Thixoforming : Semi-solid Metal Processing

Figure 6.21 (a) Schematic diagram of yield stress depending on
the internal structure of the material and (b) comparison
between measured yield stress and measured storage modulus
(internal strength) for Sn–15%Pb alloy.
In addition to Cheng s conclusion [15] one has to differentiate between isostructural,
dynamic and static yield stress. The isostructural yield stress occurs immediately
after shearing, since the slurry s structure remains unchanged and it is equivalent to
the structure during shearing. Creep tests with loads up to 10 Pa were performed in
order to determine the yield stress immediately after shearing. As illustrated in
Figure 6.22a, the deformation increases continuously up to a maximum value. This
indicates that the stress at the beginning of the creep test is higher than the stability of
the internal structure. As hardening proceeds, the deformation rate decreases until the
internal strength dominates (equals constant deformation). The time span t required
to reach the plateau value increases with increase in shear stress. Plotting the shear
stress against the corresponding time span (Figure 6.22) shows the time-dependent
Figure 6.22 (a) Creep tests at different shear stresses and
(b) yield stress versus t extracted from the creep tests. Material:
Sn–15%Pb.
6.1 Empirical Analysis of the Flow Behaviourj187