# Duralumin Alloys in World War II Heritage Aircraft: Correlations Between Manufacturing Parameters and Alloys' Properties

Agathe Duclos, Elena Bernardi, Luc Robbiola, Christophe Deshayes, Luiza de Souza Machado, Carla Martini, Cristina Chiavari, Andrea Balbo, Cesar Adolfo Escobar Claros, Elodie Guilminot, et al.

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## To cite this version:

Agathe Duclos, Elena Bernardi, Luc Robbiola, Christophe Deshayes, Luiza de Souza Machado, et al. "Duralumin Alloys in World War II Heritage Aircraft: Correlations Between Manufacturing Parameters and Alloys' Properties." Heritage, 2024, 7 (12), pp.6772-6790. 10.3390/HERITAGE7120313. hal-04832520

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Article

# Duralumin Alloys in World War II Heritage Aircraft: Correlations Between Manufacturing Parameters and Alloys' Properties

Agathe Duclos<sup>1,2</sup>, Elena Bernardi<sup>3 ID</sup>, Luc Robbiola<sup>4 ID</sup>, Christophe Deshayes<sup>1</sup>, Luiza de Souza Machado<sup>1,2</sup>, Carla Martini<sup>5 ID</sup>, Cristina Chiavari<sup>6 ID</sup>, Andrea Balbo<sup>7 ID</sup>, Cesar Escobar<sup>8</sup>, Elodie Guilminot<sup>8 ID</sup>, Benoît Malard<sup>2 ID</sup> and Magali Brunet<sup>1,* ID</sup>

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**Citation:** Duclos, A.; Bernardi, E.; Robbiola, L.; Deshayes, C.; de Souza Machado, L.; Martini, C.; Chiavari, C.; Balbo, A.; Escobar, C.; Guilminot, E.; et al. Duralumin Alloys in World War II Heritage Aircraft: Correlations Between Manufacturing Parameters and Alloys' Properties. *Heritage* **2024**, *7*, 6772–6790. https://doi.org/10.3390/heritage7120313

Academic Editor: João Pedro Veiga

Received: 13 September 2024

Revised: 8 November 2024

Accepted: 25 November 2024

Published: 28 November 2024

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**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).

**Abstract:** Duralumin-type alloys have been employed for structural and non-structural parts of aircraft since the early 1920s up to now. In the European project "PROtection and Conservation of Heritage AirCRAFT" (PROCRAFT), up to 34 aircraft wrecks from World War II from different nations (United States of America, United Kingdom, France, Germany, Italy) were identified. On most of them, the collection of numerous fragments was possible, thus constituting a large sampling group. A metallographic analysis was carried out, and elemental composition, microstructure and hardness were measured. This article focuses firstly on the elemental composition of this group of historical Al alloys. From these data, some fine characteristics specific to the manufacturing countries will be investigated through principal component analysis. Then, on a restricted group constituted of German alloys, we will discuss how some manufacturing parameters, such as micro-alloying and thermo-mechanical treatments, influence the mechanical properties of the assessed aircraft components. Other influencing parameters, linked to the archaeological nature of the parts, are considered and addressed based on a specific study of an alloy collected on a crashed JU88 aircraft originating from the German company Junkers.

**Keywords:** aluminium alloys; metallography; aircraft history

## 1. Introduction

Aluminium alloys have been strategic materials in aeronautics all through the 20th century. The case of Duralumin is of historical interest: discovered in 1906 by Alfred Wilm, a German engineer, this light alloy was very early at the heart of aircraft [1,2]. The excellent mechanical properties of Duralumin (high tensile strength and hardness, comparable to mild steel) are based on the age-hardening mechanism. For Duralumin and, more generally, for aluminium alloys from the 2xxx family, mastering the properties is complex because many alloying elements (Cu, Mg, Mn, Si...) need to be tuned precisely. Furthermore,

<footnote n="1">1 CEMES-CNRS, Université de Toulouse, 31055 Toulouse, France; agathe.duclos@cemes.fr (A.D.); christophe.deshayes@cemes.fr (C.D.); luiza.desouza-machado@ensiacet.fr (L.d.S.M.)</footnote>

<footnote n="2">2 CIRIMAT, Université de Toulouse, CNRS, INPT, 31030 Toulouse, France; benoit.malard@ensiacet.fr</footnote>

<footnote n="3">3 Department of Industrial Chemistry "Toso Montanari", University of Bologna, Via Piero Gobetti 85, 40129 Bologna, Italy; elena.bernardi@unibo.it</footnote>

<footnote n="4">4 TRACES UT2J, Université de Toulouse, CNRS, 31000 Toulouse, France; luc.robbiola@univ-tlse2.fr</footnote>

<footnote n="5">5 Department of Industrial Engineering, University of Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy; carla.martini@unibo.it</footnote>

<footnote n="6">6 Department of Cultural Heritage, University of Bologna, Via degli Ariani 1, 48121 Ravenna, Italy; cristina.chiavari@unibo.it</footnote>

<footnote n="7">7 Centro di Studi sulla Corrosione et Metallurgia "A.Dacco", Engineering Department, University of Ferrara, 44121 Ferrara, Italy; andrea.balbo@unife.it</footnote>

<footnote n="8">8 GPLA-Arc'Antique, Département de Loire Atlantique, 44300 Nantes, France; cesar.escobar@loire-atlantique.fr (C.E.); elodie.guilminot@loire-atlantique.fr (E.G.)</footnote>

<footnote>* Correspondence: magali.brunet@cemes.fr; Tel.: +33(0)562257896</footnote>
thermo-mechanical treatments are extremely important in order to obtain an optimum hardening response. Because of this complexity, the development of the 2xxx alloys took different pathways. First, a patented German industrial alloy, Duralumin, with an original composition 3.5–4.5 wt.% Cu, 1 wt.% Mn, 0.5 wt.% Mg, was extensively employed by all aircraft manufacturers worldwide [1,2]. Very early, each supplier mastering this technology started to tailor the composition of Duralumin to improve its properties. All developments were done by a trial-and-error process. In 1934, the North American company Alcoa [3] set up the 24S alloy with a higher content of Mg (around 1.5 wt.%) which exhibited better mechanical properties than the 17S (around 0.5 wt.% for the original Duralumin) as reported in Table 1, characterised by a higher specific strength, a sufficient damage tolerance (fracture toughness) and a good formability, as well as a relatively good corrosion resistance. Alcoa was soon followed by most countries worldwide, which developed their own version of Duralumin with higher Mg content, also called Super-Duralumin. It was quickly discovered that a small amount of strain-hardening (by cold work) applied to Super-Duralumin was beneficial to the yield strength (in MPa) without sacrificing the ductility (elongation in %). This temper was originally designated by the symbol “RT” in Alcoa’s system [3,4].

