GRAIN BOUNDARY DESIGN FOR DESIRABLE
MECHANICAL PROPERTIES
T. Watanabe
To cite this version:
T. Watanabe.
GRAIN BOUNDARY DESIGN FOR DESIRABLE MECHANICAL
PROPERTIES. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-507-C5-519.
<10.1051/jphyscol:1988562>. <jpa-00228059>
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JOURNAL DE PHYSIQUE
Colloque C5, supplement au nolO, Tome 49, octobre 1988
GRAIN BOUNDARY DESIGN FOR DESIRABLE MECHANICAL PROPERTIES
T
. WATANABE
Department of Materials Science, Faculty of Engineering, Tohoku
t m i v e r s i t y , Sendai 980, Japan
Abstract - Grain boundary design for desirable mechanical properties of polycrystalline materials is discussed on the basis of the intrinsic structuredependent mechanical properties, particularly deformation and fracture of
grain boundaries.
The importance of the grain boundary character distribution (GBCD) is emphasized which is a new microstructural parameter controlling
the properties of polycrystals.
The results of recent studies of GBCD are
presented.
The potential for grain boundary design for strong and ductile
polycrystals through the control of GBCD and boundary inclination is discussed.
1. Introduction - The control of microstructure is a basic approach to materials
development established by modern materials science which is based on the principle of
structure-property-performance relationship (1-3).
Grain boundaries ( including
interphase boundary ) are important elements of the microstructure of polycrystalline
They can affect considerable effect on the properties and performance of
materials.
the materials.
So the design and control of grain boundaries are strongly required
to develop high performance engineering materials with desirable properties ( 4 , 5 ) .
Recent rapid progress in our understanding of grain boundary structure and properties
(6-9) appears to enable us to achieve a new approach to materials development.
It has been revealed that the intrinsic grain boundary properties strongly depend on
It is very likely that structurethe type and structure of grain boundaries.
dependent grain boundary properties can be effectively utilized in materials development by manipulating grain boundaries to endow polycrystalline materials with desirable
bulk properties.
2. Desirable Mechanical Properties for Structural or Functional Materials.
Mechanical properties are the most important for structural materials which are
normally required to meet the following requirements: high strength, high ductility,
high fracture resistance, elastic and plastic homogeneity, low susceptibility to
environmental effects, high stabililty of microstructure and properties and so on.
These requirements are related moreor less to the presence and effect of grain boundFor instance high strength and high ductility are often obtained by grain
aries.
refinement, i.e. by incorporation of a high density of grain boundaries in polycrystalline materials.
Normally almost all engineering materials whether structural or
functional, must receive some kind of working and shaping before they are put in
In the past there were a number of cases where materials could not be
service.
developed because of their undesirable mechanical properties, particularly brittleness
or low ductility associated with intergranular fracture. It is well known that there
is a general tendency for strong materials to become more sensitive to intergranular
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988562
C5 -5 08
JOURNAL DE PHYSIQUE
f r a c t u r e and show a l o s s i n d u c t i l i t y a f t e r some s t r e n g t h e n i n g t r e a t m e n t .
This is a
long-pending problem which m a t e r i a l s s c i e n t i s t s and e n g i n e e r s must s o l v e .
There h a s
been n o g e n e r a l r u l e o r working p r i n c i p l e t o o b t a i n s t r o n g and d u c t i l e p o l y c r y s t a l l i n e
materials.
T h i s i s a v e r y c h a l l e n g i n g s u b j e c t r e g a r d i n g t h e development of h i g h performance m a t e r i a l s w i t h d e s i r a b l e and e x c e l l e n t mechanical p r o p e r t i e s .
3 . S t r u c t u r a l E f f e c t s on I n t e r g r a n u l a r Mechanical P r o p e r t i e s
B a s i c knowledge of s t r u c t u r a l e f f e c t s on i n t e r g r a n u l a r mechanical p r o p e r t i e s i s needed
f o r l a t e r d i s c u s s i o n on g r a i n boundary d e s i g n f o r d e s i r a b l e mechanical p r o p e r t i e s .
I n o r d e r t o q u a n t i t a t i v e l y a n a l i s e observed s t r u c t u r a l e f f e c t s we need t o c h a r a c t e r i z e
g r a i n boundaries e x i s t i n g i n r e a l materials.
I n g e n e r a l , t h e r e a r e two groups of
boundaries: low-energy b o u n d a r i e s and high-energy b o u n d a r i e s . We c o n s i d e r t h a t lowa n g l e b o u n d a r i e s and low C c o i n c i d e n c e b o u n d a r i e s belong t o t h e f i r s t group and h i g h
a n g l e g e n e r a l s o c a l l e d random b o u n d a r i e s t o t h e second. Throughout t h i s paper we
c o n s i d e r t h e s e t h r e e t y p e s of b o u n d a r i e s .
