Probing defects in Al–Mg–Si alloys using muon spin relaxation Sigurd Wenner ∗ and Randi Holmestad Department of Physics, NTNU, Høgskoleringen 5, Trondheim NO-7491, Norway Kenji Matsuda and Katsuhiko Nishimura Department of Materials Science and Engineering, University of Toyama, Gofuku 3190, Toyama-shi, Toyama 930-8555, Japan Teiichiro Matsuzaki and Dai Tomono Advanced Meson Science Laboratory, RIKEN Nishina Center for Accelerator Based Science, RIKEN, Wako, Saitama 351-0198, Japan Francis L. Pratt ISIS Facility, Rutherford Appleton Laboratory, Chilton OX11 0QX, UK Calin D. Marioara Materials and Chemistry, SINTEF, Box 4760 Sluppen, Trondheim NO-7465, Norway (Dated: August 24, 2012) Muon spin methods are very sensitive to nanoscale defects such as trace elements and vacancies in metals. This sensitivity is required when investigating Al–Mg–Si alloys, a complicated system in which diffusion-controlled phase transformations are responsible for the most important hardening mechanisms. We present muon spin relaxation experiments conducted on Al–Mg–Si alloys at mea- surement temperatures in the range 20–300 K. Varying the alloy composition and heat treatment, we find differences in muon depolarization in several temperature regimes. This reflects differences in concentration of several types of muon-trapping defects. We identify free solute atom and vacancy regimes, and confirm that the concentration of these defects decrease when an alloy is annealed at low temperature. We further attribute one regime to Mg–Si–vacancy clustering, a mechanism re- quired for precipitation hardening during aging. After storage at room temperature, muon trapping in this regime is more pronounced for a Mg-rich alloy than a Mg–Si-balanced alloy. I. INTRODUCTION Muon spin methods have been used for probing the microscopic properties of a wide range of materials, in- cluding superconducting and magnetic materials, 1–3 bio- logical molecules, 4 and semiconductors. 5,6 Most of the re- search activity using muons on aluminium and other pure metals took place in the late 1970s and early 1980s. 7–9 In non-magnetic materials, polarized positive muons (µ + ) can be used as probes for atomic-scale magnetic fields. The methods have been proven very sensitive to point defects such as trace element atoms. 8,10 The property of muons known as asymmetric decay is central to how the measurements are conducted: Muons are unstable, and decay to positrons, whose directions of motion tend to be parallel to the muon spin. The detection of these positrons enables us to follow the time evolution of the average muon polarization inside the material. In this paper, the acronym µSR refers to muon spin relaxation, with which no external magnetic field is applied. Aluminium alloys containing Mg and Si as main alloy- ing elements (6xxx series alloys) are used extensively as structural materials due to their formability, mechanical strength, and corrosion resistance. These alloys are heat- treatable, which means that their microstructure changes when thermal and mechanical treatment is applied to them. Typically, 6xxx alloys are given a solution heat treatment (SHT) before subsequent aging, to distribute the solute elements evenly in the Al matrix, and to intro- duce a high concentration of vacancies, which is necessary for later substitutional diffusion of the solute elements. A unique feature of Al–Mg–Si alloys is that room tempera- ture (RT) storage between SHT and artificial aging (AA) at higher temperatures has a significant effect on the hardness of the material. 11,12 This effect is called natural aging (NA), and can degrade the mechanical properties of dense alloys (with solute content above 1%). 11,13 NA is caused by the clustering of solute atoms. 14,15 Upon ag- ing above ≈ 150 ◦ C, metastable phases with well-defined crystal structures precipitate in the Al matrix. 16 The pro- cesses of clustering and precipitation are very sensitive to parameters such as alloy composition, storage time, aging temperature, and heating/cooling rates. We have applied µSR to samples of Al–Mg–Si alloys with various compositions and heat treatments. The evo- lution of the muon polarization is measured at a range of temperatures, as the trapping of muons by nanometer- sized defects is temperature-dependent. This enables the estimation of properties of several types of defects as av- eraged over a macroscopic volume of the material under study. To our knowledge, measurements of this kind have not before been conducted on aluminium alloys. This paper presents results from µSR studies of alloys with typical industrial Mg- and Si-content. Section II re-