Mechanical Properties of Metals and Composites

1. MATERIALSPHENOMENA
183
5.7. Mechanical Properties of Metals and Composites
5.7.1. Introduction
Topics such as elastic properties of materials, fracture
toughness, fatigue crack propagation, plastic yielding at
room temperature, at elevated temperatures, creep,
viscous flow etc. all belong to the concept of “Mechanical
Properties”.
Furthermore, several metal families are involved, of which
the main ones are those based on steel, on aluminium, on
copper, on titanium and on magnesium. The number of
researchers working in all these fields, and the number of
industries involved, is vast. In this text, focus will be main-
ly on metals and on plastic properties at room tempera-
ture, including strength and ductility. Even then the varie-
tyofactivities,needsandpotentialitiesisgreat.Thepresent
discussion is written with an eye to the future, and will
therefore take the future needs of society as its starting
point, rather than the present state of the art.
5.7.2. Future Needs of Society Regarding Materials for
Structural Applications
(a) Weight and energy saving
By “structural applications” we understand all applica-
tions for which the main function of the material is to
carry a load. In the transport sector (cars, trucks, trains,
aircraft), weight savings will become increasingly impor-
tant, because they are intricately connected to energy
savings by reduction of fuel consumption. But there are
other sectors as well for which weight savings are impor-
tant. For examples, by using lighter but stronger materials,
it may be possible to make bridges wider without having
to replace the foundations. Not surprisingly, the most
stringent demands for weight reductions come from
expensive products, such as sporting goods for top-level
sports, and of course aerospace.
In the decade to come, car body parts from mild steel will
partly be replaced by aluminium alloys, which on their
turn may be replaced later by special high-strength
magnesium alloys. Reinforcement parts from mild steel
are already being replaced by parts from high-strength
steels, allowing considerable weight savings.
The second decade to come will see further weight reduc-
tions by the replacement of some steel parts by carbon-
fibre reinforced plastic (composite material) or by alu-
minium-based metal matrix composites. Parts from high-
strength steel may be replaced by high-strength titanium
alloys.Finallymanypartsfromcarbonsteelwillbereplaced
by stainless steel, which not only lasts longer but also has
a higher strength.
In the hulls of recent military aircraft, the share of alu-
minium is reduced to as less as 16% and steel to 6%. They
have been replaced by advanced Ti-alloys and by polymer-
based composite materials. In the civilian aircraft indus-
try, evolution is much less drastic although the evolution
is the same.
(b) Micro- and “nano”-technology
The importance of structures and devices with sizes in
themicrometre-rangesuchasinterconnects,microvalves,
microswitches, often also deposited on Si substrates, will
become more important in the future. (This field is often
reckoned to belong the “nanotechnology” field, although
its scale obviously is that of micrometres). State of the
art is that it is now possible to make such structures or
devices. Optimization of properties has just begun. More
focus will have to be given in future to the mechanical
behaviour of these structures or devices. In interconnects
for example (material: copper or aluminium), the lifetime
and functionality are greatly affected by the residual
stresses present. The latter depend on the processing of
the component, but also on the material properties of the
interconnect, such as elastic constants, and especially the
lattice misorientations at grain boundaries. The study of
thisrequiresadvancedelectron-microscopicaltechniques
which allow for the simultaneous acquisition of topologi-
cal and crystal texture data, combined with advanced
material models such as crystal-level elasto-plastic finite
element codes. In micromachines (material: often sili-
con), the grain boundaries are equally important, as they
are potential sources of stress concentration possibly
leading to premature failure of the device.
P. Van Houtte | Catholic University Louvain, Louvain, Belgium

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MAX-PLANCK-INSTITUTFÜRMETALLFORSCHUNGSTUTTGART
5.7.3. State of the Art and Research Needs
(a) Steels
Fig. 5.7 shows the combinations of strength/ductility
which are available today for steel sheet for car bodies. The
two material families at the right (TRIP, high-strength
Dual Phase) are still under development. Such steels are
very complex; they contain several phases; the ductility is
enhanced by special effects which strongly increase work
hardening, such as transformation-induced plasticity.