3 . 1 Deformation - The p r e s e n c e of g r a i n b o u n d a r i e s can a f f e c t p l a s t i c deformation i n
c r y s t a l l i n e s o l i d s through t h e g e n e r a t i o n o r a b s o r p t i o n o f l a t t i c e d i s l o c a t i o n s , o r by
p r o v i s i o n o f b a r r i e r s t o d i s l o c a t i o n motion ( 1 0 , l l ) .
At h i g h t e m p e r a t u r e s g r a i n bounda r i e s can p l a y more r o l e s i n p l a s t i c deformation a s s o u r c e s o r s i n k s of p o i n t d e f e c t s ,
and more d i r e c t l y by g r a i n boundary s l i d i n g .
So we can e x p e c t t h a t t h e r e w i l l b e
more s i g n i f i c a n t e f f e c t s of g r a i n boundary s t r u c t u r e on h i g h t e m p e r a t u r e deformation
t h a n low t e m p e r a t u r e deformation.
I n f a c t , i t h a s been r e p o r t e d t h a t t h e e f f e c t o f
g r a i n boundary s t r u c t u r e on low t e m p e r a t u r e deformation can b e observed i n a v e r y
e a r l y s t a g e of deformation b u t n o t i n l a t e r s t a g e (12,13).
Whereas a t h i g h temperat u r e s t r u c t u r a l e f f e c t s o f g r a i n boundary a p p e a r r e t a i n a b l e o v e r t h e whole range o f
p l a s t i c deformation up t o t h e f i n a l f r a c t u r e .
So l e t u s s e e t h e e f f e c t o f g r a i n
boundary on h i g h t e m p e r a t u r e d e f o r m a t i o n i n z i n c b i c r y s t a l s .
F i g u r e 1 shows c r e e p c u r v e s of z i n c b i c r y s t a l s c o n t a i n i n g <10i0> t w i s t b o u n d a r i e s a t
45O t o t h e specimen a x i s . T e n s i l e c r e e p t e s t s were conducted a t c o n s t a n t s h e a r s t r e s s
f o r b a s a l p l a n e s o f t h e g r a i n s f o r a l l t h e b i c r y s t a l specimens i r r e s p e c t i v e of t h e
It i s c l e a r t h a t a s i n g l e g r a i n boundary can d r a s t i c a l l y a f f e c t c r e e p
t w i s t angle.
deformation w i t h d i f f e r e n t magnitudes dependening on t h e t w i s t a n g l e .
Surprisingly
g r a i n boundary s l i d i n g h a r d l y o c c u r r e d i n a l l t h e b i c r y s t a l specimens e e s t e d .
Of p a r t i c u l a r i n t e r e s t was t h a t t h e specimens which c o n t a i n e d 51" o r 54' t w i s t bounda r i e s which a r e regarded a s s l i g h t l y o f f C9 n e a r c o i n c i d e n c e b o u n d a r i e s showed more
d i f f i c u l t c r e e p deformation.
The observed m i s o r i e n t a t i o n dependence on c r e e p deforma t i o n may be r e l a t e d t o t h e e f f e c t i v e n e s s o f boundary a s d i s l o c a t i o n b a r r i e r dependent
The a b s o r p t i o n o r emmision o f d i s l o c a t i o n s by
a n t h e t y p e and : s t r u c t u r e o f boundary.
g r a i n boundary have been found t o b e s t r o n g l y dependent on boundary s t r u c t u r e ( 1 4 ) .
Regarding t h e s t r u c t u r a l e f f e c t on d i s l o c a t i o n p a s s a g e a c r o s s g r a i n boundary, Lim and
Raj (15) have done e x c e l l e n t e x p e r i m e n t a l work on n i c k e l b i c r y s t a l s w i t h <110> symmetr i c tilt boundaries.
They q u a n t i t a t i v e l y s t u d i e d t h e e f f e c t of g r a i n boundary s t r u c t u r e on t h e c o n t i n u i t y o f s l i p a c r o s s c o i n c i d e n c e b o u n d a r i e s w i t h d i f f e r e n t C v a l u e s .
F i g u r e 2 shows t h e observed e f f e c t o f boundary C on t h e c o n t i n u i t y of s l i p .