<table><caption>**Table 1.** Typical example of characteristics of Alcoa’s first wrought aluminium alloys: expected nominal weight percent of alloying elements (Al and normal impurities constitute remainder) and specific mechanical properties. UTS: Ultimate Tensile Strength, YS: Yield Strength, E: Elongation [3].</caption><thead><tr><th rowspan="2">Al Base Alloy</th><th colspan="4">Alloying Elements (wt.%)</th><th colspan="4">Mechanical Properties (Typical Values)</th></tr><tr><th>Cu</th><th>Si</th><th>Mn</th><th>Mg</th><th>UTS (MPa)</th><th>YS (MPa)</th><th>E (%)</th><th>Hardness Brinell (HB)</th></tr></thead><tbody><tr><td>17S-T</td><td>4.0</td><td>...</td><td>0.5</td><td>0.5</td><td>400</td><td>241</td><td>20</td><td>100</td></tr><tr><td>24S-RT</td><td>4.2</td><td>...</td><td>0.5</td><td>1.5</td><td>467</td><td>365</td><td>13</td><td>116</td></tr><tr><td>14S-T</td><td>4.4</td><td>0.8</td><td>0.75</td><td>0.35</td><td>448</td><td>344</td><td>10</td><td>130</td></tr></tbody></table>

Silicon was not considered as an alloying element in 17S and the 24S [5], but as an impurity of the bauxite ore. However, another alloy was also developed in 1928 by Alcoa (Table 1): 14S, which is a true quaternary alloy, with a controlled content of Si between 0.5 and 1.2 wt.%, used mainly for forgings because of its high formability and resistance to high temperatures [6].

In the meantime, the Germans created their own Duralumin range: 3115 corresponding to 17S-T4, and 3125 corresponding to 24S-T4 (see below), with the following composition and properties. These compositions were modified at the end of World War II (WWII) due to international blockade [7,8]. From 1943, Cu was required in smaller quantities and replaced by more Mg [9,10].

As only a few archival documents remain, it is still unknown to what extent the Germans considered Si as a “controlled” micro-alloying element in their Duralumin and Super-Duralumin alloys. The official German standards present a specific range allowed for wt.% Si in strain-hardening aluminium alloys [9] (see Table 2), but the way the overall elemental composition and the mechanical properties were achieved remain uncertain (due to the use of ersatz material and the international blockade).

<table><caption>**Table 2.** Percent of alloying elements (Al and normal impurities constitute the remainder). Typical expected mechanical properties of German wrought aluminium alloys following Reichsfahrtministerium standards from 1937. UTS: Ultimate Tensile Strength, YS: Yield Strength, E: Elongation [9].</caption><thead><tr><th rowspan="2">Alloys</th><th colspan="4">Alloying Elements (wt.%)</th><th colspan="4">Mechanical Properties</th></tr><tr><th>Cu</th><th>Si</th><th>Mn</th><th>Mg</th><th>UTS (MPa)</th><th>YS (MPa)</th><th>E (%)</th><th>E (%)</th></tr></thead><tbody><tr><td>3115</td><td>3.7–4.7</td><td>0.1–0.7</td><td>0.3–0.7</td><td>0.4–1.0</td><td>400</td><td>250</td><td>15</td><td>15</td></tr><tr><td>3125</td><td>3.5–4.5</td><td>0.2–0.9</td><td>0.9–1.5</td><td>0.9–1.5</td><td>430</td><td>270</td><td>10</td><td>10</td></tr></tbody></table>
In 1954, a new international designation system was introduced [11]: 17S, 24S and 14S were designated as 2017, 2024 and 2014, respectively. Similarly, thermal treatments were uniformed:

* T4 for solution-treatment, followed by a quenching and natural aging (T in Alcoa's system);

* T3 for strain-hardening before natural aging (RT in Alcoa's system).

The mechanical properties are indeed dictated by the metallurgical microstructure, which is itself influenced by the composition of the alloy and the manufacturing conditions [12].

Over the years, although many new alloys and new thermo-mechanical treatments were set up, the 2024-T3 alloy became the most widely used wrought alloy in the aeronautic industry, especially for the wings or the fuselage of aircraft. The 2017-T4 alloy was abandoned for aerospace applications and is nowadays employed for common applications. Thanks to its welding and forging performance [13], the 2014 alloy is still dedicated to specific aeronautical parts such as landing gear hubs. With a T6 treatment, 2014 exhibits very high specific strength (UTS = 485 MPa, YS = 415 MPa, E = 13%) [14].

While previous work allowed the retrieval of information on the use of this family of alloys (Duralumin, Super-Duralumin) for the construction of aircraft during WWII [15], in this work, we analyse an even broader number of excavated metal parts that are part of crashed WWII aircraft from five different nations: USA, France, Germany, UK and Italy, which constitute the PROCRAFT sampling group. The metal parts are mostly wrought products, i.e., plates and stringers made of aluminium alloys.

The first analyses consist in determining the elemental composition of the alloys. Then a focus on the case of German manufacturers (Junkers and Focke-Wulf) is made. The study of microstructure (grain size and shape, size and density of intermetallic compounds) is a first step revealing the manufacturing parameters that influence the mechanical properties of aluminium alloys: heat treatment, rolling, cooling. By observing the microstructure, a better understanding can be gained on the industrial choices made in the manufacture and processing of duralumin aerospace alloys during WWII and the consequences of these choices on the mechanical behaviour of aircraft parts.

Eventually, the impact of the archaeological origin of the samples is evaluated by studying the modification of the nanostructure of these alloys (dislocations and precipitation).

## 2. Materials and Methods

### 2.1. Materials

All the studied artefacts (see Supplementary Materials, Table S1) were found after extensive searches from archaeological excavations by the associations which are third parties of the European project PROCRAFT [16]: Aérocherche, near Toulouse (France); Breton Air Remembrance Association (ABSA 39-45), in the Brittany and Normandy regions (France); Volandia and Air Club and Fun in Italy. This was performed under permits delivered by the archaeological department of the French Regional Cultural Affairs Directorate (DRAC) and Subaquatic and Submarine Archaeological Research Department (DRASSM). The aircraft were identified thanks to testimonies and contemporary reports. The artefacts have stayed buried in the ground for around 80 years and were stored indoors once excavated. In this work, fragments from 34 WWII aircraft were collected and studied in the framework of the PROCRAFT project. This way, a large panel of alloys from different nations was probed via the analysis of their elemental composition.

For the study on German manufacturers, metal parts (all wrought aluminium alloys) from five aircraft were studied and compared to modern industrial 2017A (standard European EN AW-2017 AlCu₄MgSi alloy) and 2024 aluminium (standard European EN AW-2024 AlCu₄Mg₁ alloy) alloys. Two plane wrecks were Junkers 88 bombers and three were Focke-Wulf 190 fighters, both widely deployed by Germany in WWII. A large number of these aircraft crashed in the Toulouse area (France), where the study was carried out. The Junkers aircraft were built in 1940 and 1942, and crashed between 1943 and 1944. We
do not know the date of manufacturing of the Focke-Wulf planes, but as they crashed
in 1943, we will assume in this study that they were built before the change of German
standards for elemental composition in 1943 (corroborated by elemental analysis). The
specific localisation of the samples on the aircraft is also unknown.