Fundamental research is needed in order to obtain even
better materials, for which the bands are shifted either to
the top or to the right. These materials will probably
be multiphase materials as well, or materials with a
nanostructure, or a combination of both. For example, it
is conceivable to develop pearlitic steels with interlamellar
distances in the lower nanometre range, which will
probably have superior properties due to the effect of the
lamellar structure on both strength and work hardening.
Other types of steels which would also exploit the “lamellar
effect” are also conceivable.A discussion on the scientific
issues related to multiphase alloys is given below.
(b) Aluminium alloys
The art of using several alloying elements in order to ob-
tain much higher strength in aluminium alloys is well de-
veloped. One of the most interesting ways to achieve this
is by precipitation hardening. This “art” is now evolving to
become a “science” by doing extensive studies on these
precipitates, their crystal structure, the degree of their co-
herence with the matrix, their stability. This research will
lead in the decade to come to further substantial improve-
ments of these alloys. One of the questions to be addressed
is the study of the stability of age-hardened alloys (an appli-
cation of precipitation hardening) at temperatures in the
range of 150°C - 200 °C. High-strength aluminium alloys
which would be stable in that range would allow for impor-
tant weight and energy savings in aerospace applications.
Many advanced aluminium alloys are also multiphase
alloys. See the discussion on multiphase alloys below.
For the rest, more work has to be done on developing reli-
able constitutive models to be used in design applications.
For some alloy systems, wide-range models exist for work
hardening as a function of strain rate and temperature [1],
but not for all alloy systems. Moreover, the existing model,
how complex it may appear [1], should still be extended to
include anisotropy and strain path effects.
(c) Titanium alloys
Aerospace industry has developed some truly high-
strength Ti-based alloys with very interesting microstruc-
tures (some of them lamellar). Some of these develop-
ments are unfortunately not in the public domain (military
secrets). Companies which build both military and civil-
ian aircraft can of course apply these alloys also in civilian
application, which cannot be done by companies which
only build civilian aircraft. From a more scientific point of
view, many issues remain unresolved, again because not
enough is know of the behaviour of multiphase materials
at the micro-to nanometre scale. (See the discussion on
multiphase materials).
(d) Copper alloys
The knowledge acquired for other metals (see above) would
be used to develop copper alloys which combine a high
electricalorthermalconductivitywithahighstrength,which
might have very important technological applications.
(e) Composites
Composite materials, either based on a polymer-matrix or
on a metal matrix, and reinforced by fibres have a high po-
tential to develop materials with an extremely favourable
ratio between strength and weight. The state of the art is
there that these materials can be produced, but that pro-
cessing still needs a lot of work to improve. The true under-
standing of the mechanical properties is now emerging,
but still has to be done in many cases.Again they will very
much depend on the interaction between phases, devel-
opment of stresses, and of course on the understanding
of the behaviour of the phases themselves. In view of the
high potential of these materials, a strong effort for further
fundamental research to raise it to the same scientific
level as the research on metals, is imperative.
Fig. 5.7. Combinations of strength and ductility (elongation) for various
steel families.
200 300
0
10
20
30
40
50
60
400 500 600 700 800 1000 1100 1200
BH
P-Alloyed
DP
TRIP
MA
IF HSS
Tensile Strength (MPa)
ElongationA80(%)

3. MATERIALSPHENOMENA
185
5.7.4. Scientific Trends
(a) Multiphase and nano-structured materials
Before the material scientists at steel companies, alumin-
ium companies etc. can really design new materials which
derive a high strength from either a multiphase structure
or a nanostructure, the institutions for fundamental mate-
rials research must provide answers to a certain number of
questions.A few examples:
• By which dislocation mechanisms is work hardening
achieved in the individual phases of multiphase materials?
• What are the relations between morphology (e.g. lamel-
la orientations) and crystallography at the time that the
microstructure is first formed?
• How do the phases interact with each other during plas-
tic deformation?
• To what extent is the work hardening pressure depen-
dent?(itmightberelevantincasesforwhichphasetrans-
formations may contribute to plastic deformation)
• Down to which grain size/interlamellar distance would
dislocation glide remain the dominant mechanism of
plastic deformation? By what is it replaced in the very
low nanometre range? How does this affect work hard-
ening and resistance against crack propagation?