It i s
e v i d e n t t h a t t h e d e g r e e o f s l i p c o n t i n u i t y d e c r e a s e s w i t h i n c r e a s i n g C , i n o t h e r words
d i s o r d e r of boundary s t r u c t u r e f o r s l i p induced by screw d i s l o c a t i o n s , b u t n o t a t a l l
T h e i r work c l e a r l y shows t h a t t h e manner and t h e
f o r s l i p by mixed d i s l o c a t i o n s .
degree o f i n t e r a c t i o n between g r a i n boundary and d i s l o c a t i o n s depend on t h e t y p e and
s t r u c t u r e o f g r a i n boundary and on t h e t y p e o f d i s l o c a t i o n .
Low s l i p c o n t i n u i t y a t
high C b o u n d a r i e s h a s been e x p l a i n e d by t h e e a s e of a b s o r p t i o n of l a t t i c e d i s l o c a t i o n s
and a h i g h e r recombination energy f o r d i s s o c i a t i o n p r o d u c t s o f l a t t i c e d i s l o c a t i o n s .
It h a s been w e l l e s t a b l i s h e d t h a t g r a i n boundary s l i d i n g t a k e s p l a c e by movement of
l a t t i c e d i s l o c a t i o n s o r t h e i r d i s s o c i a t i o n p r o d u c t s a l o n g t h e boundary p l a n e .
Many i n v e s t i g a t o r s have shown by e x p e r i m e n t s t h a t g r a i n boundary s l i d i n g i s a s t r o n g l y
An example of s l i d i n g behaviour
s t r u c t u r e - d e p e n d e n t i n t e r g r a n u l a r phenomenon (16,17).
s t r o n g l y dependent on boundary t y p e observed i n aluminium (18) i s shown i n Fig.3.
We c a n r e c o g n i z e a l a r g e d i f f e r e n c e i n s l i d i n g b e h a v i o u r between ransom b o u n d a r i e s
( G.B.No 11 and 1 2 ) and c o i n c i d e n c e b o u n d a r i e s ( G.B.No 4 and 7 ) .
S l i d i n g can
t a k e s p l a c e s i g n i f i c a n t l y w i t h weak s l i d e h a r d e n i n g a t random b o u n d a r i e s , b u t i t i s
d i f f i c u l t a t c o i n c i d e n c e b o u n d a r i e s because o f prominent s l i d e hardening. It i s w e l l
a c c e p t e d t h a t g r a i n boundary s l i d i n g i s d i f f i c u l t a t low-angle b o u n d a r i e s and low C
coincidence boundaries, on t h e b a s i s of c o n s i d e r a b l e expdrimental evidence (16,17,19).
Grain boundary s l i d i n g i s a mechanism of h i g h temperature deformation and s u p e r p l a s t i c i t y occurring i n polycrystals.
The s t r u c t u r a l e f f e c t on s l i d i n g may be r e l a t e d t o
t h e g e n e r a t i o n of t h e h e t e r o g e n e i t y of deformation and of s t r e s s c o n c e n t r a t i o n a t g r a i n
boundary i r r e g u l a r i t i e s o r t r i p l e p o i n t s .
There i s c o n s i d e r a b l e evidence f o r import a n t r o l e s of s l i d i n g i n i n t e r g r a n u l a r f r a c t u r e a t h i g h temperature, a s shown l a t e r .
-
3.2 F r a c t u r e
Recently t h e e f f e c t of boundary s t r u c t u r e on i n t e r g r a n u l a r f r a c t u r e
h a s been e x t e n s i v e l y s t u d i e d on o r i e n t a t i o n - c o n t r o l l e d b i c r y s t a l s of m e t a l s and a l l o y s .
The m i s o r i e n t a t i o n dependence of f r a c t u r e s t r e s s and s t r a i n has been determined on
molybdenum ( 20-22), z i n c (23) and aluminium (24) b i c r y s t a l s .
It i s commonly observed
t h a t low-angle b o u r d a r i e s and low C coincidence boundaries have h i g h e r f r a c t u r e s t r e s s
and s t r a i n t h a n t h c s e f o r random boundaries.
This i s s o l i d evidence t h a t g r a i n bounda r i e s can be strong and d u c t i l e depending on t h e boundary type and m i s o r i e n t a t i o n .
High temperature i r t e r g r a n u l a r f r a c t u r e i s expected t o have much s t r o n g e r s t r u c t u r a l
e f f e c t t h a n low t e n p e r a t u r e i n t e r g r a n u l a r f r a c t u r e because i n t e r g r a n u l a r f r a c t u r e
mechanisms i n v o l v e more s t r u c t u r e dependent boundary phenomena such a s d i f f u s i o n ,
s l i d i n g and m i g r a t j on.