For the aim of the study, one sample (JU88-B3) was heat-treated at 500 °C for 2 h, quenched in water at room temperature and strength-hardened at room temperature for 5 days. This treatment is equivalent to a T4 treatment usually applied to Duralumin since the discovery of the first structural hardening alloys [1,2]. It was carried out to erase the uncontrolled thermal and microstructural changes having occurred during the life of the parts (80 years): take-off and landing cycles, weather conditions, crash of the aircraft. The treated part was brought back to its manufacturing state in order to be compared with modern industrial 2017 and 2024 duralumin alloys, but also to be compared with its long-term aged counterpart.

## 2.2. *Experimental Techniques*

The adopted methodology for determining the elemental composition of the alloys involved physicochemical characterization correlated to archive data and can be found in a previous study [17]. The first allows precise identification, while the latter helps the examination of related historical and technical aspects. Then, conventional metallographic studies are carried out on the metal parts through the observation of the microstructure at different scales.

Various techniques, both destructive and non-destructive, were used to study the artefacts with a multiscale approach. These techniques required tailored sample preparation.

For the observation of alloys' microstructure by optical microscopy (OM) and scanning electron microscopy (SEM), each specimen was prepared along the three main planes (rolling-L, long transversal-LT, short-transversal-ST). The three corresponding parts were embedded in epoxy resin and mechanically polished on water-lubricated abrasive papers (silicon carbide—SiC)—P600 then P1200. They were then polished on cloths with diamond suspension (from 3 µm down to 1 µm). This preparation also served for microhardness testing.

SEM micrographs of as-received and heat-treated samples were acquired using a Helios NanoLab600i FIB/SEM operating at 15 kV with backscattered electron imaging (BEI), paired with an X-MaxN Silicon (Lithium) Drift Detector energy dispersive X-ray spectrometer (EDS) from Oxford Instruments. Quantitative elemental composition data were collected over three 500 µm × 500 µm zones on the three different planes of the alloys, resulting in an average value obtained over 9 scans. These dimensions make it possible to scan homogeneous zones despite the precipitates. A minimum of 10⁶ counts on each spectrum was acquired. Before each measurement, the beam intensity was optimised on pure Co standard. Quantitative analysis was performed with internal standards (pure K-line Al, Si and Fe standards). Results were normalised to 100 wt.%. The compositions of two alloys were corroborated using Inductively Coupled Plasma—Optical Emission Spectroscopy (see Supplementary Materials, Table S5). Thanks to this comparison, the error is estimated to be around 10% maximum for the composition acquired by EDS.

Microhardness tests were carried out with a HM-200 Mitutoyo machine with a load of 300 g for 10 s on the polished surface, following the ASTM E 384 standard [18]. Average results on five indents were obtained in hardness Vickers numbers (here noted HV₀.₃).

To characterise the microstructure (measure the shape and size of grains) in the alloys by OM, the samples were etched using Keller's reagent (HF 1 mL, HNO₃ 2.5 mL, HCl 1.5 mL in demineralised H₂O 95 mL).

For transmission electron microscopy (TEM) observations, the preparation followed several steps. In order to bring the sample thickness down to 25 µm, a mechanical polishing with SiC paper grade up to grit 2400, was first performed. The specimens were then electrochemically thinned using a Tenupol-5 Struers apparatus operating at 60 V in a
solution of methanol and nitric acid (3:1) at -15 °C. Lastly, bright-field (BF) TEM images were taken with an orientation slightly off the [011]Al zone axis (ZA).

TEM micrographs of as-received and heat-treated samples were acquired using a JEOL JEM-2100 microscope operating at 200 kV. To measure the samples' thickness, energy-filtered TEM (EFTEM) images were acquired on a Philips Tecnai F20 microscope operating at 200 kV. EFTEM was performed on the same zones as the ones observed with the JEOL JEM-2100 microscope, and the thickness was measured with the Gatan Digital Micrograph software (EELS tool, version 3.60.4435.0) and the mean free path estimator developed by Mitchell and based on Malis et al. [19]. The estimation of dislocation density in the studied alloys was carried out by the intercept method, described by Smith and Guttman, 1953 and Bailey and Hirsch, 1960 [20,21] with Equation (1)

$$ \rho_{[110]}^{\text{visible}} = \frac{2n}{lt} \qquad (1) $$

on three micrographs coming from the same grain near the [011]Al ZA. n stands for the number of intersections between dislocations and a grid with a mesh of 100 nm. l stands for the total length of the grid and t for the thickness of the studied zone in the sample. The evaluation of a mean density was made compared to the extinction conditions of the ZA. Near the [011]Al ZA, the dislocations are extinct on three active diffraction vectors $\vec{g}$ of (001), (1-10) and (1-11) lattice planes, out of six preferential dislocations in fcc structure, as $\vec{g} \cdot \mathbf{b} = 0$ for these dislocations, with b the magnitude of the Burgers vector $\vec{b}$. The density was calculated out of the visible dislocations. It was then multiplied by two as the dispersion of dislocations was considered isotropic in a first approach [22], thus giving

$$ \rho_{[110]}^{\text{total}} = \rho_{[110]}^{\text{visible}} = 2 \frac{2n}{lt} \qquad (2) $$

A multivariate analysis of the experimental data was carried out by means of Principal Component Analysis (PCA). To this aim, the R-based freeware Chemometric Agile Tool (CAT) was used: data were pre-treated by centring and scaling, then PCA with Varimax rotation was applied.

# 3. Results and Discussion

## 3.1. Alloys' Elemental Composition

From the elemental compositions reported previously of the ancient alloys collected from WWII aircraft wrecks [15,16,23] on post-WWII aircraft (Breguet 765) [24] and on the newly collected parts detailed above (German aircraft), a PCA was carried out. Percentage concentrations (wt.%) of the main alloying elements were considered as variables: Cu, Mn, Mg and Si. The first two factors were found to explain 70.1% of the variance among the samples: the Biplot, including both loadings and scores, is shown in Figure 1. On this PCA graph, the first principal axis (factor 1) explains the 41.7% of the variance, and is mainly influenced by wt.% Si (at positive loadings) and wt.% Mg (at negative loadings), and the results are almost anti-correlated, i.e., an increase in wt.% Si corresponds to a decrease in wt.% Mg, and vice versa. Factor 2 explains 28.4% of the variance and is mainly influenced by wt.% Mn at positive loadings. Alloys from WWII aircraft are shown with different-coloured marks (green for Germany, light blue for USA, and black for France). Magenta marks are "modern" state-of-the art alloys retrieved from the literature [25-28].

