Final Report Summary - USMS (Ultra Strong Materials)
The improvement of strength and toughness is one of the main goals in materials science. An
increase in tensile strength of metallic materials is usually associated with a decrease in
ductility and fracture toughness. The limit of the strength of engineering alloys in each metallic
system - Fe, Al, Mg, Ti, Ni, and Cu based alloys or composites - seems to reach a magic limit. For
example, high strength spring steels approach about 2200 MPa, tool steels about 2500 MPa, high
strength Ti alloys about 1200 MPa or high strength Al alloys 850 MPa. All these high strength
materials have already a somewhat reduced ductility compared to their low or medium strength
alloys, which have tensile strengths between 10 and 60% of the above mentioned values. A further
increase of the tensile strength usually reduces the ductility to unacceptable values for most
engineering applications; they exhibit a ceramic-like defect sensitive behavior. This magic limit
is about 1% of the Young's modulus or about 10% of the theoretical strength. The gap between the
current high strength metallic materials and the theoretical limit is large.
There is one exception, which shows that this gap can be significantly reduced: cold drawn
pearlitic steel wires. They are basically a nano-lamellar arrangement of Fe and cementite, Fe3C.
These high strength wires are used as cable wires and steel cord wires. The latter are the most
technically developed ones and reach strength values of about 4500 MPa and are the strongest used
engineering bulk materials. Recently on laboratory scale the strength of these pearlitic wires
could be increased between 6 and 7 GPa which is 1/3 of the theoretical limit or about 3 times
higher than the standard highest strength steels.
The goal of the proposed project was to obtain similar exceptional combinations of strength and
ductility in other metal based nano-composites. In order to obtain this ambitious goal we have
analyzed the ductility and the fracture toughness controlling mechanisms in the heavily drawn
pearlitic wires with the actually highest strength by new micro-mechanic techniques, compression,
fatigue and crack growth experiments on samples with dimensions of few micrometers. For the first
time we could explain the exceptional combination of properties. The most essential feature is the
anisotropy of the fracture toughness. We could furthermore show that this anisotropy is a
requirement to obtain strength values with 1/4 of the theoretical strength and more. At least in
one crack propagation direction one needs a significant lower fracture toughness. For the crack
propagation perpendicular to this relative brittle loading direction crack deflection or delamination
takes place which induces a significant reduction of the stresses in front of the crack and as a consequence a
relative good damage tolerance for these loading directions. It could be shown that the toughening mechanism
in these ultra strong material is similar to biological materials like wood or bone.
Beside these basic analyses of the ductility controlling phenomena in these pearlitic wires the
generation by severe plastic deformation and understanding of the underlying phenomena of similar
nano-structures have been the main focus of the project. In single phase materials we made big
progress to understand the generation of heavily elongated about 100nm thick lamellar grains. This
nano-structures are not as strong as the pearlitic wires but they are more ductile, some of them
shows an exceptional combination of strength and ductility. Special progress has been obtained in
the second half of the USMS project to generate even finer lamellar grain structures by doping with
impurities. The other group of materials which we synthesized were nano-composites, with the goal to reach lamellar thicknesses of 10 nm similar as in
the strongest pearlitic wires. Very different systems are investigated metal-metal,
metal-inter metallic, metal-metallic glass, metallic glass - metallic glass, and metal - polymer
which are transformed to metal- ceramic nano-composites. The focus of the first part of the project
was to develop an understanding of the deformation processes during severe plastic deformation of
the composites in order to generate processing maps to obtain a desired nano-structure. We
have significantly improved the processing knowledge to generate such nano-lamellar composites and developed
a physical understanding for the underlying phenomena during the heavy plastic deformation. This allow
us to synthesis a large quantity of unique new nano-composites, which has been analyzed in detail. It could be shown that in most
of the generated nano-lamellar and nano-fibrous structures a similar combination of strength and ductility can be obtained.
