Complementarity of atom probe, small angle scattering and differential

scanning calorimetry for the study of precipitation in aluminium alloys

Frederic De Geuser1, a*, Thomas Dorin1,2,b, Williams Lefebvre3,c,

Baptiste Gault4,d and Alexis Deschamps1,e 1SIMaP, Grenoble INP – CNRS – UJF, BP. 75, 38402 Saint Martin d’Hères Cedex, France

2Constellium, Voreppe Research Centre, BP 27, 38341 Voreppe Cedex, France

3Groupe de Physique des Matériaux, UMR CNRS 6634, Université de Rouen, BP 12, Avenue de

l’Université, 76801 Saint Etienne de Rouvray, France

4Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK

afrederic.de-geuser@simap.grenoble-inp.fr,

bthomas.dorin@simap.grenoble-inp.fr,

cwilliams.lefebvre@univ-rouen.fr,

dbaptiste.gault@materials.ox.ac.uk,

ealexis.deschamps@simap.grenoble-inp.fr

* corresponding author

Keywords: Atom probe tomography, small-angle scattering, DSC, precipitation.

Abstract. Two examples of precipitation studies (in Al-Li-Cu and Al-Li-Mg alloys) are shown to

demonstrate the complementarity of atom probe tomography, small-angle-scattering and differential

scanning calorimetry for precipitation studies. It will be used to unravel an unexpected two-step

precipitation behavior of T1 in Al-Li-Cu and to ascertain precipitates size in Al-Li-Mg. through a

model free comparison between atom probe and SAXS.

Introduction

Transmission electron microscopy (TEM) plays a central role for the study of nanometer size

precipitates in structural hardening alloys. It is due to its unique properties combining an excellent

spatial resolution for direct space imaging of small particles, together with diffraction analysis

providing information on morphology, crystallography, distribution, size… To a lesser extent,

analytical TEM can provide insights of the composition of precipitates. In many practical cases

encountered in age hardened aluminium alloys, however, the chemical information of the TEM is

too limited. Furthermore, while the field of view of TEM usually contains many precipitates, it may

not always be sufficient to gather significant average values of size density, especially when the

dispersion is relatively large. A third possible limitation of TEM as a tool for precipitation studies is

the lack (or very limited possibilities) of in situ measurements which limits the investigation of

precipitation kinetics to interrupted ageing measurements.

While TEM will most certainly remain the characterization method of choice for many

precipitation studies, this paper will give examples of studies where the combined use of small-

angle X-ray scattering (SAXS), atom probe tomography (APT) and differential scanning calorimetry

(DSC) can overcome the three considered limitations of TEM, namely limited atomic scale

analytical information, limited probed size, and lack of kinetics study capabilities. In particular, we

will show that a combination of techniques is a way to make the best use of each technique

advantages.

These aspects will be illustrated by two aluminium alloys examples. First, we will show with an

Al-Li-Cu alloy that, although DSC can be used to quantify the volume fraction of precipitates

formed during an ageing process, in situ SAXS reveals that interpreting the DSC signal may not be

as straightforward as thought at first. Then, the example of an Al-Li-Mg alloy will be used to show

the complementarity between SAXS and APT and how they can be quantitatively compared.

Materials Science Forum Vols. 794-796 (2014) pp 926-932© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.794-796.926

Figure 1: DSC thermograms for 3 Al-Li-Cu samples at different stages of precipitation (curves

are shifted from 0.05 W/g for clarity). Blue: T3 sample (no precipitates). Red: thin T1 precipitates

only. Green: thicker T1 precipitates. Also shown are SAXS images from selected temperatures

during heating rate in the DSC.

Experimental details

The two alloys studied were by Constellium Voreppe Research Centre. The first one is an

AA2198 alloy (Al-3wt%Cu-1wt%Li as main alloying elements) and the second one is a model Al-

Li-Mg alloy containing Al–1.85wt.%Li–4.94wt.%Mg.

The SAXS was performed either on a lab setup mounted on a rotating anode with a Cu target,

working at 8keV or at the D2AM-BM02 synchrotron beamline at the ESRF (Grenoble) at 7.7keV.

In both case, we used a CCD camera as 2D detector. The images were corrected for read-out noise,

distortion, flat-field and background noise. The data was normalized using a reference glassy carbon

sample as a secondary standard for absolute calibration. The SAXS samples were prepared by

mechanically grinding the sheets of alloys to a thickness of about 70µm and mirror-polished.