It is shown that among the sampling group, each nation has its own alloy range, al-though this range is quite scattered. The North American plane wrecks that were found are all Super-Duralumin, with a composition similar to the modern 2024. This result was expected, as modern standards for 2024 alloys are derived from the standards for the American 24S alloy [11,13]. These alloys are characterised mostly by the Mg and Si content, as expected for a standard Super-Duralumin. All French samples are Duralumin alloys. Their composition is close to a 2017A alloy. They are characterised by lower Mg and higher
Si contents. As the French standard allowed range for Mn was quite large at the time (between 0.3 wt.% and 0.9 wt.% [29]), the French sampling group is expected to be slightly scattered along Factor 2.

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Biplot with loadings and scores from the PCA model related to elemental composition of the aircraft parts. Factors 1 and 2 are reported: factor 1 is mainly related to the influence of Mg (at negative loadings) and Si (at positive loadings), factor 2 is mainly related to the influence of Mn (at positive loadings). Each sample is identified by a coloured mark representing the manufacturing country. Three distinctive groups of samples can be distinguished (blue, green, black). “GE” marks surrounded by squares are pieces coming from FW190 aircraft. “GE” marks surrounded by circles are pieces coming from JU88 aircraft.</figcaption></figure>

A look at the German plane wrecks shows a strong gradient in composition along the wt.% Mn axis, but not clear differentiation along the Factor 1 axis. Apart from a Messerschmitt sample that can be clearly identified as a Super-Duralumin alloy, we cannot differentiate 3115 from 3125 alloys solely based on the Mg and Si contents. Both 3115 and 3125 alloys are driven the same way by the composition and cannot be distinguished by PCA. The distinction between the two types of alloys was made according to the difference in Mn. The samples influenced positively by the Mn content all come from Focke-Wulf planes, whereas the samples influenced negatively by Mn are mostly Junkers. We can thus observe that these two German manufacturers were most probably supplied from different manufacturers of aluminium alloys.

As the German alloys were scattered in composition, they were analysed more closely. The twelve parts from five different German planes (three Focke-Wulf FW190 and two Junkers JU88) found in the sampling group are detailed in Table 3.
<table><caption>**Table 3.** Elemental composition (EDS quantitative measurements) of the aircraft wrecks and corresponding modern alloys. The year of production of samples marked (/) is not known. The error on the EDS measurements is about 10%. The results are presented with the standard deviation obtained after averaging 9 measurements (see Section 2.2).</caption><thead><tr><th rowspan="2">Manufacturer—Plane (Year of Production)</th><th rowspan="2">Sample</th><th rowspan="2">Al</th><th colspan="6">Composition (wt.%)</th></tr><tr><th>Cu</th><th>Mg</th><th>Si</th><th>Mn</th><th>Fe</th><th>Other</th></tr></thead><tbody><tr><td>-</td><td>2017A</td><td>Base</td><td>4.0 ± 0.1</td><td>0.7 ± 0.1</td><td>0.7 ± 0.1</td><td>0.6 ± 0.1</td><td>0.3 ± 0.1</td><td>Zn: 0.2 ± 0.1</td></tr><tr><td>-</td><td>2024</td><td>Base</td><td>4.3 ± 0.1</td><td>1.5 ± 0.1</td><td>0.1 ± 0.1</td><td>0.3 ± 0.1</td><td>0.1 ± 0.1</td><td>Zn: 0.1 ± 0.1</td></tr><tr><td>Focke-Wulf—FW190 (/)</td><td>FW190-A1</td><td>Base</td><td>3.9 ± 0.3</td><td>1.1 ± 0.1</td><td>0.6 ± 0.1</td><td>1.2 ± 0.1</td><td>0.4 ± 0.1</td><td>/</td></tr><tr><td>Focke-Wulf—FW190 (/)</td><td>FW190-A2</td><td>Base</td><td>4.3 ± 0.3</td><td>1.0 ± 0.1</td><td>0.5 ± 0.1</td><td>1.1 ± 0.1</td><td>0.4 ± 0.1</td><td>/</td></tr><tr><td>Focke-Wulf—FW190 (/)</td><td>FW190-A3</td><td>Base</td><td>4.3 ± 0.1</td><td>0.8 ± 0.1</td><td>0.2 ± 0.1</td><td>0.6 ± 0.1</td><td>0.3 ± 0.1</td><td>Zn: 0.1 ± 0.1</td></tr><tr><td>Focke-Wulf—FW190 (/)</td><td>FW190-B1</td><td>Base</td><td>4.0 ± 0.3</td><td>0.8 ± 0.1</td><td>0.5 ± 0.1</td><td>0.5 ± 0.1</td><td>0.4 ± 0.1</td><td>/</td></tr><tr><td>Focke-Wulf—FW190 (/)</td><td>FW190-C1</td><td>Base</td><td>4.4 ± 0.1</td><td>1.3 ± 0.1</td><td>0.3 ± 0.1</td><td>1.2 ± 0.1</td><td>0.4 ± 0.1</td><td>/</td></tr><tr><td>Junkers—JU88 (1942)</td><td>JU88-A1</td><td>Base</td><td>3.5 ± 0.1</td><td>1.1 ± 0.1</td><td>0.3 ± 0.1</td><td>0.7 ± 0.1</td><td>0.4 ± 0.1</td><td>Zn: 0.1 ± 0.1</td></tr><tr><td>Junkers—JU88 (1942)</td><td>JU88-A2</td><td>Base</td><td>3.5 ± 0.1</td><td>1.3 ± 0.1</td><td>0.4 ± 0.1</td><td>0.6 ± 0.1</td><td>0.4 ± 0.1</td><td>Zn: 0.1 ± 0.1</td></tr><tr><td>Junkers—JU88 (1942)</td><td>JU88-A3</td><td>Base</td><td>4.7 ± 0.5</td><td>0.9 ± 0.1</td><td>0.6 ± 0.1</td><td>0.7 ± 0.1</td><td>0.4 ± 0.1</td><td>/</td></tr><tr><td>Junkers—JU88 (1940)</td><td>JU88-B1</td><td>Base</td><td>4.6 ± 0.1</td><td>1.4 ± 0.1</td><td>0.1 ± 0.1</td><td>0.5 ± 0.1</td><td>0.2 ± 0.1</td><td>/</td></tr><tr><td>Junkers—JU88 (1940)</td><td>JU88-B2</td><td>Base</td><td>4.9 ± 0.2</td><td>0.9 ± 0.1</td><td>0.4 ± 0.1</td><td>0.5 ± 0.1</td><td>0.3 ± 0.1</td><td>/</td></tr><tr><td>Junkers—JU88 (1940)</td><td>JU88-B3</td><td>Base</td><td>4.5 ± 0.2</td><td>0.9 ± 0.1</td><td>0.5 ± 0.1</td><td>0.5 ± 0.1</td><td>0.4 ± 0.1</td><td>Zn: 0.1 ± 0.1</td></tr><tr><td>Junkers—JU88 (1940)</td><td>JU88-B4</td><td>Base</td><td>4.3 ± 0.3</td><td>0.8 ± 0.1</td><td>0.4 ± 0.1</td><td>0.4 ± 0.1</td><td>0.2 ± 0.1</td><td>/</td></tr></tbody></table>

The parts vary in elemental composition, but correspond to the German Reichsluft-fahrtministerium (RLM) standards in effect between 1937 and October 1942 [9]; see Table 2.

