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USMS Report Summary

Project ID: 340185
Funded under: FP7-IDEAS-ERC
Country: Austria

Mid-Term 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 is 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 micromechanic 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 were the material behave brittle.
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 nanostructures 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 nanostructures are not as strong as the pearlitic wires but they are more ductile, some of them shows an exceptional combination of strength and ductility. The other group of materials which we synthesized were nanocomposites, 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-intermetallic, metal-metallic glass, metallic glass – metallic glass, and metal – polymer which are transformed to metal- ceramic nanocomposites. 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. Till now we have significantly improved the processing knowledge and the underlying phenomena. This will allow us to synthesis some unique new nanocomposites, which will be analyzed in detail in the second part of the project.

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