Verlag des Forschungszentrums Jülich
JUEL-4281
Roitsch, Stefan
Microstructural and Macroscopic Aspects of the Plasticity of Complex Metallic Alloys
111 S., 2008
Plastic deformation refers to the irreversible shape change of a piece of solid matter
due to an externally applied force. In contrast to elastic deformation, plastic deformation
is permanent and corresponds to a relative displacement of parts of the deformed
material.
The plasticity of structurally simple materials has been investigated since the 1920s. It
was proposed by Orowan (1934), Taylor (1934), and Polanyi (1934) that onedimensional
defects, termed dislocations, are the carriers of plastic deformation in most
crystals. Motion of dislocations in atomic planes causes successive opening of atomic
bonds and is therefore an energetically favoured process of plastic deformation in
contrast to simultaneous opening of all bonds in one plane.
In the last decades, the plasticity of a multitude of metals and intermetallic compounds
has been investigated and a variety of models basing on the process of dislocation
motion has been proposed. Pioneering works on different types of dislocations and their
movement were established, for example, by Peierls (1940) and Nabarro (1967).
However, the understanding of the plastic-deformation behaviour of matter is still
limited to structurally simple materials. Further progress in crystal plasticity is
necessary in order to comprehend deformation mechanisms of more complex phases.
The class of complex metallic alloys (CMAs) comprises systems with giant unit cells
containing up to more than a thousand atoms per cell (Urban and Feuerbacher, 2004).
Despite the fact that CMAs have been studied since several decades in crystallography,
physical properties and especially the plasticity of these phases are essentially
unexplored. This fact is astonishing since the plastic deformation behaviour of CMAs is
of particular interest: In these systems deformation mechanisms known from
structurally simple materials are prone to failure. Due to the large lattice parameters
(which usually exceed 10 A in CMAs), perfect dislocations would require unphysically
large elastic line energies. New concepts of microstructural processes are expected to
appear in order to get effective and energetically favourable deformation mechanisms.
Indeed, novel mechanisms were revealed in µf-Al-Pd-Mn (Klein et al., 1999), Al13Co4
(Heggen et al., 2007), and c2-Al-Pd-Fe (Heggen, 2003).
The intention of the present thesis is to gain comprehensive insight into the
deformation behaviour and underlying mechanisms of CMA phases. For this purpose
three selected phases, µ-Al-Mn, Mg32(Al,Zn)49, and s-Al-Mg are investigated. The
plasticity of these materials is examined for the first time. The work deals with three
different crystal lattices, body-centred cubic, face-centred cubic, and hexagonal closepacked.
In conjunction with investigations on orthorhombic CMAs reported in the
literature (Klein et al., 1999, Feuerbacher et al., 2001, Feuerbacher and Caillard, 2004,
Heggen et al., 2007), the most important crystal lattices in this materials class are
covered.
The combination of macroscopic and microstructural investigations allows for a
versatile and detailed view on the plasticity of the examined phases. The use of highquality
single-crystalline sample materials, grown in the frame of the present thesis,
ensures that effects of impurities, secondary phases, and grain boundaries can be
excluded and accordingly, only the intrinsic mechanical properties are examined.
The studied alloys are brittle at room temperature but show a brittle-to-ductile
transition at elevated temperatures between 65 and 82 % of the respective melting
temperature. The stress-strain behaviour exhibits remarkable features like high fracture
stress or pronounced yielding behaviour. Thermodynamic activation parameters of the
deformation processes were analyzed for all three materials.
The microstructural deformation behaviour of the two phases µ-Al-Mn and
Mg32(Al,Zn)49 was successfully elaborated by means of transmission electron
microscopy (TEM). Distinct differences to mechanisms known from structurally simple
materials are observed. Both phases possess deformation mechanisms which are
primarily based on dislocation climb. The interaction of different involved dislocation
types by means of a chemical stress is a basic feature in the deformation processes of
both CMAs.
The first chapter introduces the materials class of CMAs. Basic structural
characteristics as well as the most common types of local order are described. The
relations between these phases with structurally simple and quasicrystalline materials
are discussed. One important example of a novel type of defect found in CMAs, the
metadislocation, is briefly revisited.
The production of high-quality single-crystalline material is of decisive importance
for reliable results of deformation experiments. The basics of single-crystal growth, the
growth techniques applied, as well as details on the phase diagrams of the investigated
materials are given in chapter 2.
In chapter 3 the fundamentals of crystal plasticity and the concept of dislocationmediated
deformation are reviewed. The theory of thermal activation is outlined and
experimental procedures employed in this study are described in detail.
Chapters 4, 5, and 6 address the phases µ-Al-Mn, Mg32(Al,Zn)49, and s-Al-Mg,
respectively. The structure of the respective phase is illustrated in each chapter and
results of macroscopic as well as microstructural investigations are presented and
discussed.
In chapter 7, finally, a comprehensive discussion of the macroscopic and
microstructural deformation behaviour of the investigated phases is presented. The
results are compared with investigations on other CMAs in order to gain an overview
and to find possible general characteristics in the plasticity of this materials class.
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