Figure 2 shows the categorisation of German alloys according to their Cu/Mg and Mg/Si ratios. These ratios allow to visualise the nuance of the alloys: Super-Duralumin or Duralumin. The allowed ratios are calculated after the German RLM elemental composition standards from 1937 (see Table 2). They are represented by boxes in Figure 2: the alloy 3115 corresponding to a Duralumin (blue box) overlaps the 3125 (red box) corresponding to a Super-Duralumin.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Composition of different alloys (modern and ancient) classified according to their Cu/Mg and Mg/Si ratios. The blue and red boxes respectively correspond to the ranges defined by RLM compositions standards of 1937 for 3115 and 3125 alloys [9].</figcaption></figure>

Several alloys have a composition which is difficult to classify under one specific category (either Duralumin or Super-Duralumin), as presented in Figure 1. Some of the alloys collected in the sampling group overlap the two aluminium grades. It is not yet understood if they belong to a specific category (not reported in the official German standards but possibly in a manufacturer's own standard) or if it is due to variability/ranges in the existing known standards.
Alloys from Junkers 88 wrecks clearly form two different groups, along the Cu/Mg ratio, identified as 3115 (red round) and 3125 (red square) alloys. As for the Focke-Wulfs, the separation was made via the Cu/Mg ratio and confirmed by the content of Mn (see Figure 1): higher for the Super-Duralumin than for the Duralumin. The Super-Duralumin plates separate in two groups: Junkers alloys exhibit a smaller Cu/Mg ratio and a larger Mg/Si ratio than the Focke-Wulf ones. This is a confirmation that the aluminium suppliers were different for these two aircraft manufacturers.

## 3.2. *Mechanical Properties (Hardness)*

### 3.2.1. Micro-Hardness Testing

As a first approach, the micro-hardness was measured and analysed according to the metallurgical parameters: primarily the elemental composition of the alloys. Micro-hardness results for the group of wrecks are presented in Figure 3. They were divided into two groups: Duralumin alloys to the left (French in black and white stripes; German in dark green) and Super-Duralumin alloys to the right (German in light green and American in blue). They were compared to their modern equivalents (2017A and 2024) with a threshold corresponding to the modern alloys' measured values. The main observation is the overall high hardness of the ancient Duralumin alloys compared to 2017A. Moreover, some values, especially among German samples, are even higher than a 2024 reference alloy. Similar hardnesses to the modern alloys are to be expected [30]. The measured hardnesses of modern 2017A and 2024 correspond to the literature and manufacturer tests [23].

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Micro-hardness ($HV_{0.3}$) values for as-received aircraft wrecks from the global sampling group and corresponding modern 2017A (dark grey) and 2024 (light grey) alloys. French aircraft are in black and white stripes (Duralumin); German aircraft are in dark green (Duralumin) and light green (Super Duralumin) and American aircraft are in blue (Super Duralumin).</figcaption></figure>
### 3.2.2. Contributions to Mechanical Properties

The microstructure contains, at the microscopic scale, grains of the polycrystalline alloy and coarse intermetallic particles (also called second-phase constituents). At the nanoscale, it includes dispersoids (submicronic intermetallic particles containing alloying elements, typically Mn, spread out inside the grains) as well as fine precipitation which mainly drives the hardening mechanisms [31]. This fine precipitation originating from the solid solution can range from atomic-size Cu-Mg clusters [32] or Cu-rich atomic monolayers called Guinier-Preston Zones (GPZ) [33,34] to nano-size precipitates and is oriented along the main crystalline planes of the Al matrix. Additionally, dislocations, which are linear defects of the crystalline matrix introduced during the mechanical treatment of the part (mainly by quenching [35,36] and plastic deformation such as rolling), also influence the overall mechanical properties. The grain size and distribution of second-phase particles affect strength and ductility, and fine precipitation and high dislocation densities significantly contribute to improve both the tensile strength and yield strength of the alloy.

The mechanical properties of the alloy can be presented as the sum of all the parameters contributing to the overall strengthening described by the material's yield strength, written as:

$$ \sigma_y = \sigma_O + \sigma_{gb} + \sigma_d + \sigma_s \quad (3) $$

where $\sigma_O$ is the base strength of the annealed material without any supplementary strain (e.g., cold work dislocations), $\sigma_{gb}$ is the contribution of the grain boundaries according to the Hall-Petch effect [37,38], $\sigma_d$ is the contribution of dislocations and $\sigma_s$ is the contribution of solute clusters and precipitates [39,40].

The contribution of the grain size to the mechanical properties is described by the Hall-Petch effect [37,38]. This contribution can be written as an additional part of the material's yield strength $\sigma_{gb}$:

$$ \sigma_{gb} = k_{HP} d^{-1/2} \quad (4) $$

where $d$ is the grain size (m) and $k_{HP}$ is the Hall-Petch constant (grain boundary locking term), which has been calculated at 0.062 MPa.m$^{1/2}$ for several naturally aged Al-Cu-Mg alloys [39,41] and 0.06 MPa.m$^{1/2}$ for Al-4Cu [42]. Hereafter, we will use this last value.

The grain size and shape of the ancient German alloys as well as modern alloys are presented in Table 4 (see Supplementary Materials, Tables S2 and S3 for details). For a rolled plate of same thickness, the shape of the grains is similar between ancient and modern alloys, i.e., the ratio L/LT between the longitudinal or rolling (L) and transversal (LT) directions is around 1 in accordance with a recrystallisation annealing treatment after rolling.

In the group studied, 2017A and 2024 are the alloys with the smallest grains (38 µm long). For these alloys, according to Equation (4), $\sigma_{gb} = 9.7$ Mpa. This value corresponds, respectively, to 3.4% and 2.7% of their total yield strength ($\sigma_{y(2017A)} = 282$ Mpa, $\sigma_{y(2024)} = 359$ Mpa (supplier inspection certificates)), following Equation (3).

