Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 53

Muon spin relaxation study of solute-vacancy interactions during
natural aging of Al–Mg–Si–Cu alloys†
K. Nishimura,∗1,∗2 K. Matsuda,∗1,∗2 S. Lee,∗1 I. Watanabe,∗2 T. Matsuzaki,∗2 S. Wenner,∗3 and R. Holmestad∗4Age-hardenable aluminum alloys are strengthened bynanometer-sized atomic clusters and precipitates that arecoherent with the aluminum matrix. These are formedby the segregation of atoms (e.g., Mg and Si) from asolid solution, where the solute atoms occupy substitutional sites in fcc-Al. Such a solid solution comprisinga few at. % of alloying elements is achieved by solution
heat treatment (SHT)—an annealing step at 770–870 K.
The solid solution is highly unstable after quenching in
ice water after SHT (SHTQ). During further heat treatments at a lower temperature (artiﬁcial aging) and evenroom-temperature storage (natural aging), vacancies andsolute atoms co-diﬀuse and produce atomic clusters, withall atoms remaining at fcc-Al positions.1,2) The most important techniques to detect and quantify precipitatesin the Al-Mg-Si system have been transmission electron
microscopy (TEM) and atom probe tomography (APT).
However, small atomic clusters are diﬃcult to detect inTEM, APT is unsuitable to investigate the crystallography of precipitates, and neither technique can map vacancies. Muon spin relaxation (µSR) is one techniquethat can map vacancies, and it investigates short-range
magnetic ﬁelds and the presence of point defects in solids.
This work builds upon earlier literature on aluminumwith trace elements and Al–Mg–Si alloys,3–7) and it extends the analysis to the same alloys with Cu additions.
Figure 1 shows the zero-ﬁeld µSR spectra observed forAl-1.0%Mg2 Si-0.2%Cu-AQ (in the SHTQ condition) atmeasurement temperatures in the range of 20–280 K andfor Al-1.0%Mg2 Si-0.2%Cu-350C (annealed at 623 K for1000 min after SHTQ) at 280 K. The plot demonstrates
that the muon spin depolarization rate depends heavily on temperature. Similar µSR spectra were acquiredfor all samples. We have interpreted the measured µSR
spectra using a Monte Carlo simulation, in which four ﬁtting parameters were employed: the dipolar width (∆),
trapping rate (νt ), detrapping rate (νd ), and fraction ofFig. 2. Fitted trapping rates from isotheral relaxation spectra. Samples are quenched after SHT and kept at 280 K
throughout the measurements. The trapping rates decrease with time for all alloys except the quaternary Al–Mg–Si–Cu alloy, the trapping rate for which increases withtime.

initially trapped muons (P0 ).3,8)
From previous µSR studies, muons are considered tobe trapped by single vacancies or vacancy-solute pairs
near room temperature.3–6) Therefore, in most cases, theνt values decrease with time during natural aging because excess vacancies are lost at imperfections, such asgrain boundaries, voids, and surfaces. The change intrapping rate during storage at 280 K after SHTQ wasmeasured for Al-1.0%Mg2 Si-0.2%Cu, Al-1.6%Mg2 Si, Al0.5%Si, Al-0.2%Cu, and base Al, as shown in Fig. 2.
The horizontal axis denotes the time from SHTQ on alogarithmic scale. The νt values for the Cu-free samples decrease with time. The speed of the decrease is inversely correlated with the solute concentration, which isexpected because muons have more defects to be trappedin and more time to undergo spin relaxation. The reason
why the νt values of Al-0.2%Cu start out lower than in
the other alloys is unclear. A surprising result from theisothermal experiment is that the νt values (black circles)
for the Cu-added sample increase with time. If muons aretrapped only at vacancy-associated sites, the result indicates that the number density of vacancies increases withtime after quenching when all the elements Mg, Si, andCu are present in the alloy composition. This suggeststhat new vacancies are continuously diﬀusing into thematerial and binding to solute atoms, shifting the equilibrium concentration of vacancies to higher levels thanin the alloys with fewer elements. Further investigation
to clarify this phenomenon using positron annihilation
spectroscopy is in progress.
References

Fig. 1. Muon spin relaxation spectra for an as-quenched (AQ)
Al-1.0%Mg2 Si-0.2%Cu sample at 20–280 K and a sample
annealed at 623 K for 1000 min (350C) at 280 K.
†∗1
∗2
∗3
∗4

Condensed from the article in Metall. Mater. Trans. A 50, 3446
(2019)
Department of Materials Science and Engineering, U. Toyama
RIKEN Nishina Center
SINTEF Industry, Norway
NTNU Norway

1) K. Matsuda et al., Mater. Trans. 58, 167 (2017).
2) M. Werinos et al., Mater. Design 107, 257 (2016).
3) S. Wenner et al., Phys. Rev. B 86, 104201 (2012).
4) S. Wenner et al., Acta. Mater. 61, 6082 (2013).
5) S. Wenner et al., Metall. Mater. Trans. A 45, 5777 (2014).
6) K. Nishimura et al., Arch. Metall. Mater. 60, 925 (2015).
7) K. Nishimura et al., J. Phys. Soc. Jpn. Conf. 21, 011030(2018).
8) T. Hatano et al., Hyper. Inter. 17, 211 (1984).

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