RRFM 2009 Transactions - European Nuclear Society

4. Discussion
The main contribution of this study, in our point of view, is the underlining of two main types
of ILs in the U-Mo/Al-Si interaction system, depending on the Si content in the Al and
annealing conditions. Assuming that the reaction product growth occurs at the U-Mo/reaction
layer interface, as proposed by Mazaudier et al [11], the oldest part (that formed fistly) of the
ILs is that located close to the Al side of the couple. In both types of ILs, it is constituted by
U(Al,Si) 3 + UMo 2 Al 20 (with a more or less important Si enrichment of the U(Al,Si) 3 phase. This
tends to show that, at the beginning of the interaction, in all the studied cases, enough Si is
present close to the reactional interface for leading to the U(Al,Si) 3 + UMo 2 Al 20 phases
formation, instead of UAl 4 + UMo 2 Al 20 when no Si is present [11-12]. This result confirms that
Si acts as an inhibitor of the UAl 4 formation, as shown first by Thurber et al [16] and
confirmed later by many other authors. At this stage of the interaction process, Al and Si
have to diffuse through this U(Al,Si) 3 + UMo 2 Al 20 layer in order to react with U-Mo. According
to Savchenko et al [17], Si diffuses through U(Al,Si) 3 phase much faster than Al. Then, two
cases have to be considered in our point of view :
- case 1 : Si depletion in Al, in front of the IL, becomes too important for maintaining a
sufficient diffusion flux of this specie, through the IL, and the interaction system becomes
dominated by Al diffusion (i.e. identical to that observed in U-Mo/Al diffusion couples),
- case 2 : despite of a progressive Si depletion in Al, a sufficient Si diffusion flux through
the IL can be kept (at least during a certain time), leading to the formation of a silicide phase
(U 3 Si 5 ) close to the U-Mo side.
On this basis, the case 1 should logically lead to a type 1 IL and the case 2, to a type 2 IL.
Complementary experiments including the characterization of couples obtained after different
annealing durations could give interesting clues about these hypotheses.
Remembering that, in U-Mo7/Al-Si5 couples ("threshold couples" in this study, according to
SEM+EDS results), variations of the IL thickness seem to be linked to the Al grain size, it
seems reasonable to assume that the local distribution of Si plays probably also a major role
in these phenomena. Studying couples obtained with Al-Si alloys which are characterized by
significantly different Si precipitates distributions (if its possible to maintain significant
differences at the annealing temperature) could also give additional information about the
mechanisms to be considered.
Other very important parameters to be taken into account are obviously temperature and
time because they influence directly the activation of diffusion processes and especially the
diffusion kinetics of Al and Si in Al and in the IL (more especially in the UX 3 phase). To our
knowledge, data concerning these kinetics are lacking. It would be very interesting to obtain
such data and to use them for modelling the thermally activated U-Mo/Al-Si interaction
process.
To summarize, the occurrence of two types of ILs in U-Mo/Al-Si diffusion couples seem to be
directly linked to (i) the Si precipitates distribution in the Al alloy, (ii) the diffusion conditions
(temperature and time). That's the reason why the formation of one type of IL or the other
cannot be simply associated to a Si threshold value. Discrepancies between the results
obtained in this study and those of other authors (see for example [2-3, 18-19]) could be
explained on this basis.
5. Conclusions
A set of U-Mo7/Al-Si diffusion couples, with silicon content going from 2 wt% to 10 wt%
were obtained by thermal annealing. These couples were systematically characterized by
SEM+EDS and by µ-XRD. Thanks to these characterizations, we have shown that two main
types of interaction layers (ILs) are evidenced, depending on the silicon content in the
aluminium alloy, with a threshold value of about 5 wt% (in the conditions tested in this
study).
Below the threshold value, the ILs are three-layered, with : a first sub-layer, close to U-Mo,
containing the UAl 3 + U 6 Mo 4 Al 43 phases, an intermediate one with the U(Al,Si) 3 phase and a
third one, close to Al-Si, with the U(Al,Si) 3 + UMo 2 Al 20 phases. Above this threshold, the ILs
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