• What are the glide systems in phases like cementite,
bainite, martensite? What is the values of the critical
resolved shear stresses?
• Why can the fracture of cementite upon plastic defor-
mation apparently be avoided in certain materials?
• Whichphasestressesdevelopinthevariousphases,how
can they be measured, how can they be modelled? How
do they affect ductility and strength?
• How is the thermodynamic equilibrium of the various
phases (some of them metastable) affected by the pres-
ence of high stresses and high dislocation densities after
plasticdeformation?(importantquestionastothestabil-
ity in time of properties of a material strengthened by
work hardening).
(This list is not exhaustive). To be able to answer these
questions, advanced experimental techniques have to be
developed, difficult measurements have to be carried out,
and the best “tools” of fundamental material physics need
to be used. Some questions will require the use of molecu-
lar dynamics-type approaches, or, on a slightly more coarse
scale, dislocation dynamics. Scale transition schemes will
have to be used to translate the conclusions into workable
models for design applications (see below).
(b) Atomistic modelling
A promising path is the use of atomistic modelling/molec-
ular dynamics for the study of mechanical properties.
Studies are being done on
- the behaviour of dislocations in a crystal;
- the propagation of a crack in a crystal;
- the plastic deformation of nanostructured polycrystal-
line material, when the grain size becomes so small that
dislocation glide no longer is the microscopic deforma-
tion mechanisms.
(c) Dislocation dynamics
At a somewhat higher scale (106 atoms), one studies the
behaviour of several hundreds dislocations by using specific
models for the behaviour of a dislocation in a crystal lattice.
This might some day lead to a full understanding of the dis-
location patterns which are observed in deformed materials.
(d) Multiscale modelling
A successful model at atomistic scale cannot really be used
for engineering applications. Schemes based on scientific
principles have to be developed to derive a model which
can be used at a particular length scale from a model de-
veloped from a lower length scale. The transition form the
atomistic level to the dislocation level, then to the subgrain
level, then to the grain level, finally to the macroscopic lev-
el will be achieved in this way during the decade to come.
Laser induced thermal shock measurement: influence of extreme
material loading by temperature changes are simulated.

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MAX-PLANCK-INSTITUTFÜRMETALLFORSCHUNGSTUTTGART
5.7.5. Application of Modelling in Design of Components
Advanced high-strength/light weight materials will not be
used to reduce weight in cars and other systems unless de-
sign procedures in industries are adapted. This is illustrat-
ed by the following example. Suppose that a pressed part
of a car (for example, a reinforcement column used for bet-
ter protection of the passengers) is normally made from
low-carbon steel. By making it from an advanced alu-
minium alloy or a high-strength steel, one can reduce the
thickness of the plate while maintaining the same strength
before fracture. But this would strongly reduce the elastic
bending stiffness (this issue has nothing to do with frac-
ture!) if the design is not adapted for the rest. However, it
is possible to obtain a sufficient bending stiffness by ade-
quately adapting the design. Only, the designers of indus-
try are not familiar with this. Their design principles have
been optimized by decades of experience with low carbon
steel. Unfortunately, there is no time left to acquire experi-
ence in the same way with the new advanced lightweight
materials. The answer of materials scientists and research
managers to this problem is to modernize the design pro-
cess by incorporating reliable constitutive models for the
materials behaviour into finite element models for the me-
chanical behaviour of components. This makes it possible
to optimize a product and the production process needed
for it by means of computer-aided design and computer-
aided manufacturing, thereby essentially gaining one or
two decades in the introduction of advanced lightweight
materials in industrial applications.
Fig. 5.8 illustrates the role of the constitutive model for
the mechanical behaviour of the materials in such system.
The state of the art is as follows:
• advanced material models which can realistically
describe the mechanical behaviour of single-phase poly-
crystalline materials (steel, aluminium alloys, copper
alloys, ␣-Ti alloys or ␤-Ti alloys) including important
aspects such as texture-based anisotropy and work
hardening (also at changing strain paths) are under
development at present.
