Thixoforming : Semi-solid Metal Processing

60j 3 Material Aspects of Steel Thixoforming
Figure 3.9 Schematic explanation of the matrix character depending on the volume fraction [44].
values between 0 and 1 are possible. It is only dependent on the position of the phase
boundaries, not on the grain boundaries. The matrix character is a measure of the
degree of connectivity of the investigated structural constituent. By means of this, it
is possible to evaluate the position, specifically the transition between entrapment
character and matrix character of a phase. If the matrix character is depicted
depending on the volume fraction of the observed phase, the structural arrangements
shown schematically in Figure 3.9 can be explained as follows. In image 3, the matrix
character is observed in the light phase. Despite a volume fraction of 50% of light
phase, the matrix character is low, as the globulites are located isolated in the dark
phase. In image 2, the matrix character of the dark phase in about 50% because this
phase pervades the structural arrangement like at network. The matrix character in
image 1 amounts to nearly 90%, as the matrix of the structural arrangement is formed
by the light phase.
When metals recrystallize, a large amount of high-angle grain boundaries are
formed, which melt preferably during heating. The higher the deformation degree
and, resulting from that, the smaller the grain size, the smaller the grain size of
the globulites becomes. To intensify this effect, elements can be alloyed which
decrease the solid–liquid interfacial energy (g sl). The melting of a grain boundary
takes place as soon as the condition 2g sl