So f a r s e v e r a l workers have made
<ro To> rwi- zinc eicrystab
systematic investigations i n t o t h e
8
1.623~. r=o.NMm
e f f e c t o f boundary t y p e on c r e e p
i n t e r g r a n u l a r f r a c t u r e (17,19,25-28).
The p r o p e n s i t y f o r c r e e p i n t e r g r a g 6 2f e
n u l a r f r a c t u r e s t r c n g l y depends on
5.
boundary type.
.s
A s can b e seen fron Fig.4 random
.
boundaries ( denoted by R ) were
found t o be p r e f e r e n t i a l s i t e s f o r
2
f r a c t u r e , but n o t t o r iow-angie o r
low C coincidence boundaries ( deno- 6
t e d by L o r C p l u s numeral, respect i v e l y ) i r r e s p e c t i v e oi- boundary
i n c l i n a t i o n , i n o t h e r words w i t h o r
o
IOQ,
r ~ a ,
without i n v o l v i n g g r a i n boundary
sliding i n intergranular fracture.
r i m /min
P r e s e n t l y i t i s w e l l accepted t h a t
F i g . 1 E f f e c t of bcundary misorientation on c r e e p
creep intergranular
deformation in <1010> twist zincb i c r y s t a l s .
f r a c t u r e should have s t r u c t u r e dependence o r i g i n a t e d from s l i d i n g .
z:th'
-
*
150
Nickel
5 7 3 K . C- 2%
C=3 x
f eo..-Ca
-c
a-
kmw
U =I MPa
T = W K
G.B.No
12 -011 -0-
1114) f
.-a
40-
$
LOW1 Of
0
00
0
0
I
t 3
I
1
I
1
1
1
I
It11
19
Loa
Z
Fig.2 The e f f e c t of boundary C on t h e s l i p
c o n t i n u i t y i n i s o - a x i a l <110> symmetric t i l t
b i c r y s t a l s of n i c k e l ( Lim and Raj (15) ) .
Time1 h
Fig.3 S l i d i n g Curves o b t a i n e d f o r
random boundaries and coincidence
boundaries i n aluminium (18).
C5-5 10
JOURNAL DE PHYSIQUE
U n t i l r e c e n t l y t h e r e was no e x p e r i m e n t a l evidence t h a t t h e boundary t y p e can a f f e c t
i n t e r g r a n u l a r f r a c t u r e occurring a t t h e boundaries lying perpendicular t o the s t r e s s
a x i s , t h a t i s , i n t h e s i t u a t i o n i n which any c o n t r i b u t i o n of boundary s l i d l n g is expected.
Q u i t e r e c e n t l y Watanabe (28) h a s shown t h a t c r e e p i n t e r g r a n u l a r f r a c t u r e
o c c u r r i n g a t t h e s e b o u n d a r i e s i s a l s o dependent on t h e t y p e of boundary.
Table 1 p r e s e n t s s t a t i s t i c a l d a t a o f s t r u c t u r e - d e p e n d e n t c r e e p i n t e r g r a n u l a r f r a c t u r e
observed on iron--0.8 mass% t i n a l l o y p o l y c r y s t a l s w i t h d i f f e r e n t g r a i n s i z e s . It i s
c l e a r t h a t most of f r a c t u r e d boundaries were random boundaries.
T h e r e f o r e i t can be
s a i d t h a t c r e e p i n t e r g r a n u l a r f r a c t u r e i s dependent on boundary t y p e whether s l i d i n g induced f r a c t u r e mechanism o r vacancy-condensation ( d i f f u s i o n a l ) mechanism i s operative.
Don and Majumdar have found t h a t t h e p r o p e n s i t y f o r c r e e p i n t e r g r a n u l a r c a v i t a t i o n depends on t h e d e v i a t i o n from n e a r c o i n s i d e n c e o r i e n t a t i o n and C v a l u e i n 304
Random b o u n d a r i e s which have t h e d e v i a t i o n a n g l e A 0 exceedin
s t a i n l e s s s t e e l (26).
t h e maximum v a l u e a s c o i n c i d e n c e boundary g i v e n by Brandon's c r i t e r i o n A0c=~/12(1)-172,
t e n d t o b e c a v i t n t e d s e v e r e l y , j u s t s i m i l a r l y a s was observed by Lim and R a j f o r
c a v i t a t i o n i n low-cycle f a t i g u e a t h i g h t e m p e r a t u r e i n n i c k e l ( 2 9 ) .
