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On the mechanism of intelcrystalline cracking
LETTERS
TO THE
On the Mechanism of Intercrystalline Cracking * In the January f956 issue of Acta ~e~ullur~ic~, R. D. Gifkins proposed a mechanism for the formation of interorystalline cracks when boundary sliding occurs.(l) In his mechanism, tensile stresses are developed at suitably oriented jogs along the grain boundary, as a result of slip in the grains which does not completely pass through the boundaries. After sufficient dislocation pile-up at grain boundaries, these tensile stresses may exceed the fracture stress. If boundary sliding occurs concomitantly, the fractured surfaces at the jog are separated to produce intercrysta~ine cracks. As a result of an experimental investigation conducted in the previous year,(z) we had come to a different conclusion about the mechanism of intercrystalline void formation, which, however, requires, as does Gifkins’s, the existence of boundary sliding and boundary jogs. In particular, Gifkins calls upon dynamic slip in one of the bounding grains, which is partially accommodated by slip in the other bounding grain, to produce the grain boundary jogs. Further, he calls upon the piled-up dislocations at the boundary jog to produce the tensile stress that is supposed to produce t,he Iocal fracture at the jog.
FIG. 1, Stresses induced by boundary sliding in bhe direction shown at jogs.
On the other hand, we believe that the boundary slip {inability of the grain boundary to maintain shear traction} at high temperature will lead to the formation of dilationa stresses parallel to the boundary at regions where the boundary slip is impeded, such as at abrupt jogs along the boundary or at grain corners. The latter possibility was originally ACTA
METALL~RGICA,
VOL.
4, ~O~E~~B~R
1956
EDITOR
conceived by Zener(3) and recently experimentally demonstcated by Chang and Grant.(*) There are two possible directions to bo~dary jogs relative to a horizontal boundary. Namely, up or down as the boundary is traversed from left to right. If the top grtlin is sliding to the right relative to the bottom
(3)
SEPAffATE
FRACTURED
SURFACES.
ORIGINALLY CONTINUWS 9’*‘ LINER\‘,
(11 DEVELOP
TENSILE
STRESSES
(2)
DEVELOP
FRACTURE
ACROSS JOG.
FIG. 2. Steps in the development of int~~crystalline cracks due to increasing boundary slip, according to the mechanism by Chen and Meehlin.(*)
grain, then compressive stresses are produced across “up” jogs and tensile stresses are produced across “down” jogs. Fig. 1 illustrates this point. The mag~tude of the stress developed across a jog depends upon the shear relaxation Iength between jogs and the boundary area of the jog itself. Thus, in our mechanism, the boundary sliding accomplished two necessary conditions for the production of intercrystalline cracks. It produces the tensile stresses that result in fracture across the jogged interface, and also it separates the fractured surfaces. Fig. 2 illustrates this sequence. In Gifkins’s mechanism, on the other hand, boundary slip is required solely to separate the fractured surfaces. The experimental evidence we have obtained relates to the necessity of having graid-boundary sliding in order to produce intercrystalline voids. Bicrystals were grown from 99.999% copper. Specimens were cut from one bicrystal so that the grain orientation relative to the boundary is constant for all specimens. For one series of such specimens, shear t,raction was applied parallel to the bicrystal boundary at 1200°F for 20 hours in a dead loading apparatus. For another series of such specimens, a tensile stress of 1000 p.s.i. was applied normal to 635
656
ACTA
METALLURGICA,
VOL.
4,
D’apres
lui,
diffraction
X
spon~aient cristaux
1956
les
fragmentations
obtenues
des
dans les deux
taches
pas au mQme ph~nom~ne.
d’aluminium
polygoniseraient Nous voudrions
Des mono-
t&s pur faiblement
uniquement
de
cas ne corredeform&
vers 630°C.
p&ciser exactement
notre point de
vue. 11 est d’abord
certain
que la methode
que nous
avons proposee(3)
pas&de une sensibiliti sup&ieure B celle de Guinier-Tennevin.c4) M. de Beaulieu peut
FIG. 3. Spscimen subjected to shear parallel to grain boundary. Voids have developed along grain boundary. Conditions: 600 p.s.i., 482”C, 20 hours in H,, as polished, x 150.
the boundary
for 10 hours at 1200°F.
series of specimens, parallel stress,
shear traction,
to the boundary, applied
For a third
applied
was succeeded
as above,
normal
as before by tensile
to the boundary.
