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SUMMARY:MEchanics GAthering -MEGA- Seminar: Talk1 - New theory for crack-t
 ip dislocation emission and twinning in fcc metals\; Talk2 - Multi-scale m
 odeling of the austenite-martensite transformation in steels
DTSTART:20181108T161500
DTEND:20181108T173000
DTSTAMP:20260922T015640Z
UID:a36b60f3fa2c542b31f7302eaf24f710da39da17149e6955677842b7
CATEGORIES:Conferences - Seminars
DESCRIPTION:Predrag Andric and Francesco Maresca\, LAMMM\, EPFL\nNew the
 ory for crack-tip dislocation emission and twinning in fcc metals  by Pr
 edrag Andric\, LAMMM\, EPFL\nAbstract Dislocation emission from a crack 
 tip is a necessary precursor to crack blunting and toughening. Intrinsical
 ly ductile fcc metals under mode I loading first emit a partial dislocatio
 n followed either by a trailing partial («ductile» behavior) or a twinni
 ng partial («quasi-brittle» behavior). The critical stress intensity fac
 tor KIe at which these processes occur is usually estimated by the Rice a
 nd Tadmor/Hai theories. Atomistic simulations show these models to be reas
 onable but not highly acccurate for predicting KIe. Analysis of the energy
  changes during nucleation reveals that the first and trailing partial emi
 ssion are always accompanied by creation of a surface step\, while twinnin
 g 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 t
 he necessity of step formation. The absence of the step during twinning mo
 tivates 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 cleav
 age and then emits the twinning partial at the new crack tip.\n\nMulti-sca
 le modeling of the austenite-martensite transformation in steels by Franc
 esco Maresca\, LAMMM\, EPFL\nAbstract 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 ste
 els has shown that nanolaminate austenite/matensite microstructures contri
 bute to high material toughness and resistance to hydrogen-embrittlement. 
 Despite its relevance for applications\, there is no established theory fo
 r the transformation capable to predict the contribution of the austenite-
 martensite phase tranformation to ductility.\nTo 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 ste
 els.\nThe atomistic findings have guided the formulation of a new\, predic
 tive theory of martensite crystallography. Theory predictions show that th
 e fcc/bcc lattice parameter ratio is the key factor controlling the shape 
 deformation (i.e. the in-situ transformation strain)\, which can achieve m
 ore 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.
LOCATION:MED 2 2423 https://plan.epfl.ch/?room=MED22423
STATUS:CONFIRMED
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