Most ancient alloys exhibit larger grains than the modern references. According to 1937 RLM standards, the expected yield strength $\sigma_y$ at factory state was around 245 MPa [7], close to the value of modern 2017A. Among the studied parts, the $\sigma_{gb}$ value range is about 7–19 MPa, corresponding to a contribution to yield strength of about 3–8%. We can then consider the Hall-Petch effect too weak to impact the mechanical properties of these alloys. This conclusion is in accordance with the results obtained by [42]. The Hall-Petch effect is also not observable on FW190-A1 (71 µm long) and FW190-A2 (56 µm long), which exhibit grain sizes similar to 2017A and 2024 alloys for a similar thickness.
<table><caption>**Table 4.** Microstructure of the aircraft wrecks and corresponding modern alloys.</caption><thead><tr><th>Aircraft</th><th>Sample</th><th>Alloy</th><th>Plate Thickness (mm)</th><th>Grain Length L (μm)</th><th>L/LT</th><th>L/ST</th><th>Intermetallics Density (/mm²)</th><th>Intermetallics Surface Percentage (%)</th><th>Microhardness HV<sub>0.3</sub> (as-Received State)</th></tr></thead><tbody><tr><td>-</td><td>2017A-T4</td><td>2017A</td><td>1.0</td><td>38</td><td>1.2</td><td>2.0</td><td>12,666</td><td>1.9</td><td>122</td></tr><tr><td>-</td><td>2024-T3</td><td>2024</td><td>1.0</td><td>38</td><td>1.2</td><td>2.5</td><td>10,120</td><td>1.2</td><td>144</td></tr><tr><td>Focke-Wulf—FW190</td><td>FW190-A1</td><td>3125</td><td>1.4</td><td>71</td><td>1.3</td><td>3.2</td><td>13,093</td><td>3.2</td><td>153</td></tr><tr><td>Focke-Wulf—FW190</td><td>FW190-A2</td><td>3125</td><td>1.2</td><td>56</td><td>1.0</td><td>2.6</td><td>8741</td><td>2.4</td><td>141</td></tr><tr><td>Focke-Wulf—FW190</td><td>FW190-A3</td><td>3115</td><td>1.9</td><td>157</td><td>0.9</td><td>1.0</td><td>12,760</td><td>1.8</td><td>145</td></tr><tr><td>Focke-Wulf—FW190</td><td>FW190-B1</td><td>3115</td><td>0.7</td><td>147</td><td>1.0</td><td>2.1</td><td>9363</td><td>1.6</td><td>140</td></tr><tr><td>Focke-Wulf—FW190</td><td>FW190-C1</td><td>3125</td><td>2.6</td><td>307</td><td>1.3</td><td>5.1</td><td>6548</td><td>2.9</td><td>147</td></tr><tr><td>Junkers—JU88</td><td>JU88-A1</td><td>3125</td><td>1.0</td><td>62</td><td>0.9</td><td>1.8</td><td>4393</td><td>1.8</td><td>146</td></tr><tr><td>Junkers—JU88</td><td>JU88-A2</td><td>3125</td><td>1.9</td><td>285</td><td>1.1</td><td>3.6</td><td>11,124</td><td>2.2</td><td>156</td></tr><tr><td>Junkers—JU88</td><td>JU88-A3</td><td>3115</td><td>1.4</td><td>126</td><td>1.1</td><td>2.7</td><td>10,964</td><td>2.8</td><td>142</td></tr><tr><td>Junkers—JU88</td><td>JU88-B1</td><td>3125</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Junkers—JU88</td><td>JU88-B2</td><td>3115</td><td>1.2</td><td>126</td><td>1.1</td><td>2.7</td><td>7961</td><td>1.8</td><td>-</td></tr><tr><td>Junkers—JU88</td><td>JU88-B3</td><td>3115</td><td>1.0</td><td>105</td><td>1.0</td><td>2.0</td><td>9345</td><td>1.7</td><td>138</td></tr><tr><td>Junkers—JU88</td><td>JU88-B4</td><td>3115</td><td>1.3</td><td>54</td><td>1.3</td><td>2.2</td><td>4039</td><td>1.2</td><td>148</td></tr></tbody></table>

It is known that Mn helps in controlling the grain size [43,44] through the formation of dispersoids within the grains, leading to better mechanical properties [37,38]. In FW190-A1 and -A2 alloys, the content in Mn is about 1.2% and 1.1%, respectively, higher than in modern alloys (0.7% and 0.3%). It could explain the lower grain size in the sampling group.

For all Junkers samples, the Mn contents are comparable and do not seem to affect the grain size and hardness. Although FW190 and JU88 samples have different Mn contents, their hardness is equivalent.

The PCA carried out, including microstructural parameters and composition of the samples, shows that the presence of Mn and the proportion of intermetallics is strongly correlated (see Supplementary Materials, Figure S1). This fact has already been demonstrated [43,45]. In the FW190-A1, FW190-A2 and FW190-C1 plates, in which the Mn content is high, the intermetallics surface fraction is about 3%, whereas it is about 2% for the rest of the samples. Although higher intermetallics content is supposed to deteriorate some mechanical properties, such as the fracture toughness and fatigue behaviour of the alloy [43], it does not contribute to its hardening. As shown in Table 4, these samples indeed exhibit the same level of hardness as the whole sampling group.

This first investigation allows us to observe a wide range of elemental composition among the collected German alloys. However, the measured hardness is very similar for each plane wreck (3115 and 3125) and higher than the current 2017A and 2024 alloys. A first analysis of the microstructure (grain size and morphology) does not help to explain the differences in hardness measured on the archaeological parts. We need to further investigate the micro and nanostructure to understand this phenomenon.
### 3.3. *Mechanical Properties and Nanostructure: Case Study of the JU88 Alloy*

In this context, for a better understanding of the behaviour of a Duralumin-like German alloy, one alloy (JU88-B3, Table 4) was selected to be studied with a multiscale approach. Its composition is close to a 2017A alloy and was produced at the beginning of WWII (before 1943 [9]). It is therefore well representative of the alloys of this time. However, the microstructural state and mechanical properties of this alloy could have been altered by the history of the material: its use, crash and long natural ageing. Thus, on this as-received archaeological plate, a T4 treatment (see the Section 2.1 above) was carried out to erase the microstructural changes having occurred for the past 80 years, restoring the metallurgical properties close to a manufacturing state. This alloy is compared to its modern equivalent.

#### 3.3.1. Contribution to Strengthening of Microstructure

Figures 4 and 5 respectively show the size of grains and the size of coarse intermetallics. The micrographs of the grains were taken in the rolling plane of the alloy. They show that the grains have an equiaxed shaped in this plan, with different orientations. The micrographs of the intermetallics show rounded bright particles (Al₂Cu imtermetallics) and darker angular particles (AIMnFeSi intermetallics). Their size remains the same after the heat treatment. These micrographs overall show that the microstructure does not evolve with the thermal treatment (see Supplementary Materials, Table S4).

