Scheil Solidification Simulations

Scheil Solidification Simulations are non-equilibrium transformations or partial-equilibrium transformations that result in an estimation of the solidification range of an alloy, performed in Thermo-Calc using the Scheil Calculator.


Scheil for the Solidification Process

Thermo-Calc is often used to perform equilibrium calculations, which assume complete diffusion in the liquid and solid states. These types of calculations are good for a wide range of applications. However, during the solidification process, diffusion in the solid state is restricted. For this, Scheil Solidification Simulations offer a better alternative by assuming infinitely fast diffusion in the liquid phase and zero or limited diffusion in the solid phases. 

For typical cooling rates encountered in foundries, the assumptions in Scheil Solidification Simulations have been shown to be quite good, and often much better agreement between experiment and calculation is achieved with a Scheil-Gulliver solidification simulation compared to an equilibrium solidification calculation. 


The Scheil Calculator Allows You to Calculate:

  • The solidification range of an alloy
  • Depression of the solidus temperature due to segregation
  • Composition of the last liquid to solidify in interdendritic region
  • Phases formed on final solidification in segregation pockets
  • The composition gradient in the primary solid phase(s) (segregation profile)


Three Models for Scheil Solidification Simulations

Thermo-Calc includes three models for Scheil Solidification Simulations.

  • The Classic Scheil simulation is based on the well known Scheil-Gulliver model and assumes zero diffusion in the solidified material.
  • A second model is available where one or more elements can be defined as “fast diffusers”.
  • Our third and most advanced model uses diffusion data to calculate Back Diffusion in the Primary Phase. 

Scheil Solidification Simulation For An AA7075 Alloy Comparing Back Diffusion With Classic

A Scheil Solidification Simulation of the Aluminium Alloy AA7075 comparing a Classic Scheil Simulation with two Scheil with Back Diffusion simulations and to experimental data. 

Classic Scheil Model

The most well known example of a non-equilibrium calculation is the Scheil-Gulliver solidification simulation, or Scheil simulation for short. This is the model used for our Classic Scheil simulations. 

Classic Scheil Simulations Assume: 

  • Diffusion of all elements in the liquid phase is infinitely fast
  • Diffusion of all elements in the solid phases is zero
  • The liquid/solid interface is in thermodynamic equilibrium

Classic Scheil Solidification Simulation For Aluminum 4043
Classic Scheil Solidification Simulation of Aluminium Alloy 4043 compared to equilibrium. 

This calculation procedure is equivalent to assuming that there is no diffusion in the solid phase and infinitely fast diffusion of all elements in the liquid phase. It has been shown to be a good approximation of the solidification of most alloys such as Ni-superalloys, Cu alloys, Al alloys, Mg alloys and others. However, it does not work well for steels. For this reason a variant of the Scheil simulation was developed and implemented in Thermo-Calc where one or more elements can be defined as “fast diffusers”. 


Scheil Simulation with Fast Diffusers

Scheil with Fast Diffusers is a variant of the Classic Scheil simulation that was developed mainly for steel applications. 

Scheil Simulation with Fast Diffusers Assume: 

  • Diffusion of all elements in the liquid phase is infinitely fast
  • Diffusion of all elements in the solid phases except the ones defined as “fast diffusers” is zero
  • Diffusion of the elements defined as “fast diffusers” is infinitely fast in the solid phase
  • The liquid/solid interface is in thermodynamic equilibrium

Carbon is one of the most important alloying elements in steels. It dissolves interstitially and, as a result, has very fast diffusion rates. The assumption of no diffusion in the solid phase during solidification is thus not correct for carbon at most industrial or lab-scale solidification rates. Scheil with Fast Diffusers allows one or more elements to be defined as “fast diffusers”, correcting for the problems addressed above. Typically, C is defined as a fast diffuser, but also other elements such as N, O or others can be defined.


Scheil with Back Diffusion - Our Most Advanced Scheil Model 

Our most advanced Scheil model uses diffusion data from a mobility database to calculate back diffusion in the primary phase. 

Scheil with Back Diffusion Assumes: 

  • Diffusion of all elements in the liquid phase is infinitely fast
  • Diffusion of all elements in the primary solid phase is quantitatively calculated using mobility data, a cooling rate and a domain size (typically this will be the secondary arm spacing)
  • The liquid/solid interface is in thermodynamic equilibrium

Ex T 10 Scheil With Back Diffusion In The Primary Phase
A Scheil Solidification Simulation with Back Diffusion of Al-Cu-Si alloy comparing cooling rate of 10 K/s (left line) to cooling rate of 0.005 K/S (right line). As the plot shows, the lower cooling rate is closer to equilibrium as the solidification progresses. 

As with the Scheil-Gulliver model, this model assumes infinitely fast diffusion in the liquid material, but it allows for limited diffusion in the primary phase of the solid material. Using diffusion data from a mobility database, it quantitatively takes into account the real back diffusion of all elements in the primary solid phase (typically the FCC or BCC phase). This model also allows setting selected components to be "fast diffusers" and these are treated as if there is infinite diffusivity in the solid phases.

Scheil with back diffusion requires the use of a mobility database as it is using diffusion data to complete the calculation. 


Scheil Solidification Simulation Examples

All Thermo-Calc installations, including the free DEMO, come with several examples of Scheil Solidification Simulations which are accessed easily from within the software in the Help menu > Examples files > Thermo-Calc

There are also several videos available to help you learn how to set up Scheil Solidification Simulations. 


 

 


How to Buy the Scheil Calculator

All Thermo-Calc installations include the Scheil Calculator in both Console and Graphical modes. Scheil with Back Diffusion was added in Thermo-Calc 2020a and requires a mobility database to use.

If you do not already have a Thermo-Calc license or you are interested in expanding your license, fill out the form below and we will contact you to discuss which license is right for you.