• in the decade to come, it is expected that this work will
be extended to advanced multiphase materials. That
means that all the knowledge acquired by studying the
physical problems listed above (see for example the sec-
tion on multiphase and nano-structured materials)
needs to be brought to a level of quality where it can be
implemented in quantitative models. Models of this
quality level will hopefully be transferred from the la-
boratories of universities and research institutions to the
industry during the second decade of the 21st century.
• implementation of advanced non linear model like these
into FE-codes is a formidable problem by itself. Existing
codes tend to become unstable if realistic materials mod-
els are used. To solve this problem, the best that theoreti-
cal mechanics can offer is needed. Several leading groups
in the field of FE modelling are now working at this.
Fig. 5.8. Scheme of an FE-software system which can be used to design deep drawing products.
Preprocess
Postprocess
Modelling of tool geometry
and time increments
Meshing of materials into
finite elements
Constitutive model
of materials
Final shape of product Stress and strain distribution Assessment of formability
Material data baseComputer-Aided Design (CAD)
Finite element calculation

5. MATERIALSPHENOMENA
187
5.7.6. Advance of the United States as compared to Europe
A detailed analysis of this issue for all the topics listed
above would require a much longer document. So only the
most striking aspects will be given.
The Unites States certainly has an important advantage in
the following fields:
• Study of advanced Al- and Ti-alloys and composite
materials. Any institution or company working simulta-
neously on military and civilian applications, and hence
having access to “military” results for civilian applica-
tions, has a great advantage.
• Implementation of advanced material models in FE-
codes. This work too is strongly supported by the needs
of military and nuclear industry.
• “Micro” and “Nano”-technology. The United States are
currently doing a great research effort, also in the sub-
field of “microstructural applications”.
5.8.1. Present Status
Basic research in materials science includes the study of
electronic structure as the basis for all properties of mate-
rials employed in applications. Examples are electrical
properties, magnetism, optical properties, the crystal
structure, etc. Therefore, the study of microscopic elec-
tronic properties complements investigations of macro-
scopic quantities, which often are aimed directly at
improving specific properties of materials utilized in
applications. Also, the goal of understanding materials
properties comprehensively by applying variable scale
modelling requires input about electronic structure.
Materials whose specific features of the microscopic elec-
tronic structure are employed directly in applications are
semiconductors, superconductors, and magnetic materi-
als. But the electronic structure is also the root of almost
any other material property, e.g. the extraordinary me-
chanical properties of some carbides and nitrides, optical
properties, catalytic activity, etc. Therefore, the study of
electronic structure both by experimental and theoretical
investigations will be of central importance for exploring
basic phenomena, which may lead to novel applications.
For the study of electronic structure, there is a broad range
of highly developed spectroscopic methods available. The
most direct probe of electronic structure is photoemis-
sion. This is complemented by techniques such as photo-
absorption, linear and non-linear optical spectroscopy,
inverse photoemission, etc. Also on the theoretical side
considerable progress has been achieved over the last
years. This includes a higher degree of reliability and pre-
cision with respect to the ground state electronic struc-
ture, not only for simple systems, but also for ever more
complex materials. Also, the spectroscopic properties,
which inevitably involve excited states, can be described
so that in many cases very good agreement is achieved
between experiment and theory.
5.8. Electronic Structure and Correlation
J. Kirschner, U. Hillebrecht | Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle (Saale), Germany
M. von Löhneysen | Universität Karlsruhe, 76128 Karlsruhe, Germany
There is no single review paper covering all the topics listed above. A
complete literature list would be many pages long. So only a few refer-
ences will be given, to allow for an estimate of the complexity of the is-
sues depicted in the present contribution.
About work hardening:
E. Nes, Progress in Materials Science, 41 (1997) 129-194.
About advanced FE modelling of polycrystalline materials (work of Paul
Dawson et al., Cornell University, Ithaca, N.Y., USA):
P.R. Dawson and A.J. Beaudoin, “Finite element simulation of metal
forming”, in “Texture and Anisotropy. Preferred Orientations in Poly-
crystals and their Effect on Material Properties,” Kocks, U.F., Tomé, C.N.,
Wenk, H.-R., Eds., . Cambridge University Press, Cambridge, U.K. (1998)
533-560.
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2.
References