3D atom probe study of solute atoms clustering during natural ageing and pre-ageing of an Al-Mg-Si alloy

Philosophical Magazine Letters (Impact Factor: 1.09). 04/2006; 86(4). DOI: 10.1080/09500830600643270
Source: OAI


The pair-correlation function applied to 3D Atom Probe reconstructed volumes has been used to study the influence of a pre-ageing treatment (363 K) on the early stages of precipitation at 458K in an Al-Mg-Si 6016 alloy. Mg-Si short-range positive pair correlation (clustering) is shown to form after a pre-ageing treatment. The hetero-atomic clusters are thought to act as preferential nucleation sites and lead to a finer dispersion of precipitates after ageing.

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Available from: Didier Blavette
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    • "In the limiting case of a very small evaporation field difference, this trajectory aberration is minimized. Methods for identifying clusters (nanometer-scale concentrations of one or more element) in atom-probe data have been the subject of various approaches (Blavette et al. 1988; Vurpillot et al. 2004; Miller and Kenik 2004; Miller et al. 2007; De Geuser et al. 2006; Cerezo and Davin 2007; Marquis and Hyde 2010; Stephenson et al. 2011; Serizawa and Miller 2013). The full details of these approaches are beyond the scope of this paper. "
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    ABSTRACT: Atom-probe tomography (APT) and secondary ion mass spectrometry (SIMS) provide complementary in situ element and isotope data in minerals such as zircon. SIMS measures isotope ratios and trace elements from 1–20 μm spots with excellent accuracy and precision. APT identifies mass/charge and three-dimensional position of individual atoms (±0.3 nm) in 100 nm-scale samples, volumes up to one million times smaller than SIMS. APT data provide unique information for understanding element and isotope distribution; crystallization and thermal history; and mechanisms of mineral reaction and exchange. This atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage, and can test for intracrystalline element mobility. Nano-geochronology is one application of APT in which Pb isotope ratios from sub-micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating. Based on SEM imaging and SIMS analysis, 11 needle-shaped specimens ~100 nm in diameter were sampled from one Archean and two Hadean zircons by focused ion-beam milling and analyzed with APT. The three-dimensional distribution of Pb and nominally incompatible elements (Y, REEs) differs at the atomic scale in each zircon. Zircon JH4.0 (4.007 Ga, Jack Hills, Western Australia) is homogeneous in Pb, Y, and REEs. In contrast, Pb and Y and REEs are clustered in sub-equant ~10 nm diameter domains, spaced 10–40 nm apart in zircons ARG2.5 (2.542 Ga, Grouse Creek Mountains, Utah) and JH4.4 (4.374 Ga, Jack Hills). Most clusters are flattened parallel to (100) or (010). U and Th are not collocated with Pb in clusters and appear to be homogeneously distributed in all three zircons. The analyzed domains experienced 4 to 8 × 1015 α-decay events/mg due to U and Th decay and yet all zircons yield U-Pb ages by SIMS that are better than 97% concordant, consistent with annealing of most radiation damage. The 207Pb/206Pb ratios for the 100 nm-scale specimens measured by APT average 0.17 for ARG2.5, 0.42 for the JH4.0, and 0.52 for JH4.4. These ratios are less precise (±10–18% 2σ) due to the ultra-small sample size, but in excellent agreement with values measured by SIMS (0.1684, 0.4269, and 0.5472, respectively) and the crystallization ages of the zircons. Thus Pb in these clusters is radiogenic, but unsupported, meaning that the Pb is not spatially associated with its parent isotopes of U and Th. For the domain outside of clusters in JH4.4, the 207Pb/206Pb ratio is 0.3, consistent with the SIMS value of 0.2867 for the zircon overgrowth rim and an age of 3.4 Ga. In ARG2.5, all Pb is concentrated in clusters and there is no detectable Pb remaining outside of the clusters. The Pb-Y-REE-rich clusters and lack of correlation with U in ARG2.5 and JH4.4 are best explained by diffusion of Pb and other elements into ~10 nm amorphous domains formed by α-recoil. Diffusion distances of ~20 nm for these elements in crystalline zircon are consistent with heating at temperatures of 800 °C for ~2 m.y. Such later reheating events are identified and dated by APT from 207Pb/206Pb model ages of clusters in JH4.4 and by the absence of detectable Pb outside of clusters in ARG2.5. SIMS dates for the zircon rims independently confirm reheating of ARG2.5 and JH4.4, which were xenocrysts in younger magmas when rims formed. It is proposed that most domains damaged by α-recoil were annealed at ambient temperatures above 200–300 °C