Volumen II - SAM

In order to determine the configuration of defects which are interacting with dislocations as a function of
temperature, we performed ER, DTA and SANS studies. The ER of two irradiated samples of different
orientation as a function of the annealing temperature was measured at RT, samples (f) and (g) in Table 1,
Figure 4 (left axis). The DTA behaviour as a function of temperature for a sample (f), is also plotted in
Figure 4 in the right axis.
It should be mentioned that the behaviour of the damping response in the MS tests is in agreement with that
exhibited by the ER, DTA and SANS studies. In fact, the increase in the ER values from 300K up to 600K in
both and samples, coincides with the stage III in irradiated samples; which was related to
the movement of vacancies [2 - 5]. It has been reported that in neutron irradiated molybdenum at RT, a
subsequent annealing at temperatures between 473K and 573K produces an increase in the yield stress and in
the UTS; which result in agreement with our ER results. Consequently, within the temperature range of the
stage III, the appearance of internal stresses can be assumed; which lead to the increase in the ER reported in
Figure 4. In addition, the temperature intervals where DTA reactions appear are in reasonable agreement
with the salient features of the ER curve. The first endothermic peak at around 650K can be related to the
maximum in ER which corresponds to the stage III of recovery. In fact, the development of internal stresses
in the Stage III can be thermodynamically related to the increase of internal energy from a metastable state
after irradiation to another metastable state at a higher energy valley, when vacancies start their movement.
The exothermic reaction can be related to the decrease in the ER values from 600 K and with the
development of the stages IV and V of recovery. In these stages the movement of vacancies towards the
dislocations takes place in order to decrease the free energy of the structure. Therefore, the appearance of
internal stresses in stage III, due to the reorganization of defects out of thermodynamic equilibrium, leads to
the locking of dislocations during the first run-up in temperature in the MS test. It gives rise to the damping
background, which is amplitude dependent [2, 6], without the appearance of the damping peak (see curves A
and D, during the first heating run in Figure 1). These vacancies during the first run-up in temperature in the
MS test migrate to the dislocations and produce the appearance of the damping peak in the cool down run,
after annealing at temperatures higher than 973K, which corresponds to temperatures where the ER curves
start to decrease and to the exothermic reaction in DTA.
g1
g2
(a) (b)
g2 g1
Figure 3. (a) TEM micrograph for a sample of (d)
type. (b) TEM micrograph for a sample of (a) type.
Figure 4. Behaviour of the ER and the DTA
thermogram. Circles: sample (f). Triangles: Sample
(g). Empty symbols: prior to irradiation
It should be highlighted that in stage III in irradiated samples the movement of vacancies out of
thermodynamic equilibrium was verified by means of SANS studies, where the evolution of the arrangement
of vacancies as a function of temperature was determined. Figure 5 (lower and left axis) shows the behaviour
of the radius, R, of the agglomerates of vacancies, assumed to be spheres, as a function of temperature
calculated from Guinier plots for the scattering curves measured during the SANS tests [5]. At temperatures
of about 550 K the agglomerates achieve the largest size, which is in agreement with the ER, DTA and MS
results. In fact, if we increase the temperature of the sample for values above those for the maximum in the
ER curve (approx. 600 K), the size of agglomerates of vacancies decreases. Moreover, using SANS studies,
we also have verified that both: (a) for temperatures higher than about 850K, the dissolution of agglomerates
take place and (b) for temperatures higher than 920K the concentration of vacancies out of thermodynamic
equilibrium decreases markedly, as it is revealed by the decrease in the scattering intensity curves (I vs. q)
for temperatures higher than about 900K, Figure 5 (upper and right axis) [5]. These facts lead to the decrease
in internal stresses of the microstructure and allows, after a heating to 973K, to the appearance of the
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