at those times. If you want to show the results at different times, you can separate the times with a space. You can choose to set the time in seconds, minutes, hours, or days.

For this example:

• Set the X-axis unit to Micrometer.
• Change the unit from second to minutes.
• Add the following times: 1.0, 60.0 (1 hr), 1440.0 (1 day), 10080.0 (1 week), 43800 (1 month), 525600 (1 year), 1051200 (2 years), 2629800 (5 years).

Click at the bottom of the Plot Renderer window and see that the event log starts populating. When the calculation is finished, the result will show in the Visualizations window

### Interpretation of the Result

In the plot, it is shown that when the diffusion couple is held at an elevated temperature (1200K)  for a more extended period of time, the nickel will diffuse from the nickel-rich to the nickel-poor material and we get a composition gradient on both sides. It is also shown that it takes around 5 years for the diffusion couple to get a homogeneous nickel content, that is the nickel content in both materials is the same.

The calculation is now finished! We hope you found this guide helpful.

Read about Useful Tips & Tricks in the Thermo-Calc Getting Started Guide, such as how to customize your plot, rename the nodes, and more.

## Where to Find Help and Resources

In the online help, you can browse the menus and search for keywords and phrases to find the information you are looking for. It contains the same documentation that is also installed in folders with your software. The online help system opens in your internet browser, but the files are stored locally on your computer, so you do not need an internet connection to use it.

### Quick Guide

A quick guide is available both in the Online Help and as a PDF in the Manuals Folder included in your installation package. It introduces you to the basic concepts, steps, and theory to start using the Diffusion Module (DICTRA). An understanding of this information is necessary to ensure accurate calculations.

• The PDF version is found by navigating to the help menu in the software and clicking Manuals Folder in the drop-down list. Open the Diffusion Module (DICTRA) folder and then the Diffusion_Module_DICTRA_Quick_Guide-Graphical.pdf

• The Online Help version is found in the Online Help, hover over Diffusion Module (DICTRA) in the top menu, hover over Graphical Mode, and click Quick Start Guide.

### Diffusion Tutorial

This comprehensive tutorial teaches you about the role of diffusion in materials and how the Diffusion Module (DICTRA) can be applied to materials design and processing.

The tutorial is intended for engineers interested in using the Diffusion Module (DICTRA), as well as students learning about the role of diffusion in materials. It is designed to be useful at many levels, from undergraduate studies to someone with a Ph.D. and experience in a related field.

Access the Diffusion Tutorial

### Example Calculations

All Thermo-Calc installations include example files for both Graphical Mode and Console Mode. The Graphical Mode examples are available for Thermo‑Calc and the Add-on Diffusion Module (DICTRA), Precipitation Module (TC-PRISMA), Additive Manufacturing Module, and Process Metallurgy Module. There are also examples for Property Models, including the Steel Model Library and more. The Console Mode examples are available for Thermo‑Calc and the Diffusion Module (DICTRA).

The example files are found by navigating to the Help menu > Example Files… > Diffusion Module – DICTRA

If you are in Console Mode, the console mode examples will open, and if you are in Graphical Mode, the graphical mode examples will open.

### Video Tutorials

We provide videos explaining how to make different thermodynamic calculations, diffusion simulations, and precipitation simulations using Thermo-Calc software. Our videos will also teach you how to install Thermo-Calc and will keep you up-to-date on the latest releases and additions to the software and the newest thermodynamic, properties, and kinetic databases.

Visit the Video Tutorial Library

### Learning Hub

The Learning Hub includes courses for Thermo-Calc, the Diffusion Module (DICTRA), the Precipitation Modules (TC-PRISMA), and the TC-Python SDK. In addition to introductory lectures, instructors walk through and discuss real-world examples that correlate calculations with microstructures and other metallurgical concepts, spanning a wide range of material types. Our team is working on adding content for additional modules, so make sure to check back regularly.

Visit the Thermo-Calc Learning Hub

### Training Courses

Thermo-Calc Software offers Live Online Training several times throughout the year for Thermo-Calc, the Diffusion Module (DICTRA), and the Precipitation Module (TC-PRISMA). We can also provide Customized Training upon request. We also support Third Party Training and Workshops, which are listed on our website.