F i g u r e 5 shows s c h e m a t i c a l l y s t r u c t u r e - d e p e n d e n t i n t e r g r a n u l a r f r a c t u r e s caused by
s l i d i n g - a s s i s t e d mechanism o r by vacancy-condensation ( d i f f u s i o n a l ) mechanism,
on t h e b a s i s of e x p e r i m e n t a l o b s e r v a t i o n s t h a t random b o u n d a r i e s can e a s i l y s l i d e and
t r a c t u r e , b u t n o t low-angle b o u n d a r i e s and low C c o i n c i d e n c e boundaries.
From t h e above d i s c u s s i o n we a r e l e d t o a c o n c l u s i o n t h a t t h e p r e s e n c e of a h i g h
frequency of random b o u n d a r i e s w i l l g i v e r i s e t o more i n t e r g r a n u l a r f r a c t u r e i n polycrystals.
Conversely a n i n c r e a s e i n t h e f r e q u e n c y o f low-energy b o u n d a r i e s such a s
low-angle o r low C c o i n c i d e n c e boundaries w i l ~s u p p r e s s t h e o c c u r r e n c e of i n t e r g r a nular fracture.
T h i s may be a c h i e v e d by c o n t r o l l i n g t h e g r a i n boundary c h a r a c t e r
d i s t r i b u t i o n i n p o l y c r y s t a l as w i l l b e d i s c u s s e d i n l a t e r s e c t i o n .
Now l e t us c o n s i d e r a n o t h e r import a n t a s p e c t of s ~ r u c t u r a le f f e c t s
on h i g h t e m p e r a t u r e I n t e r g r a n u l a r
fracture.
I n f a c t i t h a s been
i n c r e a s z n g l y recognized t h a t t h e
s t r u c t u r e of g r a i n boundary w i t h
a given m i s o r i e n t a t i o n c a n change
from a low t e m p e r a t u r e s t r u c t u r e
t o a h i g h t e m p e r a t u r e one a t
t r a n s i t i o n t e m p e r a t u r e by i n c r e a s i n g temperature.
Such a g r a i n
boundary s t r u c t u r a l t r a n s f o r m a t i o n
may a f f e c t i n t e r g r a n u l a r deformat i o n and f r a c t u r e a t h i g h temperature.
~i ure 6 shows
cweracure
Fig.4
Structure-dependent c r e e p i n t e r g r a n u l a r
f r a c t u r e observed i n a l p h a i r o n - t i n a l l o y .
of t h e average s l i d i n g
r a i e on <10i0> t i l t z i n c b i c r y - 1
TB.&A.
Statistical Data on Structure-dependent Intergranular Creep
s t a l s r e c g n t l y r e p o r t e d (30).
Fracture tor the Boundaries Perpendicular to the Stress Axis and
The t e m p e r a t u r e dependence was
+ Grain Bwndary Character Distribution in Fe-0.8at:I.Sn(973K,iY..9MFB)
found t o change a t c e r t a i n t r a n Spec, Grain Size Number d
Fracture
Type & No.
G. B.C.D. Q t a .
s l t i o n t e m p e r a t u r e Tc which
(,U m)
studied G.B.
data
0fG.B.
Total G.B.C.D
v a r i e s w i t h t h e t i l t a n g i e . The
R:77% tractd.:lO
R:?.Z:3
h i g h e s t Tc was o b t a i n e d on a
5 4 . 2 " t i l t boundary which i s
s i i g h t l y o f t ~ 6 . 6 ~ ~ 1 0 i i ) n>e/a~r g
coincidence o r i e n t a t i o n relationship.
Moreover a 16.5' lowa n g l e t i l t boundary showed no
change i n t h e t e m p e r a t u r e depenR:77% fractd.: 7
R:7
R:78%
dence o v e r t h e e n t i r e t e m p e r a t u r e .
The observed change i n temperature
30 Z:ZSI.
not tr.:23
R:lb.E:7
L: 3%
h a s been a t t r i b u t e d t o a g r a i n
d : Random bwndary. I: : Coincidence boundaries with E3-29.
boundary s t r u c t u r a l t r a n s f o r m a t i o n .
deEendence
L : Low-angle boundary ( 8 <15').
Quite recently Shvindlerman and Straumal (31) have examined reported TEM observations
on grain boundary dislocations at various temperatures. They found that the temperature above which dislocation image disappeared at coincidence boundaries supposedly by
a boundary structural transformation, decreases with increasing C.
From their result
it is led that the coincidence boundaries -with C smaller than about 31 can be stable
at temperatures beiow 0.6Tm, Tm is the melting temperature.
Conversely we expect
that those coincidence boundaries may lose their special properties above 0.6 Tm.
High C coincidence boundaries are considered to have lower thermal stability.
