Thixoforming : Semi-solid Metal Processing

152j 5 Thermochemical Simulation of Phase Formation
Figure 5.3 Molar liquid fraction as a function of temperature for
X210CrW12 using equilibrium (solid), standard Scheil (dashdotted),
modified Scheil (dotted) and DICTRA (dashed)
calculation.
the DICTRA simulation. The correctness of these curves is probably determined
more by the accuracy of the thermodynamic database used, which for many steels,
including tool steels, can be assumed to be fairly high, than by anything else. It is also
evident that a standard Scheil calculation is not a good approximation. It is far away
from the other curves even at relatively high liquid fractions. At low liquid fractions,
there is a pronounced tail in the DICTRA simulation (and in the modified Scheil
calculation). This is mostly caused by W segregation.
5.2.3
Determination of Liquid Fraction
To determine the liquid fraction in a semi-solid sample experimentally is far from
straightforward. Two methods are frequently used: metallography and DTA. By
quenching the sample quickly, one can hope to be able to retain an image of the
original state and, thus, to distinguish the originally liquid and solid phases in a
metallographic section. This has worked well for a number of Al alloys, but for steels
it is only possible in particular cases. This is discussed in some detail in Chapter 3 and
by P€uttgen et al. [5]. The major reason can be seen in Figure 5.4, which shows a
DICTRA simulation of the quenching of a X210CrW12 sample from the semi-solid
state (50% liquid in this case). At the former solid–liquid interface there is only a
gradual change in composition. There is no step that may have been expected. For