The results of the tests were, that with shear alone some
voids were found
along
the
boundary.
With
tension alone, no voids were found at the boundary. With
shear, followed
found
along the boundary.
and etching
by tension,
techniques
were used.
are shown in Fig. 3. It is apparent, therefore, necessary
condition
line voids
(cracks).
were performed mechanism,
many
that boundary
shear is a
our experiments
to a knowledge
of Gifkins’s
and do not serve to distinguish
simple experiments
results
of intercrystal-
Unfortunately,
his concept,s and ours.
between
We are now performing
to so di~erentiate.
it is apparent that grain-boundary
were
polishing
Typical
for the formation
prior
voids
Both diamond
some
In any ease,
sliding is a necessary
prerequisite in order to obtain intercrystalline
cracking.
School of Mines,
C. W. CHEN
~olurnbia I.:niversity,
E. S. MACHLIN
New York 27, IV. Y.
difficilement
dans
ses aristaux
des
sous-
11 lui est done impossible ments
thermiques
de asvoir si, pour les traite-
effect&s
& des
temperatures
infdrieures B 630”, une sous-structrue pas deja presente.
pIus fine n’est
En fait, memo dans des cristaux
moins purs, nous avons trouve des stries nettes dans les taches de diffraction
enregistrees
apres une deformation avons
faible
immediatement
(quelques
pu suivre leur evolution
%) et nous
apres divers
traite-
ments thermiques. Nos observations
peuvent se resumer de la man&e
suivante. 1. En-dessous
d’une
certaine gamme
de tempera-
tures, les sous-grains form& directement a la tempgrature ordinaire (sans intervention du processus thermiquement active de Cahn) ne semblent pas croitre
d’une
man&e
Blimine progressivement a cot& des continues. identifie
importante. La matrice les courbures locales presentes
sous-grains
parfaits
et plus
ou moms
Ce mecanisme peut &ire probablement B Ia diffusion et & la r~organisation des
dislocations,
c’est-a-dire
a ee que l’on appelle generale-
ment la “polygonisation.” 2. Au-dessus de ces temperatures (variables suivant la pure% du metal, de la deformation, etc.), une croissance reguliere de certains individus se dkveloppe
References
avec conservation
1. R. D. GIFKINS Actn Met. 4, 1955. 2. C. W. CHEN and E. S. MACHLIN
On the Mechanism of Intercrystalline Fracture, to be submitted to A.S.M. 3. C. ZENER The Micro-Mechanism of Fracture. Fracturing of Metals, A.S.M. (1947), p. 3. 4. H. C. CHANQ and N. J. GRANTMechanism of Intercrystalline Fracture Journ.aEof Metals February, 1956. * This research was supported by the United States Air Force through the Wright Air Development Center. Received April 25, 1956.
chacun
des
Au Sujet de la Mise en Evidence de la Polygonisation de 1’Aluminium par la Methode des Rayons X et par la Micrograp~ie~ Xous le m&me titre, M, de Beaulieu
a recemment
obtenus au laboraavec 110s propres
dune haute perfection
interne pour
sous-grains.
Dans les cas observes, cette &ape se deroule dans une matrice deja entierement polygonisee et peut 6tre citracterisee simple processus de croisssnce.
par un
3. Par contre, an voisinage du point de fusion, une sous-structure b larges domaines moins parfaits se forme
quelquefois
l’aluminium);
compare les importants resultats toire du Professeur Chaudron observations.(2)
detector
grains d’une taille inferieure au dixieme de millimetre.