<figure><img src="image_9.png" /><figcaption>**Figure 4.** JU88-B3 sample: (a) before and (b) after receiving T4 treatment—Optical microscopy microstructure observation with polarised light—magnification x10.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 5.** JU88-B3 sample: (a) before and (b) after receiving T4 treatment—SEM microstructure observation with backscattered electrons—magnification ×2000.</figcaption></figure>
### 3.3.2. Contribution to Strengthening of Dislocations

After fine precipitation, dislocation densities are the most important driver of hardening [31]. Examples of images of dislocations observed by TEM in a 2017A alloy and in the JU88 sample at the received state and after T4 treatment are presented in Figure 6. Numerous helicoidal dislocations with their axes following [011]Al can be observed in the sample at received state. These dislocations, mainly visible near the [011]Al ZA, are created during quenching of the alloy [35,36]. After T4 treatment, the dislocations are mainly helicoidal. Their fraction in the alloy is quite low. In the 2017A alloy, very few helicoidal dislocations can be seen, but there are many other types of dislocations.

The dislocation densities of ancient and modern alloys are presented in Figure 7a. The density for a modern alloy was calculated following Equations (1) and (2) to be around $(2.8 \pm 0.2) \times 10^{14} \text{ m}^{-2}$. The as-received ancient alloy has a density of $(3.5 \pm 0.2) \times 10^{14} \text{ m}^{-2}$, whereas after a T4 treatment, the dislocation density is divided by 2 at $(1.8 \pm 0.1) \times 10^{14} \text{ m}^{-2}$. This significant difference can be compared with the dislocation density decrease caused by annealing in an ultra-fine-grained Al-Cu-Mg [41].

The micro-hardness results are presented in Figure 7b. The hardness of the 2017A is measured at $122 \pm 5 \text{ HV}_{0.3}$, and the ancient alloy has a $138 \pm 9 \text{ HV}_{0.3}$ hardness at the as-received state, significantly higher than expected. The application of a T4 treatment induces a hardness decrease of the ancient alloy down to $127 \pm 4 \text{ HV}_{0.3}$, a similar value compared to the modern one and slightly higher to the one expected 80 years ago (115 HV) [30].

<figure><img src="image_11.png" /><figcaption>**Figure 6.** BF-TEM images in [011] Al ZA of dislocations in: (a) 2017A-T4 alloy; (b) as-received JU88-PEC-B2 wreck; (c) JU88-PEC-B2 after T4 treatment.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 7. (a) Dislocation densities; and (b) microhardness values (HV₀.₃) for the ancient German alloy JU88-B3 (AR: as-received; T4: T4 treatment) and the corresponding modern alloy.</figcaption></figure>

The dislocation density $\rho_d$ can be described as a contribution to the yield strength $\sigma_d$ of the alloy, written as in [39–41,46]:

$$ \sigma_d = M \alpha G b \sqrt{\rho_d} \qquad (5) $$

where M is the mean orientation factor, also called Taylor factor, usually taken as 2.6 [39,41] and recently refined as 2.7 for FCC structures [47], $\alpha$ is a constant taken between 0.2 and 0.5 [39–41] and usually taken as 0.3 for aluminium alloys [48,49], G is the shear modulus (here taken as 28 GPa [39]), b is the magnitude of the Burgers vector (equal to 0.286 nm for Al) and $\rho_d$ is the dislocation density.

The calculation of $\sigma_d$ for the as-received and T4 states gives a difference in dislocation contribution to strengthening of about 44%, which means the dislocations contribute almost 1.5 times more to the strengthening after the use and end-of-life of the archaeological part. This most likely indicates that the crashing of the plane placed significant supplementary strain on the material compared to the received state.

### 3.3.3. Contribution to Strengthening of Fine Precipitation

However, the high dislocation density in the as-received sample cannot, by itself, explain the hardness. As is well known, fine precipitation is the first driving mechanism for the hardness of these type of alloys [31].

Representative bright-field (BF) TEM images in conventional mode for the 2017A-T4 and JU88-B3 samples oriented in [011]Al ZA are shown in Figure 8. In all samples, only dispersoids—precipitates from 100 nm to 1 µm long—can be observed in BF mode. The images in BF do not present any trace of coherent precipitation that could have formed during a moderate heat treatment. The alloys most probably only contain clusters and/or GPZ formed during aging at ambient temperature (T4) and which can only be observed with more advanced tools such as high-resolution TEM or atomic probing.
<figure><img src="image_13.png" /><figcaption>**Figure 8.** BF-TEM images in [011]Al zone axis of: (a) 2017A-T4 alloy; (b) as-received JU88-B3 wreck; (c) JU88-B3 after T4 treatment.</figcaption></figure>

### 3.4. Discussion

#### 3.4.1. On the German Manufacturers of Al Products

The PCA of the alloys' elemental compositions (Figure 1) suggests that Germany had a different way to produce its Duralumin and Super-Duralumin alloys than other nations at the time. No clear distinction can be made between 3115 and 3125 alloys at first sight. The Mg content was usually clearly above 1 wt.% for Super-Duralumin and below 1 wt.% for Duralumin in the 1930s and 1940s for the main aluminium producing countries [3,9,29]. However, the German group presents no distinct limit between 3115 and 3125 as far as Mg content is concerned. This slight variation in Mg composition could affect the mechanical behaviour of the alloys, as Mg plays a key role in fine precipitation. The main observation about composition is the high Mn content found in some 3125 alloys in Germany (above 1 wt.%). This high Mn content was indicated in German RLM standards for 3125 alloys [9] which authorised the addition of Mn up to 1.5%. This standard range was not found in other national standards [3,29].
The two German manufacturers studied in this work, Junkers and Focke-Wulf, have different Mn contents for their 3125 alloys. The Junkers samples even exhibit a Mn content out of the RLM standard bounds. As all the analysed planes were made before the change of the RLM standards in 1943 [10], we assume that Junkers and Focke-Wulf had different providers for their aluminium plates. This hypothesis is mainly supported by the difference in wt.% Mn content between FW190 and JU88 3125 alloys and the different Cu/Mg ratios (Table 1 and Figure 2). As said previously, this indicates a different choice of alloying elements. It is noted that all the Duralumin samples coming from the same plane (JU88-A) are very similar in composition, which encourages the hypothesis of a same provider for the overall production of this plane.

Although we do not yet have the proof for such a hypothesis, several archive documents indicate that:

1. Several national companies supplied German aeronautics with aluminium alloys.

2. Links could exist between suppliers and manufacturers.

The biggest metallic materials suppliers were located in two distinct zones: Dürener Metallwerke and Vereinigte Deutsche Metallwerke (VDM), in the Ruhr mineral district; and Aluminium-Industrie Aktiengesellschaft, at the Swiss border. Their locations could have been crucial to providing aircraft manufacturers with aluminium plates [17]. For instance, we have been able to trace back some specific commercial partnerships between VDM and the Heinkel industries, as well as between Farbenindustrie Aktiengesellschaft and an Arado complex [50]. The suppliers of aluminium alloy products for Junkers and Focke-Wulf remain to be identified.

