Scientific Advisory Board - Erich Schmid Institute

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$dissertation global and local fracture properties of metal matrix ...$

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE Plasticity and Fracture in the 41 years history of eSi, research in the fundamental understanding of plasticity, fatigue and fracture has brought the institute a lot of international recognition. Research has been devoted on the one hand to fundamental questions, such as the proper characterization of fatigue and fracture properties and the scale effects of strength, ductility, and fracture resistance. Another permanent topic has been to explore the relation between the micro- and nanostructure of materials and their deformation-, fatigue-, and fracture properties. it is clear that the durability, i.e. the resistance against the nucleation and growth of defects, plays a key role in the development of new materials and components. A deep understanding of the relations between microstructure and properties is a basic prerequisite for the optimization of the microstructures of materials or components with respect to specific desired properties. A distinguishing feature of the experimental work at eSi in the recent years has been the approach to resolve the microstructure-properties relationships by investigating the influence of the local microstructure on the local material behaviour. this is essential for the understanding about complex composite or multi-phase materials. interdisciplinary research between the fields of materials science and digital image correlation (dic) has led to the development of highly sophisticated experimental techniques that allow us to perform measurements of the local fracture initiation toughness from quantitative fracture surface analysis and to record the damage evolution in materials by in situ deformation- and in situ fracture experiments in the scanning electron microscope (Sem). Fig . 6 SEM micrograph of a TRIP 800 steel. The featureless grains consist of austenite that partially transforms to martensite during tensile loading in the horizontal (x-) direction. The graphs show the displacements in transverse (y-) direction for 2.5 and 4.5 % global strain. The left profile, which mainly goes through ferrite, exhibits negative displacements u y due to lateral contraction. The right profile, which crosses the austenite grain, exhibits positive displacements in Region II due to the phase transformation to martensite. page 20 <strong>Scientific</strong> RepoRt 2012
ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE Local Deformation and Fracture Analyzes An example for the application of these techniques is a study on the deformation behaviour of a tRip-steel, which is an advanced high-strength steel used in the automotive industry. tRip stand for “transformation-induced plasticity”. the microstructure consists of ferrite, bainite, and metastable austenite. the austenite grains transform to martensite during plastic deformation. the accompanied volume expansion leads to improved strain hardening behaviour compared to steels without tRip-effect. the aim of this study was to characterize the process of phase transformation at a microstructural level, which is very difficult because of the small grain sizes. tensile tests were carried out in a high-resolution Sem to obtain highly magnified micrographs of the microstructure at different stages of deformation. processing of these images by dic allowed us to study the evolution of the local strain fields during the tensile test. After improvement of the experimental procedure, it was possible to examine the deformation behaviour at very high spatial resolution, and we succeeded in directly observing the strain-induced transformation of single austenitic grains, see fig. 6. the transformed grains often show characteristic, new surface features; an example is presented in fig. 7a,b. With this procedure, it was possible for the first time to directly measure the local transformation strains at various deformation stages [Kasberger, diploma thesis, 2012]. Fig . 7 Microstructure of a TRIP 800 steel with austenite grains (a) before and (b) after phase transformation to martensite. (c) Steps on the surface of a TiAl-Nb-Mo microstructure that appear due to mechanical twinning in the region near a crack tip. Another example is a study on the fracture behaviour of nb- and mo- alloyed titanium aluminides at higher temperatures. these materials are possible candidates for replacing the comparatively heavy nickel based alloys that are used in components subjected to high mechanical stresses at temperatures up to 800°c. crack growth resistance curves were measured at various temperatures, and the intrinsic and extrinsic mechanisms that increase the fracture resistance, e.g. twinning (fig. 7c), crack ligament bridging and micro-cracking, were identified [Leitner, diploma thesis, 2012]. A third example is the development of new procedures to measure in a reproducible way the fracture toughness of thin-sheet fiber composites [Zechner&Kolednik, comp. Sci. technol. 2013]. the main problem in these materials is that the crack extension cannot be determined, since a long process zone develops in front of the crack tip. this process zone may extend over the whole ligament of the specimen before the first fibers at the crack tip start to fail, and so the usual measurement of the crack extension is impossible. two new approaches were <strong>Scientific</strong> RepoRt 2012 page 21
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Scientific Advisory Board - Erich Schmid Institute

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