It is reasonable to conslder that temperature-induced structural transformation wiil
affect high temperature intergranular deformation and fracture in polycrystals depending on the frequency of structurally stable or unstable boundaries. Uniortunately
Structural modifications of grain
no consideration has been taken into this aspect.
boundaries may also be induced by segregation (32,33) and by incorporat~onof lattice
dilocations (34,35).
4.
4.1
Bridging between Bulk Properties of Polycrystal and Intrinsic Boundary Properties.
Importance of Grain Boundary Character Distribution,GBCD.
Up to now, when we want to predict the bulk properties of polycrystals from available
basic knowledge of intrinsic boundary properties, there is no way because so far no
There
consideration was taken into in discussing bulk properties of polycrystals.
is a gap between polycrystals and bicrystals.
Accordingly useful informations on
intrinsic structure-dependent properties of grain boundar~eshave not been fully
utilized in materials development. Recent studies have revealed that low-energy
boundaries and high-energy boundaries behave very differently, but very systemat~cally
depending on their boundary character which can be determined experimentally.
Now what we need to know is statistical data concerning the distribution of low-energy
or high-energy boundaries in real polycrystalline materials.
This kind of statistical information has been called the grain boundary character distribution (5,3b-38).
Although it is still premature, the bulk properties are expected to be controlled by
the grain boundary character distribution, GBCD.
Furthermore basic research which
reveals the relationship between GBCD and bulk properties of polycrystals is needed.
GBCG IS considered a new microstructural parameter which is necessary for explaining
The GBCU can bridge between the intrinsic proand pred~ctingthe bulk properties.
pertles of grain boundaries and the bulk properties of polycrystals.
700
4.2 Recent Studies of GBCD in Polycrystals.
More recently systematic invesigations into GBCD
for polycrystalline materials produced by differrent fabrication methods, as partly reviewed by
the present author (5,36,37).
1
Tcmperature.TlK
600
550
56)
650
.
450
I
1
cioio, T ~ l t
0-
:,
iJ
16'
I
.e
0
z
u
.
.
-
2
18,
f
i
0=16.5'
I
P
(a) S l i d ing-Assisted Mechanism
t
0(b)
Diffusional Mechanism
Fig.5 Schematic representation of structuredependent intergranular fracture caused by
(a) sliding-induced mechanism and (b) vacancy
condation-diffusional mechanism.
L
1.6
1.8
2.0
2.2
1000/(TlK )
Fig.6 Temperature dependence oi
the average slidlng rate on <i010>
tilt zinc bicrystals (30).
C5-5 12
JOURNAL DE PHYSIQUE
Regarding GBCD the following aspects need to be clarified. (ij the type of boundary
actually existing in real polycrystals,(ii) the frequency of occurrence tor individual
types of boundaries, (iii) factors affecting GhCD and (iv) relationship betwen GBCD
and bulk properties ot polycrystals produced by well defined and controlled processing.
So far we have studied the GBCD on aluminium polycrystals produced by annealing compressed single crystals, iron and its alloys conventionally cast,rolled and annealed
( Fe,Fe-Si, Fe-Sn and Fe-Co ) , cast iron-chromium alloy with columnar grain structure,
rapidly solidified and annealed iron-6.5mass% silicon alloy ribbons, iron-cobalt alloy
polycrystals annealed in magnetic field.
The electron channelling pattern (ECP)tecnnique for orientation anaiysis has been effectively used for the characterization of
grain boundaries in polycrystals having a wide range of grain size ( 5um-10mm ).
Here we look at some important findings obtained from our studies and those by others.
From the work on aluminium polycrystals produced from single crystals (38), it was
found that the frequency of specific types of boundary ( low-angle, low C coincidence
with C3-CL9, and random boundaries dependd on the amount of prestrain (60%,10%,80%)
The frequency
and the initial orientation of single crystal sheet, as shown by
of coincidence boundaries with L3-29 tends to increase from about 20% to 30% with
It is surprising that the polycrystals produced
increasing compressive prestrain.
from the single crystals with the initial orientations of 3 and 4 near f112) showed
a drastic d6crease in the frequency of low-angle boundaries with increasing prestrain.
The observed high trequency of low-angle boundaries was found to be associated with the
presence of (110) texture. Conversely the frequency of random boundaries increased
from about 25% to 50% with increasing prestrain from 60% to 80%.
lhere was not a
significant change in GBCD for the polycrystals produced from single crystals with tne
orientation 2 near f110).