The shapes and sizes of the grains in the plates were observed and measured on the group of German aircraft (Table 4). The grains exhibit various lengths, mostly depending on the thickness of the plate and on the Mn content, but they also exhibit the same equiaxed shape, with L/LT ratios around 1 and L/ST ratios around 2–3. These ratios correspond to the measured ratios in modern alloys, enabling us to assume that in both cases, a recrystallisation treatment was applied. The fact that the modern alloys exhibit smaller grains is probably due to the higher number of rolling passes and the applied conditions of annealing (possibly a slightly lower temperature and/or shorter time of annealing).

### 3.4.2. On the Hardness in Long-Term Naturally Aged Plates

We believe that the peculiar high hardness of ancient Duralumin alloys was not intended during the manufacturing of aluminium plates. Figure 7a shows that in a T4 state, the hardness comes back to a regular value similar to that of a modern plate (from $138 \pm 9 \text{ HV}_{0.3}$ to $127 \pm 4 \text{ HV}_{0.3}$). Several archive documents from the 1935–1950 period describe the usual heat treatment of Duralumin as a quench followed by maturation [9,10,30]. We still have to note that the T3 treatment (pre-straining preceding the maturation) was known and used on 24S-RT in the USA [4] and possibly on 3115 in Germany (by renaming the alloy 3115.5, the 1937 RLM standards indicate "ausgehärtet und nachgerichtet", which could be translated as "hardened and straightened" [9]). But T3 treatment was still at the experimental stage at the time [51], which is why, regarding the period of production and the standards we consider in the following, a usual T4 treatment was applied [52].

All alloys from the sampling group come from crashed aircraft, having thus suffered a supplementary amount of strain. The strain caused by the crash surely comes from the displacement and creation of stress-accommodating dislocations. This participates in the increase in the dislocation density and thus to hardness. Nonetheless, these dislocations alone could not explain the high hardness of Duralumin plates.

By calculating the contribution of each strengthening parameter on an Al-Cu-Mg alloy, Shao et al. have demonstrated that dislocations and precipitation added an equivalent amount to the yield strength [39]. Hardening and strengthening being strongly linked [53], and the composition being the main influential factor in the variance of the group (see Supplementary Materials, Figure S1), the role of micro-alloying elements has to be considered. However, no fine precipitation can be seen in a 3115 alloy by conventional TEM in BF mode,
whether at the T4 or T4 + 80 years state. An analysis of the precipitation at a smaller scale thus remains necessary to understand the role of alloying elements in the high hardness values in Figure 3.

**4. Conclusions**

From different measurements and observations at various scales on ancient aluminium alloys compared to modern ones—including the elemental composition, the hardness and TEM characterization of WWII aircraft fragments—the following conclusions can be drawn:

* German aircraft manufacturers did not share the same providers for their aluminium plates but followed similar aircraft piece manufacturing routines, as evidenced by the wt.% Mn content in 3125 alloys and the shape of the grains. Although the measuring of alloying element content was well known at the time [54], there is no indication that Germany fully controlled the elemental composition of 3115 and 3125 plates. Even if the elemental content was standardised [9,10], several analysed parts could not be identified only by their Mg/Si and Cu/Mg ratios. These are, however, the elements that define which alloy belongs to the 3115 or 3125 designation.

* All Duralumin and Super-Duralumin alloys from several countries, with different compositions, exhibit a higher hardness than the modern 2017A and 2024 reference alloys. A case study on a 3115 German alloy collected on a Junkers 88 wreck crashed in 1943 showed that the influence of the grain size was minimal on this variable. The high rate of dislocations caused by the thermo-mechanical history of the sample contributes to the alteration of the mechanical properties of the alloy. We have not observed fine precipitation (GPZ and/or Cu-Mg clusters) yet. Thus, we did not evaluate the impact of the composition on hardness. However, the elemental composition is a key factor of influence on precipitation sequences and on the overall hardening of the alloy.

To further investigate how alloying elements affect hardness, more TEM observations at high resolution are needed in order to evaluate the impact of Cu-Mg clusters and GPZ on a T4 + 80 years Duralumin alloy.

To be more accurate and confirm our hypotheses, more mechanical testing is required in the future (micro-tensile tests, fracture toughness, etc.).

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10.3390/heritage7120313/s1, Table S1: Sampling group chosen for the study; Table S2a: German and modern Duralumin alloys—SEM microstructure observation (by backscattered electrons)—magnification ×500 and ×1000; Table S2b: German and modern Super-Duralumin alloys—SEM microstructure observation (by backscattered electrons)—magnification ×500 and ×1000; Table S3a: German and modern Duralumin alloys—MO microstructure observation; Table S3b: German and modern Super Duralumin alloys—MO microstructure observation; Table S4: JU88-B3 sample before and after receiving T4 treatment—SEM microstructure observation (by backscattered electrons)—Magnification ×2000; Table S5: Elemental analysis by SEM-EDS and ICP-OES for two pieces from the sampling group studied; Figure S1: PCA loadings plots for the sampling group of the study.

**Author Contributions:** Formal analysis, A.D., L.d.S.M., B.M. and M.B.; Funding acquisition, E.G., C.C., L.R., E.B., C.M., A.B. and M.B.; Investigation, A.D., E.B., L.R., C.D., L.d.S.M., C.M., C.C., A.B., C.E., E.G. and M.B.; Methodology, A.D., E.B., L.R., B.M. and M.B.; Project administration, E.G.; Resources, L.R., C.M. and C.C.; Software, E.B. and C.D.; Supervision, B.M. and M.B.; Validation, L.R., C.M., C.C., A.B., C.E. and E.G.; Writing—original draft, A.D.; Writing—review & editing, E.B., L.R., C.M., C.C., A.B., C.E., E.G., B.M. and M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study has been partially supported by Agence Nationale de la Recherche, France, with the PROCRAFT project (ANR-20-JPIC-0001) under the European JPI-CH Conservation and Protection Call (2020–2024), and by the Occitanie region, the University of Toulouse through the "Défi Clé Sciences du Passé" with the PaérO project (2022–2025). This work is under Creative Commons Attribution (CC BY) license: https://creativecommons.org/licenses/by/4.0/ (accessed on 25 October 2024).
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.

**Acknowledgments:** The authors wish to thank Alfredo Tolley from CNEA CAB, San Carlos de Bariloche, Argentina and Dominique Lamirault from CEMES, CNRS, Toulouse, France for the preparation of TEM samples; Sébastien Joulié from CEMES, CNRS, Toulouse, France for the TEM thickness measurements. They also wish to thank the volunteers of Aérocherche and particularly Gilles Collaveri for providing us with the aircraft artefacts.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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