Thixoforming : Semi-solid Metal Processing

148j 5 Thermochemical Simulation of Phase Formation
Table 5.1 Standard chemical composition (mass%) of the steels,
used for the calculations in the present chapter.
Alloy C Mn Si Cr Mo V W
100Cr6 1.00 0.30 0.30 1.50 — — —
X210CrW12 2.15 0.30 0.30 12.0 — — 0.7
HS6-5-2 0.84 0.30 0.30 4.15 4.95 1.9 6.30
calculated. This can be used, as just one example, for calculating the driving force for
the precipitation of the stable phase from a metastable state. The coupling
of thermodynamics and phase diagrams has been realized within the Calphad
method [6–8], which was essentially developed in the 1970s and is now used to deal
with a wide variety of alloy systems, oxide systems, salt systems and so on. In the
Calphad method, the Gibbs energy of each phase is parameterized in terms of
temperature and composition and, in principle pressure, but this is rarely used, based
on crystallographic and compositional data. The Gibbs energy functions contain
variable coefficients, which are optimized to experimental phase diagrams and
thermodynamic data. Crystallographic and thermodynamic data from ab initio
calculations are increasingly being used to support and extend the experimental
data. In this chapter, we use the Thermo-Calc software [9] for thermodynamic
calculations with the TC-Fe 2000 [10] and TCFE4 [11] steel databases. The more
recent TCFE4 database essentially gives the same results as TC-Fe 2000 for the steels
treated here, but additionally contains molar volume data. In the general case, the
coverage and quality of the database(s) used are a concern, but for the steels dealt with
Figure 5.1 The relation between Gibbs energies of the individual
phases and the resulting phase diagram. The dashed line shows
the temperature at which the Gibbs energy curves are drawn.