The actual monotectic microstructure within the apparently
unstructured dark grey area cannot be resolved at
the low magnification of Fig. 8 but is shown in the higher
magnification of Fig. 9. Three phases, (Al), HAl2Cu and
small white spots of (Sn) are clearly distinguished. This
monotectic microstructure can be well explained by the calculated
phase fraction chart of sample #1 in Fig. 10. Just
before the monotectic reaction the four phases
L0 + L00 + (Al) + HAl2Cu are present with amounts of
57% + 38% + 5% + 0%, respectively, and after completion
with amounts of 0% + 45% + 31% + 24%. The region originally
occupied by L0 (57%) thus transforms entirely into a
microstructure containing L00 + (Al) + HAl2Cu in amounts
of (7% + 26% + 24%)/57% or 12% + 46% + 42%, respectively.
It is important to note that this new L00 (7%) essentially
remains spatially separate from the bulk of the
initially present L00 (38%), even though it is the very same
phase. From this L00 some more (Al) and HAl2Cu precipitate
upon further cooling in the temperature range 524–
227 C, as shown in Fig. 10, before it completely solidifies
essentially to (Sn) in the eutectic. This corresponds to curve
‘‘4” and point ‘‘5” in Fig. 7a. This is the reason why we see
(Al) and HAl2Cu with some (Sn) in a very fine structure in
Fig. 9, corresponding to the monotectic decomposition
type reaction and with approximately the calculated
amounts. It is also the reason why we see the coarser microstructure
in the bulk of initially present L00, as labeled in
Fig. 8, consisting of larger (Al) crystals (darkest phase)
and HAl2Cu (medium grey) that grew freely from/into
the large liquid space L00 below 524 C.
The reaction type in the primary liquid demixing field
L0 + L00 is tricky. The solidification path, curve ‘‘1” in
Fig. 7a, suggests that starting with L0 and producing L00
is reflected by the reaction L0 ? L00. But this is only true
during the initial stage from 667 C to 628 C as shown
in Fig. 10. Further cooling from 628 C to 530 C increases
the amount of L0 at the expense of L00 and thus the reaction
type changes from L0 ? L00 to L00 ? L0. The curvature of
path ‘‘1” in Fig. 7a indicates that the direction of the