The Influence of Trace Elements (In, Sn) on the
Hardening Process of Al–Cu Alloys
Frank Lotter,* Danny Petschke,* Torsten E. M. Staab, Urban Rohrmann,
Thomas Schubert, Gerhard Sextl, and Bernd Kieback
Adding trace elements (Cd, In, Sn) to Al-Cu-based alloys can significantly
improve their strength by the growth of small and finely distributed
precipitates. However, the underlying atomic mechanisms of their nucleation
are so far only superficially understood. We follow the precipitation process,
that is changes in the microstructure, by different methods: differential
scanning calorimetry (DSC), giving information on formation and dissolution
of precipitates, 3D atom probe tomography (3DAP), giving information on
size and density of precipitates and finally, positron annihilation lifetime
spectroscopy (PALS), being sensitive especially to quenched-in vacancies and
their interaction with alloying elements. By the use of these complementary
methods we obtain information on vacancy binding to the alloying elements
and also on structure, kind and distribution of precipitates while correlating
this with hardness measurements.
The alloy system Al-Cu has important applications as high-
strength light-weight material for the fuselage in aviation (Al-Cu-
Mg, Al-Cu-Li) and as cast-alloys (Al-Si-Cu) for engine blocks in
the transport sector.
Without the second alloying element, the
structure-property relationship is to a large extent determined by
size, distribution and the crystal structure
of the main hardening precipitates, that is,
Guinier-Preston zones (GP-II)/
However, adding trace elements like
cadmium (Cd), indium (In) or tin (Sn) in
small amounts of 100–500 ppm to these
alloys can signiﬁcantly improve their
strength during artiﬁcial aging at moderate
C), while aging at
room temperature is largely suppressed.
This has been discovered already at the end
of the 1940th
and was more extensively
following decades several mechanisms have
been proposed: see for example Ref. .
Nevertheless, the underlying atomic mech-
anisms of the nucleation of just
itates being ultra-ﬁnely distributed have
been even 50 years later only superﬁcially
and are still nowadays under debate.
The solubility of those elements in aluminum is very low. For
Sn the maximum solubility lies between 0.026 at.%
For In Swanson et al.
found a solubility of 0.02 at.
% at 600
C, whereas Kemerink et al.
determined values about
two times larger.
Since in aluminum-based materials the formation of
precipitates and, thus, their increasing strength (age-hardening),
is vacancy-diffusion-controlled during or immediately after
quenching following the heat treatment, a high vacancy-binding
energy of trace elements must have a decisive inﬂuence on the
Hence, we studied the inﬂuence of the prototypical trace
elements Sn and In on the nucleation and age-hardening
behavior, that is, strength of Al-Cu-based alloys from elements
with high purity. We employed different methods previously not
used in this combination. Thus, we can take a ﬁrst step to reveal
the complex interplay between alloying and trace elements with
quenched-in vacancies that survived the heat treatment.
Three dimensional atom probe tomography (3DAP) is employed
as a suitable method to image nanometer-sized precipitates and
investigate their spatial correlation in 3D space. Differential scanning
calorimetry (DSC) gives information on the formation and re-solution
of precipitates, while positron annihilation lifetime spectroscopy
(PALS) À sensitive to quenched-in vacancies À will show differences
between the Al-Cu alloys with and without trace elements.
Based on a better knowledge of the role of trace elements in
their interplay with vacancies during nucleation and growth of
F. Lotter, D. Petschke, Dr. T. E. M. Staab, Prof. G. Sextl
Department of Chemistry
LCTM Roentgenring 11
D-97070 Wuerzburg, Germany
Dr. U. Rohrmann
Fraunhofer Project Group IWKS
Rodenbacher Chaussee 4
D-63457 Hanau, Germany
Dr. T. Schubert, Prof. B. Kieback
D-01277 Dresden, Germany
Prof. G. Sextl
D-97082 Wuerzburg, Germany
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssa.201800038.
Aluminum Alloys www.pss-a.com
Phys. Status Solidi A 2018, 215, 1800038 © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1800038 (1 of 7)