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Interaction between lattice dislocations and grain boundaries in high purity iron. Fiche cinematographique
ONERA n° 1173 ( 1986 )
B. El’Mrabat, L. Priester, R. Valle, A. Jouniaux, A. Marraud
To cite this version:
B. El’Mrabat, L. Priester, R. Valle, A. Jouniaux, A. Marraud. Interaction between lattice disloca- tions and grain boundaries in high purity iron. Fiche cinematographique ONERA n° 1173 ( 1986 ). Revue de Physique Appliquée, Société française de physique / EDP, 1988, 23 (4), pp.682-682.
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682
INTERACTION BETWEEN LATTICE DISLOCATIONS AND GRAIN BOUNDARIES IN HIGH PURITY IRON
Fiche
cinematographique
ONERA n° 1173 ( 1986 )B.
EL’MRABAT ,
L.PRIESTER(
Laboratoire deMétallurgie
Structurale U.A.(CNRS )
n°1107 , Université PARIS -SUD ,
91405 ORSAY , France )R. VALLE, A. JOUNIAUX , A. MARRAUD ( CNRS - ONERA , 29 Avenue de la Division Leclerc , 93320 CHATILLON , France )
Revue Phys. Appl. 23 (1988) 682 AVRIL 1988,
The
dynamic aspect
of the interaction bet-ween lattice dislocations and
grain
boundaries has beeninvestigated by
"in-situ" deformation tests in theHigh Voltage
ElectronMicroscope
( H.E.V.M. ).These
experiments
wereperformed
onhigh purity
ironpolycrystals
with low carbon andphosphorus
contents (C :50ppm ,
P : 40 to 100 ppmby weight) ,
both atroom
temperature
and at 620 K . Thegrain
boundariesunder
investigation
are random ones without anyprior crystallographic
characterization .During
the firststages
ofi deformation atroom
température,
threephenomena
were revealed :- thé
cross-slip of
lattice dislocationsgliding
toward the
grain boundary ;
in some cases , dis- locations werestrongly repelled
from thegrain boundary
at a distance of a few micrometers,- the role of some
grain
boundaries as efficient barriers to thepropagation of slip :
dislocationtangles
were built up in narrow zonesalong
thegrain boundary leading
to the formation of work hardenedregions compared
to thegrain
center.These observations are in
good agreement
with thehardening
mechanism which occurs in silicon,bicrystals [1]
,- the
absorption
and thegeneration
of lattice dis-locations :the
figure
1 illustrates the different sequences of thesephenomena
observed near andin a
particular grain boundary .
Stress concen-tration was first created in the
region
A nearthe
triple point
before the sourcemight operate
( Fie. 1 a ). Absorbed and emitted lattice dislo-cations glide
on twoslip planes
of the samecrystal
that seem to converge in thegrain
boun-dary plane
( a/b or c/d ) . Then , in theneigh- bouring region
B ,initially
free of extrinsic dislocations , lattice dislocations entered thegrain boundary
and becameimmediatly
dissociated ( e ) . Next . a source mechanism similar to the first one oerated in theregion
C .Figure 1
: sketchillustrating
theabsorption
andthe émission of lattice dislocations which sequen-
tially
occur in thégrain boundary
1/2 (see text).Glide of extrinsic dislocations was never observed.
This remark allows us to
put
forward thefollowing hypothesis :
the observed source mechanism islikely
to be the so-called "V-mechanism" which may
operate
when two sli
planes
intersectalong
the grain boun-dary plane[2 . . This process may occur more easily
in
body
centered cubic materials due to themultipli- city
of theslip
systems . Thegeometrical
conditionrequired
toget
a "V-mechanism" and the occurence of dissociation of theincorporated
dislocations are twoarguments
in favor of thespeciality
of thegrain boundary
1/2 (Fig.
1 ) .The
grain boundary
behavior under deformation athigh temperàture (620K )
ischaracterized-by
thespontaneous
orprogressive spreading
of extrinsicdislocations. The
spreading
kinetics may differ fromone dislocation to another one in the same
boundary.
Dislocation motion was never observed . This remark involves that the
complete
accommodation of extrin- sic dislocations in most boundaries neveroccurs [3].
The most
striking
observation is the stabili- ty of extrinsic dislocations under stress even athigh temperature .
Mostgrain
boundaries behave ashard
components
of the microstructure.This behavior may be attributed to the effect ofphosphorus
whichstrongly segregates
to thegrain
boundaries in iron andpromotes
embrittlement . At roomtempérature , phosphorus
mayimpede intergranu1ar dislocation
mo-tion
by increasing
the frictionstressai .
Thiseffect may be seen as a consequence of
he strong bonding
betweenphosphorus
and iron and may alterna-tely explain grain boundary embrittlement L5,6J.At elevated temperature , phosphorus segregation may
1ncrease the grain 6oundary
diffusion coefficient and thusimpede
climb of dislocations whithin theboundary .
Dissociation may occur as itjust requires
short range diffusion but thegrain boundary equili-
brium state is not
reached [3].
Mechanical proper- ties ofpolycrystals
arestrongly
modifiedby grain boundary segregation
Thegrain boundary
behaviorsunder stress
investigated by
static T.E.M.experen
ments’and calculations are
clearly
confirmedby
the"in situ" H.V.E.M. deformation tests.
REFERENCES
1 GEORGE A. " Mechanisms and mechanics of
plastici-
ty , AUSSOIS (1987 )
J. dePhysique
p.2 BARO
G,
GLEITER H. , HORNBOGEN E.Mater. Sci.
Eng.
3 ( 1968/69 ) 92 .3 LARTIGUE S. , PRIESTER L. Acta Metall.
31(1983)
1809 4 BISCONDI M. J. dePhysique
43(1982)
C6-293 5 HASHIMOTO H., ISHIDAY.,
WAKAYADA S. , YAMAMOTOR., DOYAMA M. Acta Metall. 32
(
1984 ) 1 6 ISHIDA Y.,MORI M. J. dePhysique
46 ( 1985 )C4 - 465 .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01988002304068200