Volumen II - SAM

uns in temperature is completely similar to the exhibited by the sample irradiated 10 hours, although in this
case the peak presents a final lower intensity due mainly to the decrease in the damping background.
The damping peak in irradiated samples practically disappears when the sample is vibrated for a few hours at
1250K. A new 10 hours re-irradiation at RT and subsequent annealing, during the thermal cycles, restore the
peak. This behaviour can be observed in Figure 2. This Figure shows the internal friction spectrum for the
sample (b) after a few hours vibrating at 1250K (spectrum marked A in the Figure). In the warming run just
after re-irradiation, the γ relaxation at 440K and a damping increase starting between 500 and 550 K are
observed, but the peak is absent. In the cooling run after the sample has been heated to 973K the peak is
again restored (spectrum B in Fig. 2). Further annealing to higher temperatures displaces the peak to around
1000K, spectrum marked C on cooling in the same Figure.
Figure 1. Damping spectra measured for
irradiated and unirradiated samples of Table 1.
Arrows indicate the warming and cooling runs.
Figure 2. (A) damping spectrum after annealing
several hours at 1250K. (B) damping peak during
the cooling after the re-irradiation. (b) type sample.
In the other hand, samples (d) and (e) oriented for single slip were cut from the same ingot and prepared in
the same manner, then deformed and one of them, sample (e), irradiated for 20 hours. Deformed samples
present a damping peak whose intensity results smaller in than in samples for the same degree
of plastic deformation [6, 7]. In addition, the irradiated sample presents similar behaviour to the unirradiated
one [2]. However, similar behaviour to the one showed in Figure 2, in a re-irradiated sample, (e) in
Table 1, was found.
The dislocation structure of deformed molybdenum has been studied by TEM in previous works [2, 6].
Samples deformed at RT in the deformation range used in the present work, show screw dislocations with a
high density of jogs but few edge dislocations. However, dislocation fragments with Burgers vectors a
and prismatic loops were present. Nevertheless, with the aim of checking the dislocation arrangement that
results after annealing during the thermal cycles in the MS measurements, we have performed TEM studies
on the same samples explored by means of MS. Figure 3.a shows the micrograph for a sample of type (d)
which presented at 980K a damping peak whose maximum had a value of about 10x10 -3 . Figure 3b shows
the micrograph for a sample of type (a). Previously to TEM examinations, the sample showed at 900K a
damping value of about 17x10 -3 . An analysis of constructive interference, allowed us to relate the (110) plane
as the sliding one and the Burgers vectors with the direction.
The effect of irradiation on the samples is to increase the number of intrinsic point defects, vacancy mainly,
because interstitials are mobile at lower temperatures than RT [2]. After irradiation, dislocations are pinned
producing a decrease in the internal friction background, see Fig. 1. In addition, samples are more
sensitive to irradiation than . In fact, the effect of irradiation in samples is to reduce the
damping peak and to move it to lower temperatures. In contrast, in samples oriented for single slip the
irradiation after deformation does not produce clear changes. The difference in the MS behaviours for
irradiated and samples can be related to different dislocation arrangement in each type of
deformed single crystal. Indeed, samples oriented for multiple slip have more active slip systems and present
higher density of dislocation, determined by counting the dislocation lines, more jogged and shorter
dislocations than the samples, as it was observed by TEM, Figures 3.a and 3.b.
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