The APT specimens were mechanically cut to rods of 0.3mmx0.3mm cross section and

electropolished into sharp tip needled (radius of curvature < 100nm) by in 2% perchloric acid (70%)

in 2-butoxyethanol at 15 V at room temperature. Analyses were performed on high voltage energy

compensated instruments at 40K (ECOTAP or LEAP 3000X Si).

For DSC, the samples were mechanically cut into squares of 0.5mm thickness and about 4mm

size. The DSC analysis was performed in a Perkin Elmer Pyris Diamond instrument with high purity

nitrogen used as purge gaz and pure aluminium crucibles. The heating ramp rate was 50 K.min-1

from -40°C to 520° so as to avoid any transient effect from the stabilization of the calorimeter.

Materials Science Forum Vols. 794-796 927

Combining DSC ramp heating and in situ SAXS on an Al-Li-Cu

The 2198 Al-Li-Cu alloy is a high strength structural hardening alloys which is mainly strengthen

by the formation of the T1 phase which precipitates as thin platelets laying on the {111} planes of

the aluminium matrix. Their aspect ratio (diameter on thickness) can be as high as 100. They have

been shown [1,2] to consist of only 1 unit cell of the T1 structure in thickness (1.3nm) for an ageing

temperature of 155°C [3]. Thickening of T1 can be obtained at higher temperature. SAXS has been

shown to be able to monitor the morphology (thickness and length) of the precipitates in situ during

the precipitation process [4]. For a quantitative measure of the precipitate volume fraction, however,

another global technique is required. Integration of heat-flow exothermic precipitation peak

obtained by DSC is a typical way of quantifying the fraction of precipitates. Figure 1 shows the heat

flow obtained by ramp heating in the DSC on 3 different alloys. The first heat flow curve (blue) was

obtained on the as received T3 sample containing no T1 (although it is likely to contain some room

temperature clusters). The second curve (red) has been obtained on an alloy aged for 18h at 155°C,

containing only 1-unit cell T1 precipitates. The third one (green) has been obtained after a two-step

ageing, 18h at 155°C followed by 70h at 190°C, where the T1 precipitates have undergone

significant thickening.

Many heat flow events are present on these curves, which are difficult to interpret. The

endotherm A is likely the dissolution of room temperature clusters/GP zones. The exotherm B is

likely to concern T1, as it is very intense in the T1-free sample (so that T1 could form during the

DSC ramp. The E endotherm can be safely associated with the dissolution of the existing phase.

Events C and D are more complicated to understand. To unravel the precipitation processes

involved during the DSC run, we have reproduced the DSC ramp heating during an in situ SAXS

experiments. The result is a continuous quantitative description of the precipitates morphology.

Selected SAXS images corresponding to the different events are shown on Fig. 1. They show the

typical star pattern [4] corresponding to T1 precipitates. The length of the streaks is inversely

proportional to the thickness of the precipitates and their orientation is characteristic of their habit

planes.

The SAXS images confirm that the B exotherm corresponds to T1 precipitation. However, they

also indicate that at the end of event B the precipitates still have the same thickness (single unit cell)

since the streaks, while more intense, have the same length. Only after event C do they thicken. This

is further confirmed by the fact that the single-cell T1 containing sample do not show event B but

show event C. The thick T1 containing sample does show the C event, but it is part of a wider

globally endothermic event so that it is likely that it can be attributed to a thickening of the

precipitates during their dissolution. This demonstrates that the T1 precipitates form through a two-

step process with only lengthening in a first step, while the thermally activated thickening step

(probably associated with growth ledge nucleation and long range diffusion) occurs at higher

temperature. This is consistent with the believed precipitation scenario occurring during isothermal

ageing [5]. A detailed description of this study will be published elsewhere [6].

Combining SAXS and APT to quantify precipitates size

This second example uses two samples of the same Al-Li-Mg which undergone different heat

treatments to demonstrate the complementarity of APT and SAXS. The two alloys have been aged

for 24h at 120°C and 8h at 150°C respectively. Their microstructure have been characterized by

TEM (not shown here) and they consist only in a fine homogeneous dispersion of ’ (Al3Li but

containing Mg) spherical precipitates with a large volume fraction [7]. The radii of the precipitates

for the two samples were estimated by TEM at 3nm and 4.8nm respectively. SAXS images were

recorded for these two samples and are shown on Fig. 2. The presence of a maximum on both

curves is related to interparticles interaction and confirms the high volume fractions [8]. Classical

SAXS data processing [9] can extract the size of the precipitates and the resulting sizes are

928 Aluminium Alloys 2014 - ICAA14

measured to be 3.04nm and 4.97nm for the 24h at 120°C and 8h at 150°C respectively, showing

excellent agreement with TEM.