The project therefore opened new ways to generate ultra strong materials for a wide range of industrial application.
increase in tensile strength of metallic materials is usually associated with a decrease in
ductility and fracture toughness. The limit of the strength of engineering alloys in each metallic
system - Fe, Al, Mg, Ti, Ni, and Cu based alloys or composites - seems to reach a magic limit. For
example, high strength spring steels approach about 2200 MPa, tool steels about 2500 MPa, high
strength Ti alloys about 1200 MPa or high strength Al alloys 850 MPa. All these high strength
materials have already a somewhat reduced ductility compared to their low or medium strength
alloys, which have tensile strengths between 10 and 60% of the above mentioned values. A further
increase of the tensile strength usually reduces the ductility to unacceptable values for most
engineering applications; they exhibit a ceramic-like defect sensitive behavior. This magic limit
is about 1% of the Young's modulus or about 10% of the theoretical strength. The gap between the
current high strength metallic materials and the theoretical limit is large.
There is one exception, which shows that this gap can be significantly reduced: cold drawn
pearlitic steel wires. They are basically a nano-lamellar arrangement of Fe and cementite, Fe3C.
These high strength wires are used as cable wires and steel cord wires. The latter are the most
technically developed ones and reach strength values of about 4500 MPa and are the strongest used
engineering bulk materials. Recently on laboratory scale the strength of these pearlitic wires
could be increased between 6 and 7 GPa which is 1/3 of the theoretical limit or about 3 times
higher than the standard highest strength steels.
The goal of the proposed project was to obtain similar exceptional combinations of strength and
ductility in other metal based nano-composites. In order to obtain this ambitious goal we have
analyzed the ductility and the fracture toughness controlling mechanisms in the heavily drawn
pearlitic wires with the actually highest strength by new micro-mechanic techniques, compression,
fatigue and crack growth experiments on samples with dimensions of few micrometers. For the first
time we could explain the exceptional combination of properties. The most essential feature is the
anisotropy of the fracture toughness. We could furthermore show that this anisotropy is a
requirement to obtain strength values with 1/4 of the theoretical strength and more. At least in
one crack propagation direction one needs a significant lower fracture toughness. For the crack
propagation perpendicular to this relative brittle loading direction crack deflection or delamination
takes place which induces a significant reduction of the stresses in front of the crack and as a consequence a
relative good damage tolerance for these loading directions. It could be shown that the toughening mechanism
in these ultra strong material is similar to biological materials like wood or bone.
Beside these basic analyses of the ductility controlling phenomena in these pearlitic wires the
generation by severe plastic deformation and understanding of the underlying phenomena of similar
nano-structures have been the main focus of the project. In single phase materials we made big
progress to understand the generation of heavily elongated about 100nm thick lamellar grains. This
nano-structures are not as strong as the pearlitic wires but they are more ductile, some of them
shows an exceptional combination of strength and ductility. Special progress has been obtained in
the second half of the USMS project to generate even finer lamellar grain structures by doping with
impurities. The other group of materials which we synthesized were nano-composites, with the goal to reach lamellar thicknesses of 10 nm similar as in
the strongest pearlitic wires. Very different systems are investigated metal-metal,
metal-inter metallic, metal-metallic glass, metallic glass - metallic glass, and metal - polymer
which are transformed to metal- ceramic nano-composites. The focus of the first part of the project
was to develop an understanding of the deformation processes during severe plastic deformation of
the composites in order to generate processing maps to obtain a desired nano-structure. We
have significantly improved the processing knowledge to generate such nano-lamellar composites and developed
a physical understanding for the underlying phenomena during the heavy plastic deformation. This allow
us to synthesis a large quantity of unique new nano-composites, which has been analyzed in detail. It could be shown that in most
of the generated nano-lamellar and nano-fibrous structures a similar combination of strength and ductility can be obtained.
The project therefore opened new ways to generate ultra strong materials for a wide range of industrial application.