3.1. Irradiation effects

Figure 1(a) shows the SEM image of the Fe-0.6%Cu alloy irradiated to 0.1 dpa by 2.5 MeV Fe ions at 573 K. The crystalline structure formed many tiny clusters on the surface, aggregating at the grain boundary especially. It was reported in Refs.[11, 15] that Cu atoms migrated with diffusion of vacancies and aggregated at the grain boundary ^{[11, 15]}. Fig. 1(b) shows that the S parameter decreased with increasing positron energy from 0.18 keV to 20 keV, corresponding to the depth of ~0.65 μm. This depth is almost close to the irradiated vacancy peak area induced by 2.5 MeV Fe ions, according to the simulation results using the TRIM code. This indicates that, comparing to the non-irradiated sample, the 0.1 dpa Fe ion irradiation increases S parameters, except the near surface in the positron energy range of 1–5 keV. At the near surface of thinner than 0.1 μm, the decrease of S parameters for the irradiated sample can be attributed to the Cu precipitates crystalline on the grain boundary, which could change the diffusion length of positron.

Fig. 1.Full-screen View

SEM image of the Fe-0.6%Cu irradiated by 2.5 MeV Fe ions (a) and S parameters of slow positron beam and vacancy distribution simulated by TRIM (b).

3.2. Thermal aging effects

The irradiated samples were thermal aged in high vacuum at 573 K for different hours. Doppler broadening analysis of slow positron beam was carried out with the samples. Fig. 2 shows the S-E and W-E curves, and S-W slots. In Fig. 2(a), over positron energy of 10.0 keV, the decreasing S parameters of the sample thermal aged for 50 hours approached the results of the non-irradiated sample. This indicates that the vacancy-defects, as main defects formed by Fe ions irradiation, had migrated and almost recovered after the long time thermal aging at 573 K^[16]. Also, the increasing W parameters in Fig. 2(b) indicate the migration and aggregating of Cu precipitates along with the thermal dynamic of vacancies defects, because of the irradiation of Fe ions. This is also confirmed by the S-W plots in Fig. 2(c), in which the slope increases with the thermal aging time as marked by the arrows. It can be assumed that the annihilation mechanism of positrons is changed.

Fig. 2Full-screen View

S and W parameters (a and b) as function of positron energy and the S-W diagram(c), for Fe0.6%Cu samples treated differently.

The annihilation of positrons below 5 keV is attributed to the defects near the surface. The thermal dynamics of defects and Cu precipitates is complicated near the surface, as shown by the Doppler broadening results of slow positron beam. In 50 and 100 h thermal aging, the S parameters show a lower trough near 1.5 keV. This phenomenon may be attributed to two factors. One is the backscattered positrons. When positrons incident on the surface of materials, most positron reflected elastically from the surface annihilate with electron outside the materials. The coefficient for the backscattered positrons increases with the atomic numbers (Z) of the metal target ^{[17, 18]}. And it seems that, in Fig. 2(a), the longer is the thermal aging, the smaller is the averaged atomic number of the specimen. Another factor is the recovery of some microvoids by dissociating their vacancies, hence the reduced probability of positron annihilation. W parameters of the specimen thermal aged for longer hours are almost fitted with non-irradiated specimen below 2 keV. It could be assumed that the W parameter is not contributed from annihilation with Cu precipitates. In the S-W diagram, the appearance of an inflection point can confirm that the annihilation mechanism of positrons is changed at near surface. However, for the irradiated sample thermal-aged for 5 h, the S and W parameters change in the same tendency, indicating that the annihilation mechanism is similar. For the samples of longer thermal aging hours, the slope of the S-W plots in subsurface area is the same as the sample of 5 h thermal aging, while in near surface area it is close to the non-irradiated. This indicates that in long time thermal aging, the annihilation mechanism of positrons is similar to the substrate in near surface area.

To distinguish the annihilation mechanism of positrons and characterize the precipitates of Cu atoms in near surface, CDB experiments, with an as-irradiated sample and the sample just after 50 h thermal aging, along with the pure Fe and Cu samples, and the control, were performed at positron energy of 1.5 keV, corresponding to the inflection point at S-W diagram. The results of ratio to pure Fe are shown in Fig. 3. At high momentum region, the information of annihilation mechanism of positrons with the core of electron is provided. For the core electrons, the 3d electron shell of Fe atom differs from that of Cu atom, so the broad peak around 25×10⁻³ m₀c characteristic of Cu 3d electrons is clearly observed for pure Cu specimen. The increase of counts in the low momentum region and the decrease in the high momentum region correspond to the inflection point at S-W diagram. If the Cu precipitates in the specimens, the Cu peak will appear in the CDB spectrum. In Fig. 3, some unobvious Cu peaks can be seen in both the as-irradiated and as-thermal aged curves. It seems that the number of Cu atoms is less, in accord with the W parameters in Fig. 2(b). In Fig.3, that the ratios of the two irradiated samples are higher than 1 in low-momentum region (<5×10⁻³ m₀c), showing that the positrons annihilated mainly with low momentum electrons, and more positrons were trapped in the irradiation-induced microvoids, vacancies, and dislocation loops. The Cu CDB signal can be observed only when the positrons are localized around the Cu atoms in the dilute alloys: trapped at vacancy-type defects bound to Cu atoms or confined in Cu precipitates ^[5]. In low momentum area (<5×10⁻³ m₀c), the spectrum of thermal aged is lower than irradiated, because the irradiated specimen had more defects and part of the microvoids recovered in the thermal aging. In high momentum region, the ratios of the thermal aged are lower than 1. A possible interpretation is that the density of Cu precipitates decreases due to aggregation of the small precipitates and positrons annihilates in the matrix ^[19]. As the precipitates grow larger, they undergo transformations such as bcc to 9R structure ^[20], corresponding to the change of S-W curves. All these indicate that density of the irradiation-induced Cu precipitates decreases in the near surface by thermal aging for 50 h.

Fig. 3Full-screen View

CDB spectrums for irradiated and thermal aged Fe-0.6%Cu alloy with 1.5 keV positron beam energy.