Conferences - Seminars

08NOV
2018
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  Thursday 8 November 2018 16:15 - 17:30 MED 2 2423

MEchanics GAthering -MEGA- Seminar: Talk1 - New theory for crack-tip dislocation emission and twinning in fcc metals; Talk2 - Multi-scale modeling of the austenite-martensite transformation in steels

By Predrag Andric and Francesco MarescaLAMMM, EPFL

New theory for crack-tip dislocation emission and twinning in fcc metals  by Predrag AndricLAMMM, EPFL
Abstract Dislocation emission from a crack tip is a necessary precursor to crack blunting and toughening. Intrinsically ductile fcc metals under mode I loading first emit a partial dislocation followed either by a trailing partial («ductile» behavior) or a twinning partial («quasi-brittle» behavior). The critical stress intensity factor KIe at which these processes occur is usually estimated by the Rice and Tadmor/Hai theories. Atomistic simulations show these models to be reasonable but not highly acccurate for predicting KIe. Analysis of the energy changes during nucleation reveals that the first and trailing partial emission are always accompanied by creation of a surface step, while twinning partial emission is not. Here, we present a new analysis in which first and trailing emissions are controlled by a crack-tip instability due to the necessity of step formation. The absence of the step during twinning motivates another new model that accounts for the fact that twin nucleation does not occur directly at the crack tip. Both theories are quantitatively validated against molecular statics simulations across a wide set of fcc metals described with EAM potentials and excellent agreement is obtained. A twinning mode is also reported wherein the crack first advances by cleavage and then emits the twinning partial at the new crack tip.

Multi-scale modeling of the austenite-martensite transformation in steels by Francesco MarescaLAMMM, EPFL
Abstract The austenite-martensite (fcc-bcc) transformation controls the formation of microstructures in a wide range of high strength steels. Recent progress in the physical metallurgy of steels has shown that nanolaminate austenite/matensite microstructures contribute to high material toughness and resistance to hydrogen-embrittlement. Despite its relevance for applications, there is no established theory for the transformation capable to predict the contribution of the austenite-martensite phase tranformation to ductility.
To clarify the mechanism of transformation, we have performed atomistic simulations of the interface reproducing the major experimental TEM and HRTEM observations in Fe alloys. The atomistic model reveals for the first time the structure and motion of the athermal and glissile fcc austenite/bcc martensite interface in steels.
The atomistic findings have guided the formulation of a new, predictive theory of martensite crystallography. Theory predictions show that the fcc/bcc lattice parameter ratio is the key factor controlling the shape deformation (i.e. the in-situ transformation strain), which can achieve more than 90%, namely three times the existing experimental estimates. The theory can thus be used for guiding design of novel and tougher advanced high-strength steels.

Organization MEGA.Seminar Organizing Committee

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