1. 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 40 MPa m1/2 with upto 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.
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.
0.5 mm
Transmission electron microscopy of the bainitic structure.