Rybin et al.(39) have also studied the GBCD on molybdenum polycrystals produced
by annealing rolled or hydroextruded single crystals, or rolled polycrystals. It is of
particular interest that an extremely high frequency ( =65%) of low-angle boundaries
was observed on the palycrystal spedimen ptoduced from rolled single crystal, while
hydroextruded artd annealed specimen had the frequency of low-angle boundaries of 11%
which is close to the value (16%) for the polycrystal specimen produced by annealing
rolled polycrystal.
It is clear that the GBCD of thermo-mechanically produced polycrystals depends: on the mode of deformation, the amount of prestrain and the initial
state ot the matrix.
Watanabe et a1.(40) studied the effect of annealing temperature on GBCD in rolled
and annealed Fe--3mass%silicon alloy.
As seen in F i g . the frequency of low-angle
boundaries and of coincidence boundaries decreases with increasing annealing temperature. In counterbalancing the frequency of random boundaries increases.
There seems
to be a general trend that the frequency of random boundaries increases as grain size
increases, as indicated by
Such trend can be expected from the recent discussion on change in boundary energy during grain growth made by Watanabe (41) on the
basis of a regular polygonal grain growth model.
Quite recently Watanabe et al.(42) have determined the GBCD tor rapidly solidified and annealed Fe-6.5massX Si alloy ribbons with high ductility and excellent
We expected that the origin of
magnetic propert.xes developmed by Arai et a1.(43).
high ductility of the material might be associated with the presence of a high frequency of low-energy boundaries resistant to fracture. As expected we observed a
high frequency of low-energy boundaries of about 45% ( low-angle boundaries;24.8%
and coincidence boundaries :20.2% ). So nearly a half of existing boundaries were
It should be mentioned that the GBCD for slightly anneafound to be of low-energy.
led specimen ( L363K, lOmin ) was almost the same as that theoretically calculated
This may suggest that the microstructure
for random grain orientation distribution.
had not been much different from that of the as-solidified state. The trequency of
low-angle boundaries should be 2.3%, however it Increased drastically after full
rhe result obtained is shown In Fig. 10 together with
annealing ( 1363K for lh ) .
GBCD data obtained for other materials.
In fact it was found that the frequency of coincidence boundaries can be well
described by an inverse cube root law of C for iron and its alloys as seen from the
For the full annealed Fe-6.5massZSi alloy ribbon the frequency of C1 ( low
figure.
angle boundary is regarded as C1 coincidence boundary ) , C5, C15, Z25 coincidence
The inverse
boundaries which occurred more frequently, fell on a straight line.
cube root law holds for the iron and its alloy polycrystals used, the slope of the
Moreover the GBCD data obtained from fcc nickel (43)
lines is different among them.
m.
w.
and 304 stainless steel (26) cannot be described by the Inverse cube root law.
This may be due to the presence of extremely high frequency of C3 twin boundaries
( more than 40% ) and C3-reiated coincidence boundaries probably produced by multipletwinning.
This suggests that the mechanisms of the formation of grain boundaries
are different between the studied bcc materials and fcc materials.
Pig.9
The frequency of coincidence
boundaries with C3-29 as a function
of grain size ig iron-3X silicon
(40)Fig.7 Grain boundary character distributions
observed on aluminium polycrystals produced
by annealing compressed single crystals with
different initial orientations (38).
Random
8
/
B a n d a v
~
-0
e Low-angle Boundary
0
'
1309
(COO
1500
1609
1
I700
Pig.8 Effect of annealing temperature on GBCD
in alpha iron-3mass% silicon alloy (40).
Fig.10 C dependence of the freqency of coincidence boundaries in
some bcc and fcc polycrystals.
JOURNAL DE PHYSIQUE
C5-5 14
The s u c c e s s of q u a n t i t a t l v e d e s c r i p t i o n of t h e i n c i d e n c e of coincidence bounda r i e s by t h e i n v e r s e cube r o o t law i m p l i e s t h e p o s s i b i l i t y of p r e d i c t i o n o f t h e m c i dence of t h e c o i n c i d e n c e b o u n d a r l e s w i t h a g i v e n C.
I n t h e c a s e of t h e Fe-6.5massX
S i a l l o y r l b b o n w i t h iiOO} t e x t u r e , we can e x p e c t high i n c i d e n c e of 11, C5, C13, 117
t o r <loo> r o t a t i o n .
A c t u a l l y we observed h i g h f r e q u e n c y of t h e c o i n c i d e n c e boundaLower C c o ~ n c i d e n c eb o u n d a r i e s
r i e s w i t h t h e C v a l u e s mentioned above, except C17.
o c c u r r e d more f r e q u e n t l y i n a s c e n d i n g o r d e r of C v a l u e , propbably associated w i t h
t h e i r boundary energy.
