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Because of outstanding thermal conductivity of 160-230 W m-1 K-1, high water corrosion resistance and good radiation tolerance, as well as comparably low-radioactivity elements, Al alloys have been used as components in fission reactors [1]. To be noted, the Al–2.7wt%Mg (AlMg3) alloy has been used until now as the beam window material for the safety-hulls of the targets of the Swiss Spallation Neutron Source (SINQ) [2]. Among Al 6xxx series, Al6061-T6 alloys, usually containing Al, Mg, Si and sometimes Cu elements, were fabricated through the artificial aging at ~ 160 – 177 °C for the solid solute solution (also T6 treatment). As a result, Al6061-T6 has the good yield strength of ~ 250 MPa (at room temperature), which is mainly due to the formation of the high-density needle-shaped precipitates during the T6 treatment.
Regarding the radiation tolerance of Al6061-T6, although previous studies focus much on the changes in mechanical properties such as toughness, strength and ductility [1], there is still lack of detailed study of microstructural evolution of Al6061-T6 under neutron irradiation. Considering significant differences between ion and neutron/proton irradiations, in this work, microstructure and strength of Al6061-T6 after neutron/proton irradiations have been investigated mainly by transmission electron microscopy (TEM) and uniaxial tensile testing, respectively.
Dog-bone shaped tensile specimens of Al6061-T6 were irradiated with neutrons and protons in SINQ Target-13 within the STIP-VII irradiation program. The maximum damage level was about 12 dpa. Meantime, about 650 appm helium and 1630 appm hydrogen were produced. The irradiation temperature was around 62 ℃. Tensile tests were conducted at 10-3 s-1 strain rate and at 22, 58 and 152 °C.
The irradiated specimens demonstrated slight irradiation hardening but quite pronounced embrittlement effects. In the unirradiated specimen, needle-shaped precipitates were observed and along the <100> directions of fcc Al matrix. The average length and the number density of which are 22.1 ± 10.3 nm and of (1.7 ± 0.8) × 1022 m-3, respectively. Pretty larger precipitates having a lower density of ~ 1019 m-3 coexisted with needle-shaped ones. In the as-irradiated specimen, most of needle-shaped precipitates were dissolved into Al matrix. Perfect dislocation loops of 1/2<110> and faulted frank loops of 1/3<111> were observed. After annealing at 152 °C/⁓1 h, namely during tensile testing, the density of perfect loops became negligible and frank loops disappeared. However, the needle-shaped precipitates appeared again. High-density He bubbles were found in both as-irradiated and post-irradiation annealed specimens. The annealing caused the growth of He bubbles from 1.7 ± 0.4 nm to 2.5 ± 0.5 nm.
[1]. K. Farrell, 5.07 - Performance of Aluminum in Research Reactors, Elsevier Inc., 2012.
[2]. Y. Dai, D. Hamaguchi, J. Nucl. Mater. 343 (2005) 184–190.
| Themes for the contribution | 3 Post-irradiation examination: |
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