before reheating and only a small number of domains were amorphous and available to concentrate Pb at the time of reheating. The size, shapes and orientations of clusters were altered by annealing after formation. The absence of enriched clusters in JH4.0 shows that this zircon was not similarly reheated. Thus APT data provide thermochronologic information about crustal reworking even for zircons where no overgrowth is recognized. The clusters in JH4.4 document Pb mobility at the sub-50 nm scale, but show that the much larger 20 μm-scale domains analyzed by SIMS were closed systems. The reliability of oxygen isotope ratios and other geochemical data from zircon can be evaluated by these means. These results verify the age of this zircon and support previous proposals that differentiated crust existed by 4.4 Ga and that the surface of Earth was relatively cool with habitable oceans before 4.3 Ga. These analytical techniques are of general applicability to minerals of all ages and open many new research opportunities.
    Full-text · Article · Jul 2015 · American Mineralogist
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    • "Depending on the material analyzed by APT, the user may focus on specific features such as interfaces, clusters, precipitates, short range order, morphology or number density of objects, etc. As far as the study of precipitation is considered, among the wide range of tools dedicated to this kind of study, one may propose a distinction between statistical tools relevant for the very early stages of precipitation (De Geuser et al., 2006; Marquis, 2002, 2007; Moody et al., 2007, 2008; Philippe et al., 2009; Sharik, 2007; Sudbrack, 2004; Sudbrack et al., 2004) and tools based on direct cluster or phase selection (Hellman et al., 2000, 2002, 2003; Hyde, 1993; Hyde and English, 2000; Hyde et al., 2009; Karnesky et al., 2007; Lefebvre et al., 2007; Marquis, 2002; Miller, 2000a,b; Miller and Kenik, 2004; Morley et al., 2009; Prakash Kolli and Seidman, 2007; Stephenson et al., 2007; Vaumousse et al., 2003; Vurpillot et al., 2004; Wolde-Giorgis et al., 2003) that are applicable as soon as regions of significantly distinct concentrations or solute densities are present. Despite the variety of methods proposed, the determination the matrix concentration is often neglected. "
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    ABSTRACT: In this study, we propose a fast automatic method providing the matrix concentration in an atom probe tomography (APT) data set containing two phases or more. The principle of this method relies on the calculation of the relative amount of isolated solute atoms (i.e., not surrounded by a similar solute atom) as a function of a distance d in the APT reconstruction. Simulated data sets have been generated to test the robustness of this new tool and demonstrate that rapid and reproducible results can be obtained without the need of any user input parameter. The method has then been successfully applied to a ternary Al-Zn-Mg alloy containing a fine dispersion of hardening precipitates. The relevance of this method for direct estimation of matrix concentration is discussed and compared with the existing methodologies. Microsc. Res. Tech., 2010. (c) 2010 Wiley-Liss, Inc.
    Full-text · Article · Mar 2011 · Microscopy Research and Technique
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    ABSTRACT: Dilute Al-0.06 at.% Sc alloys with microalloying additions of 50 at. ppm of ytterbium (Yb) or gadolinium (Gd) are studied with 3D local-electrode atom-probe (LEAP) tomography for different aging times at 300°C. Peak-aged alloys exhibit Al3(Sc1−x Yb x ) or Al3(Sc1−x Gd x ) precipitates (L12 structure) with a higher number density (and therefore higher peak hardness) than a binary Al-0.06 at.% Sc alloy. The Al–Sc–Gd alloy exhibits a higher number density of precipitates with a smaller average radius than the Al–Sc–Yb alloy, leading to a higher hardness. In the Al–Sc–Gd alloy, only a small amount of the Sc is replaced by Gd in the Al3(Sc1−x Gd x ) precipitates, where x=0.08. By contrast, the hardness incubation time is significantly shorter in the Al–Sc–Yb alloy, due to the formation of Yb-rich Al3(Yb1−x Sc x ) precipitates to which Sc subsequently diffuses, eventually forming Sc-rich Al3(Sc1−x Yb x ) precipitates. For both alloys, the precipitate radii are found to be almost constant to an aging time of 24h, although the concentration and distribution of the RE elements in the precipitates continues to evolve temporally. Similar to microhardness at ambient temperature, the creep resistance at 300°C is significantly improved by RE microalloying of the binary Al-0.06 at.% Sc alloy.
    Full-text · Article · Jan 2006 · Journal of Materials Science
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