(aussi bien dans le fer que dans
Cet &at
nous parait
correspondre
B
ce que Crussardc5) a appele “recristallisation in situ.” 11 est probable due les observations de M. de Beaulieu se rapportent b cet &at. Quoi qu’il en soit, nous n’avons jamais observe la formation de ces grands sous-grains imparfaits directement & partir de cristaux ne contenant pas au prealable une sousstructure L caractere parfait. 11 importe de remarquer
TO THE
On the Mechanism of Intercrystalline Cracking * In the January f956 issue of Acta ~e~ullur~ic~, R. D. Gifkins proposed a mechanism for the formation of interorystalline cracks when boundary sliding occurs.(l) In his mechanism, tensile stresses are developed at suitably oriented jogs along the grain boundary, as a result of slip in the grains which does not completely pass through the boundaries. After sufficient dislocation pile-up at grain boundaries, these tensile stresses may exceed the fracture stress. If boundary sliding occurs concomitantly, the fractured surfaces at the jog are separated to produce intercrysta~ine cracks. As a result of an experimental investigation conducted in the previous year,(z) we had come to a different conclusion about the mechanism of intercrystalline void formation, which, however, requires, as does Gifkins’s, the existence of boundary sliding and boundary jogs. In particular, Gifkins calls upon dynamic slip in one of the bounding grains, which is partially accommodated by slip in the other bounding grain, to produce the grain boundary jogs. Further, he calls upon the piled-up dislocations at the boundary jog to produce the tensile stress that is supposed to produce t,he Iocal fracture at the jog.
FIG. 1, Stresses induced by boundary sliding in bhe direction shown at jogs.
On the other hand, we believe that the boundary slip {inability of the grain boundary to maintain shear traction} at high temperature will lead to the formation of dilationa stresses parallel to the boundary at regions where the boundary slip is impeded, such as at abrupt jogs along the boundary or at grain corners. The latter possibility was originally ACTA
METALL~RGICA,
VOL.
4, ~O~E~~B~R
1956
EDITOR
conceived by Zener(3) and recently experimentally demonstcated by Chang and Grant.(*) There are two possible directions to bo~dary jogs relative to a horizontal boundary. Namely, up or down as the boundary is traversed from left to right. If the top grtlin is sliding to the right relative to the bottom
(3)
SEPAffATE
FRACTURED
SURFACES.
ORIGINALLY CONTINUWS 9’*‘ LINER\‘,
(11 DEVELOP
TENSILE
STRESSES
(2)
DEVELOP
FRACTURE
ACROSS JOG.
FIG. 2. Steps in the development of int~~crystalline cracks due to increasing boundary slip, according to the mechanism by Chen and Meehlin.(*)
grain, then compressive stresses are produced across “up” jogs and tensile stresses are produced across “down” jogs. Fig. 1 illustrates this point. The mag~tude of the stress developed across a jog depends upon the shear relaxation Iength between jogs and the boundary area of the jog itself. Thus, in our mechanism, the boundary sliding accomplished two necessary conditions for the production of intercrystalline cracks. It produces the tensile stresses that result in fracture across the jogged interface, and also it separates the fractured surfaces. Fig. 2 illustrates this sequence. In Gifkins’s mechanism, on the other hand, boundary slip is required solely to separate the fractured surfaces. The experimental evidence we have obtained relates to the necessity of having graid-boundary sliding in order to produce intercrystalline voids. Bicrystals were grown from 99.999% copper. Specimens were cut from one bicrystal so that the grain orientation relative to the boundary is constant for all specimens. For one series of such specimens, shear t,raction was applied parallel to the bicrystal boundary at 1200°F for 20 hours in a dead loading apparatus. For another series of such specimens, a tensile stress of 1000 p.s.i. was applied normal to 635
656
ACTA
METALLURGICA,
VOL.
4,
D’apres
lui,
diffraction
X
spon~aient cristaux
1956
les
fragmentations
obtenues
des
dans les deux
taches
pas au mQme ph~nom~ne.
d’aluminium
polygoniseraient Nous voudrions
Des mono-
t&s pur faiblement
uniquement
de
cas ne corredeform&
vers 630°C.
p&ciser exactement
notre point de
vue. 11 est d’abord
certain
que la methode
que nous
avons proposee(3)
pas&de une sensibiliti sup&ieure B celle de Guinier-Tennevin.c4) M. de Beaulieu peut
FIG. 3. Spscimen subjected to shear parallel to grain boundary. Voids have developed along grain boundary. Conditions: 600 p.s.i., 482”C, 20 hours in H,, as polished, x 150.
the boundary
for 10 hours at 1200°F.
series of specimens, parallel stress,
shear traction,
to the boundary, applied
For a third
applied
was succeeded
as above,
normal
as before by tensile
to the boundary.
The results of the tests were, that with shear alone some
voids were found
along
the
boundary.
With
tension alone, no voids were found at the boundary. With
shear, followed
found
along the boundary.
and etching
by tension,
techniques
were used.
are shown in Fig. 3. It is apparent, therefore, necessary
condition
line voids
(cracks).
were performed mechanism,
many
that boundary
shear is a
our experiments
to a knowledge
of Gifkins’s
and do not serve to distinguish
simple experiments
results
of intercrystal-
Unfortunately,
his concept,s and ours.
between
We are now performing
to so di~erentiate.
it is apparent that grain-boundary
were
polishing
Typical
for the formation
prior
voids
Both diamond
some
In any ease,
sliding is a necessary
prerequisite in order to obtain intercrystalline
cracking.
School of Mines,
C. W. CHEN
~olurnbia I.:niversity,
E. S. MACHLIN
New York 27, IV. Y.
difficilement
dans
ses aristaux
des
sous-
11 lui est done impossible ments
thermiques
de asvoir si, pour les traite-
effect&s
& des
temperatures
infdrieures B 630”, une sous-structrue pas deja presente.
pIus fine n’est
En fait, memo dans des cristaux
moins purs, nous avons trouve des stries nettes dans les taches de diffraction
enregistrees
apres une deformation avons
faible
immediatement
(quelques
pu suivre leur evolution
%) et nous
apres divers
traite-
ments thermiques. Nos observations
peuvent se resumer de la man&e
suivante. 1. En-dessous
d’une
certaine gamme
de tempera-
tures, les sous-grains form& directement a la tempgrature ordinaire (sans intervention du processus thermiquement active de Cahn) ne semblent pas croitre
d’une
man&e
Blimine progressivement a cot& des continues. identifie
importante. La matrice les courbures locales presentes
sous-grains
parfaits
et plus
ou moms
Ce mecanisme peut &ire probablement B Ia diffusion et & la r~organisation des
dislocations,
c’est-a-dire
a ee que l’on appelle generale-
ment la “polygonisation.” 2. Au-dessus de ces temperatures (variables suivant la pure% du metal, de la deformation, etc.), une croissance reguliere de certains individus se dkveloppe
References
avec conservation
1. R. D. GIFKINS Actn Met. 4, 1955. 2. C. W. CHEN and E. S. MACHLIN
On the Mechanism of Intercrystalline Fracture, to be submitted to A.S.M. 3. C. ZENER The Micro-Mechanism of Fracture. Fracturing of Metals, A.S.M. (1947), p. 3. 4. H. C. CHANQ and N. J. GRANTMechanism of Intercrystalline Fracture Journ.aEof Metals February, 1956. * This research was supported by the United States Air Force through the Wright Air Development Center. Received April 25, 1956.
chacun
des
Au Sujet de la Mise en Evidence de la Polygonisation de 1’Aluminium par la Methode des Rayons X et par la Micrograp~ie~ Xous le m&me titre, M, de Beaulieu
a recemment
obtenus au laboraavec 110s propres
dune haute perfection
interne pour
sous-grains.
Dans les cas observes, cette &ape se deroule dans une matrice deja entierement polygonisee et peut 6tre citracterisee simple processus de croisssnce.
par un
3. Par contre, an voisinage du point de fusion, une sous-structure b larges domaines moins parfaits se forme
quelquefois
l’aluminium);
compare les importants resultats toire du Professeur Chaudron observations.(2)
detector
grains d’une taille inferieure au dixieme de millimetre.
(aussi bien dans le fer que dans
Cet &at
nous parait
correspondre
B
ce que Crussardc5) a appele “recristallisation in situ.” 11 est probable due les observations de M. de Beaulieu se rapportent b cet &at. Quoi qu’il en soit, nous n’avons jamais observe la formation de ces grands sous-grains imparfaits directement & partir de cristaux ne contenant pas au prealable une sousstructure L caractere parfait. 11 importe de remarquer