Figure 2: SAXS intensity from the Al-Li-Mg samples after ageing 8h at 150°C (blue) and 24h at

120°C (red).

To extract volume fraction from the SAXS curves, however, the chemical composition of the

precipitates (or rather their electronic density contrast with the matrix) must be known. For this we

turn to APT. Figure 4 presents a reconstruction of the alloy aged 8h at 150°C. It shows that,

although the probed volume is vastly smaller than that explored by a single SAXS measurement, it

still contains a fair number of precipitates. From this kind of analysis, we can extract the

composition of the precipitates and that of the matrix. With this information at hand, we can now

extract the volume fractions from the SAXS curves. It is shown to be 21.2% and 20.4% for the 24h

at 120°C and 8h at 150°C respectively.

Figure 3: APT reconstruction obtained on the Al-Li-Mg aged 8h at 150°C. The ’ are outlined

by an isoconcentration surface.

While we now have all the information we seek, it could still be of interest to check whether the

large APT reconstruction we have at our disposal can confirm the size of the precipitates. There are

multiple ways of measuring a precipitate size in APT but we will suggest a rarely used technique (to

Materials Science Forum Vols. 794-796 929

that purpose) which has the huge advantage of enabling a direct, model-free comparison with the

independent SAXS experiment. It consists in calculating the so-called radial distribution function

[10,11] between the solute elements and estimating the size of the objects. The other direct

advantage of such a method is that it gives an average value for all the precipitates in the volume in

one go. While doing this has already been suggested in the APT community [12], it is far from

certain that every APT users realize how closely this formalism is associated to that of small angle

scattering, with knowledge base dating back to the end of the 19th

century and Lord Rayleigh, with

application to precipitation in aluminium alloys (GP zones…) in the 1950s.

Figure 4 shows the correspondence between the small-angle signal I(q) and the correlation

function (r), which are related through Fourier transformation:

(1)

(r) is nothing else than the RDF g(r) (or rather (r)=g(r)-1 since the RDF tends to zero towards

infinity). Its value for a spherical precipitate is well known (see e.g. [13]) and is shown on Figure 4.

It is a simple 3rd

order polynomial which initial slope is -3/(4R), which can be easily used to extract

the average size of the precipitates.

Figure 4: a) simulated SAXS curves from a sphere (and a size dispersed distribution of spheres).

b) corresponding real space autocorrelation function. c) illustration of the aspect of this curve in a

real experiment and how it relates to RDF calculated in APT.

It is of value however to also introduce the function p(r)=(r)r2. This function is called distance

distribution function in the small-angle scattering community and corresponds to the density of

probability of finding a given distance r in the object. It goes through a maximum which is very

close to the radius of the sphere in the case of spherical objects, giving a direct lecture of the size of

the particles. It also amplifies high order oscillation on the correlation function which may be

interesting for studying interparticles interaction. In order to compare directly SAXS and APT we

plot on the same graph the p(r) function obtained from APT, and that obtained from SAXS through

Fourier transform (eq. 1). The results are shown on Figure 5. While the agreement is fair (the sizes

are reasonably correlated to that obtained in SAXS and TEM) there is a consistent 15% error on the

position of the maximum. It is not unreasonable to attribute this to atom probe reconstruction

artefacts since, while the reconstruction parameters have been correctly estimated [14], the

precipitates have a low evaporation field resulting in a local contraction of the atoms. We have

attempted to correct the APT p(r) by simply rescaling r by a factor 1.15. The results is shown on

figure 5c and 5d for the 8h at 150°C sample. The agreement is now close to perfect, both in (r) and

in p(r) and the curves are even still in agreement while in the “large” r regime corresponding to

interparticles interactions which is of potential great interest for understanding of the structure factor

which is very difficult to interpret in SAXS for lack of real space data.

930 Aluminium Alloys 2014 - ICAA14

Summary

By using two examples of aluminium alloys, we have shown the potential benefits of combining

DSC, APT and SAXS for precipitation studies. SAXS combined whith DSC was shown to unravel

complicated precipitation processes in Al-Li-Cu alloys, while APT was shown to be the perfect

complement of SAXS, feeding its interpretation model with the missing chemical information,

allowing real space checking of the expected size, volume fraction, and precipitates interaction. A

method has been introduced for a direct, model-free comparison between APT results and SAXS

results.

Figure 5: model free comparison between SAXS and APT (and TEM). a) p(r) for the 8h at 150°C

alloy. b) p(r) for the 24h at 120°C. c) rescaled (r) for the 8h at 150°C and d) rescaled p(r) for the

8h at 150°C. The size determined by TEM are shown as dashed lines.

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