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This is an author produced version of a paper published in :
International Materials Reviews
Cronfa URL for this paper:
http://cronfa.swan.ac.uk/Record/cronfa28287
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Paper:
Riva, S., Yusenko, K., Lavery, N., Jarvis, D. & Brown, S. (2016). The scandium effect in multicomponent alloys.
International Materials Reviews, 61(3), 203-228.
http://dx.doi.org/10.1080/09506608.2015.1137692
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The Scandium Effect in Multicomponent Alloys
Sephira Riva*
1
, Kirill V. Yusenko
1
, Nicholas P. Lavery
1
, David J. Jarvis
2
,
Stephen G.R. Brown**
1
1 College of Engineering, Swansea University, Bay Campus, SA1 8QQ, Swansea, UK
2 European Space Agency, ESTEC, Keplerlaan 1, Noordwijk, NL
* 839245@swansea.ac.uk
** S.G.R.Brown@swansea.ac.uk
Abstract
Despite its excellent elemental properties, lightweight nature and good alloying potential,
scandium has received relatively little attention in the manufacturing community. The
abundance of scandium in the Earth's crust is quite high. It is more abundant than silver,
cobalt, lead and tin. But, because scandium is so well dispersed in the lithosphere, it is
notoriously difficult to extract in commercial quantities - hence low market availability and
high cost. Scandium metallurgy is still a largely unexplored field - but progress is being
made. This review aims to summarise advances in scandium metallurgical research over the
last decade. The use of scandium as a conventional minor addition to alloys, largely in
structural applications, is described. Also, more futuristic functional applications are
discussed where details of crystal structures and peculiar symmetries are often of major
importance. This review also includes data obtained from more obscure sources (especially
Russian publications) which are much less accessible to the wider community. It is clear that
more fundamental research is required to elevate the status of scandium from a laboratory-
based curiosity to a mainstream alloying element. This is largely uncharted territory. There is
much to be discovered.
Keywords Scandium, Alloys, Intermetallics, Phase diagrams, Mechanical properties
1. Introduction
Since the first days of powered flight, aircraft designers have focused on achieving minimum
weight, both in airframes and in propulsion systems. Historically, the selection of materials
was driven mostly by strength/weight ratio considerations. However, current design criteria
have strict requirements for processing methods, safety and material specifics.
Aluminium-lithium alloys, for example, were developed in Russia in the early 1960’s (the so-
called 1420 alloys). These alloys required heat treatment in order to increase their hardness
and mechanical properties, but their composition, combined with new welding technology,
allowed a decrease in aircraft mass of up to 20 %. Aluminium-beryllium alloys were also
proposed to provide affordable alternatives to resin matrix composites in critical airframe
components.
1
In both cases, the danger involved in applying such reactive materials limited their active use.
However, the introduction of Sc into the 1420 alloy, leading to an increase in yield strength
of 20 – 25 % while decreasing rocket bodies mass by 10-15%, opened the doors for wider
application of scandium in the aerospace industry.
Constructional parts of the giant airplane “AN-124” were produced from a modified variant
of the Sc-containing 1420 alloy, the so-called 1421 alloy (according to the Russian
classification).
Al – Cu – Li highly corrosion resistant alloys with zirconium and scandium additions were
employed for cryogenic fuel tanks for spacecraft and aircraft. Tests proved their applicability
for fuel tank applications, which decreased rocket mass by 35 %.
2
The last ten years have brought about a renewed interest in scandium as a key element for the
development of lighter materials for aerospace applications. A recent NASA report describes
the use of Al─Sc-based alloys for fuel tanks and air frames, applications in which low weight
and chemical stability to hydrogen peroxide are critical. Moreover, the recent ESA Grand
Challenge, identified by FTAP in 2012, states that lightweight, stable, high-stiffness
structures are a top priority for the Agency.
3
As such, high performance scandium alloys could find numerous applications in primary
satellite structures, cryogenic tanks, solar panel substrates, rocket nozzles and thrusters, re-
entry hot structures, Mars rovers, electronic packaging and optical benches, as well as in
armour development.
4
The last detailed evaluation of Al-based scandium alloys and compounds was published over
10 years ago: the numerous papers published since then have mostly confirmed known
results. Conversely, the purpose of this review is to collect and critically analyse recent
experimental data concerning scandium-based alloys and intermetallics, as well as rarely
accessed data in PhD theses and journals.
1.1 Scandium properties
Scandium was first discovered in 1879 by Lars F. Nilson in Uppsala, Sweden. However, its
limited availability and the intrinsic difficulties with its extraction delayed its study until fifty
years later. It was only in 1937 that the first pound of pure elemental scandium metal was
produced (Fisher et alia) by performing an electrolysis of molten scandium, lithium and
potassium chlorides in a graphite crucible with a tungsten wire, using molten zinc as the
working electrode. The first data on the investigation of scandium systems with other metals
and on the synthesis of scandium compounds were published in the early sixties.
Scandium is the first d-element, as well as being a member of the family of rare earths. Its
peculiar features link it to the lanthanides and yttrium. In particular, scandium is a light metal
with a density comparable with aluminium (2.989 gcm
-3
) and a high melting temperature of
1814 K (close to the melting point of iron).
1.2 . Elemental scandium under high-temperature and high-pressure
Elemental scandium has been studied under high-temperature and very high-pressure. Under
ambient pressure Sc has two forms: the low-temperature (α) hcp form, stable below 1337 °C,
transforms into a high-temperature (β) bcc structure with a melting temperature of 1541 °C.
Transition between alpha and beta phases have been investigated in situ using time-of-flight
neutron diffraction (figure 1.2.1).
5
Figure 1.2.1. Crystal structure of room temperature hcp α (Sc) along c-axis (left) and high-
temperature bcc β (Sc) (right).
5
Figure 1.2.2. Crystal structure of high-pressure tetragonal incommensurate Sc-II phase along
c-axis. The figure shows two crystallographically independent Sc atoms forming host
framework (yellow) and guest atoms (green).
5
Figure 1.2.3. Crystal structure of high-pressure hexagonal Sc-V phase along a-axis (left) and
c-axis (right).
5
The high-pressure behaviour of elemental scandium has been intensively investigated during
the last two decades using diffraction and magnetometry in diamond anvil cells up to 297 and
74 GPa respectively.
6
Under compression Sc shows five high-pressure modifications. None
of these can be recovered after decompression. Schematically, high-pressure properties of Sc
can be summarised as follows:
