Thixoforming : Semi-solid Metal Processing

76j 3 Material Aspects of Steel Thixoforming
the described conditions. As no nucleation is required, the present globulites will
immediately start to grow after quenching, hence a differentiation of primary and
secondary austenite seems to be impossible. The thermodynamic equilibrium calculations
provide good reference values concerning the composition of the particular
stable phases as the experimental and calculated values agree fairly well. The results
of the thermodynamic diffusion calculations could also be confirmed, because no
differentiation between primary and secondary austenite is possible by means of a
differentiation of the local element concentrations at technically realizable cooling rate.
3.4.1.4 EBSD-Measurements of Quenched X210CrW12 Specimens
In addition to the microprobe experiments, EBSD measurements were also executed
to obtain conclusions about the structure in the partial liquid state via the crystal
orientation. It became apparent that also with this method no differentiation of
primary and secondary austenite is possible (Figure 3.25). The image in (a) shows
austenitic globulites in a matrix, which consists of fine, two-phase structure of fcc
(light) and bcc or carbide components of the eutectic (dark). A differentiation of the
carbides from the body-centred components is not possible by means of the applied
measurement so that both phases are equally coloured. The image in (c)shows the
orientation distribution and the image in (b) depicts the corresponding inverse pole
figure. The grain marked with an arrow exhibits a twin boundary in the middle. It
shows in its upper part a very uneven grain boundary, presumably due to the
secondary austenite accumulated with the same orientation as the primary grain.
Figure 3.26 shows the backscattered electron (BSE) image of a sample quenched from
1330 C (a). The orientation distribution in the image in (b) shows that even the
secondary austenite of the eutectic possesses the same orientation as the neighbouring
large, primary austenite particles. Even at the high realized quenching rates of
more than 100 K s 1 , the growth rate of the phase boundary surfaces seems to be so
high that the same orientation is propagated within a certain sphere of influence
Figure 3.25 Austenite (light) and ferrite or carbide distribution
(dark) (a) and inverse pole figure with associated orientation
distribution [(b) and (c)] of a specimen quenched from about
1275 C.