Scientific Advisory Board - Erich Schmid Institute

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
Scientific RepoRt 2012 page 21