Transcript Poster Plasticidad

SHEAR RELIEF
M. J. Peet, C. García-Mateo, F. G. Caballero and H. K. D. H Bhadeshia
University of Cambridge, Department of Materials Science and Metallurgy
Phase Transformations & Complex Properties Research Group
We have discovered that bainite can be formed at very low temperatures, resulting in plates which are so thin that
the material can have a strength of 2500 MPa in bulk form, without inducing residual stresses. The microstructure
consists of a mixture of carbide-free bainite and carbon enriched retained austenite. The toughness is in excess of
1/2
40 MPa m with up to 20% ductility tensile tests. Whilst technological implications are exciting, the novel alloys
also offer the possibility of fundamental studies on the displacement associated with the transformation
mechanism.
To study these displacements, we have used atomic
force microscopy, which is the only technique capable
of resolving the individual platelets. We have
discovered that bainite can be formed at very low
temperatures, resulting in plates which are so thin that
the material can have a strength of 2500 MPa in bulk
form, without inducing residual stresses.
Atomic force microscopy of the surface relief due to bainite
transformation.
0.5 mm
Transmission electron microscopy of the bainitic structure.
Initial conclusions
We have been able to directly correlate the surface
observation using atomic force microscopy with the
scale of the features found in transmission electron
microscopy. This contradicts a large number of
observations reported in the literature, which our work
indicates are in error due to the formation of surface
oxides during the preparation of relief specimens.
We are now focussing on the relative roles of elastic
and plastic accommodation of the shape change in
determining the morphology of the transformation
product.