Thixoforming : Semi-solid Metal Processing

150j 5 Thermochemical Simulation of Phase Formation
cooling rate is irrelevant. Melting cannot be studied as a separate process in this way,
but has to be assumed to be the reverse of solidification.
5.1.4
Diffusion Simulations with DICTRA
In order to verify the equilibrium and Scheil calculations, to obtain a more detailed
picture and to investigate, for example, the influence of cooling rate, it is necessary to
use some form of diffusion simulation. There are very few reasonably general
software packages available for diffusion simulation. Diffusion simulation is numerically
a very demanding task, in particular when a moving phase interface is
present, as in solidification. Here we use the software package DICTRA [9], which
uses Thermo-Calc as a kind of subroutine to provide thermodynamic data. DICTRA
treats multi-component diffusion in one dimension with the possibility to have one
or more sharp moving interfaces. At the interface(s) local equilibrium is assumed. In
addition to the thermodynamic database TC-Fe 2000 [8], the mobility database MOB
2 [14] is used here. This database is suitable for steels and the diffusion data for Ferich
ferrite and austenite can be considered reliable, but the diffusion data for the
liquid are only approximate. A diffusion problem in DICTRA is set up by defining a
cell with the phase(s), compositions and temperature at the start of the simulation.
The cell size corresponds to the characteristic diffusion length, which in the case of
semi-solid processing is half the average distance between the centre of two
neighbouring globules. In a normal solidification problem it would typically be half
the secondary dendrite arm spacing. If the simulation is started in the liquid, the
position where the solid phase will appear has to be specified. It is possible to require
that the solid phase forms only when there is a defined driving force (undercooling)
available, thus mimicking an energy barrier for nucleation. This possibility was not
used in the present simulations. DICTRA does not take interface energy into account.
In order to do this and in order to simulate or predict morphology in two or three
dimensions, it is necessary to use other methods, in particular phase field
methods [15, 16].
5.2
Calculations for the Tool Steel X210CrW12
5.2.1
Phase Diagram
A calculated temperature versus mass% C phase diagram for the tool steel
X210CrW12 is shown in Figure 5.2. It is clear that austenite forms as the primary
phase during solidification and there is a narrow field where austenite þ M7C3
(where M is mostly Cr) form eutectically. Below about 800 C, ferrite and M23C6
(where M is again mostly Cr) become stable, but usually austenite is retained
metastably below that temperature and transforms martensitically at a much lower