F i n a l l y i t s h o u l d be mentioned t h a t i n Flg.10 t h e s t r a i g h t l i n e s f o r i r o n and it bcc
a l l o y s and t h e rimdom g r a i n o r i e n t a r i o n d i s t r i b u t i o n merge a t a p o i n t around H-173=0.3
which c o r r e s p o n d s t o C37.
T h i s may imply t h a t i t would b e p o s s i b l e t o i n c r e a s e t h e
frequency o n l y f o r t h e c o i n c i d e n c e b o u n d a r i e s w i t h C v a l u e s s m a l l e r t h a n 37. I n o t h e r
words, t h e c o i n c i d e n c e boundaries w i t h E l - C37 may be r e g a r d e d a s low-energy.
I n t h e p r e s e n t a u t h o r ' s o p i n i o n , t h i s may b e a u s e f u l c r i t e r i o n f o r t h e upper bound
o f C v a l u e whlch 1s p h y s i c a l l y meaningful.
We can e x p e c t t h a t such q u a n t i t a t l v e
d e s c r i p t i o n of GKCD p a r t i c u l a r l y f o r low-energy b o u n d a r i e s w i l l e n a b l e u s t o p r e d i c t
GBCD i n r e l a t i o n t o t h e method and c o n d i t i o n of m a t e r i a l f a b r i c a t i o n and m a t e r i a l
itself.
4.3 Comments on E t f e c t s of G r a i n S i z e and T e x t u r e - The p r e s e n t a u t h o r would l i k e
t o make comments on t h e e f f e c t s of g r a i n s i z e and t e x t u r e on mechanical p r o p e r t i e s o t
p o l y c r y s t a l s from t h e view p o i n t o t GBCD.
The mechanical p r o p e r t i e s o f p o l y c r y s t a l s
may b e more a t f e c t e d by g r a m b o u n d a r i e s t h a n p r e s e n t l y expected. More r e c e n t l y
G r a b s k i and coworker have launched a new approach t o g r a m s i z e e f f e c t on t h e flow
s t r e s s i n p o l y c r y s t a l s m t h e l i g h t o f r e c e n t knowledge of GBCD (44,45). High s t r e n g t h
and high ducti1it.y a c h i e v e d by g r a m r e f i n e m e n t a r e l i k e l y r e l a t e d t o t h e p r e s e n c e
o f a h i g h f r e q u e n c y of low-energy boundaries.
The e f f e c t o f t e x t u r e s h o u l d a l s o
r e c e i v e some r e c o n s i d e r a t i o n i n t o t h e r e l a t i o n s h i p t o t h e GBCD i n t e x t u r e d polycrys t a l s s i n c e h i g h i n c i d e n c e of low-angle b o u n d a r i e s and c o i n c i d e n c e b o u n d a r i e s o f t e n
appears i n scrongly textured p o l y c r y s t a l l i n e materials.
As t o t h e o r e t i c a ~ back ground of h i g h frequency of low-energy b o u n d a r i e s i n f i n e g r a l n e d p o l y c r y s t a l s , a s mentioned b e f o r e , Watanabe (41) h a s shown t h a t t h e boundary
e n e t g y f i l w ' t i n c r e a s e d u r i n g g r a i n growth i n r e c r y s t a l l i z a t r o n p r o c e s s because of t h e
requirement o f g e o m e t r i c a l arrangement of g r a i n b o u n d a r i e s b o r d e r i n g growing g r a i n .
I t i s e x p e c t e d t h a t f i n e - g r a i n e d p o l y c r y s t a l which normally e x p e r i e n c e d low temperat u r e a n n e a l i n g o r o n l y t h e e a r l y s t a g e of r e c r y s t a l l i z a t i o n , may c o n t a i n more s p e c i a l
low-energy boundaries.
Conversely t h e p r e s e n c e o t a h i g h f r e q u e n c y o f h i g h energy
random b o u n d a r i e s may e x p l a i n low d u c t i l i t y a s s o c i a t e d w i t h i n t e r g r a n u l a r f r a c t u r e
normally observed i n c o a r s e - g r a i n e d p o l y c r y s t a l s and t h e premature i n t e r g r a n u l a r
f r a c t u r e a s s o c i a t e d w i t h abnormally grown g r a i n i n f i n e - g r a i n e d p o l y c r y s t a l s .
5.
G r a i n Boundary Design f o r D e s i r a b l e Mechanical P r o p e r t i e s .
5.1
The C o n t r o l o f GBCD.
W e have shown t h a t GBCD i s t h e most i m p o r t a n t m i c r o s t r u c t u r a l parameter i n
discussing t h e r e l a t i o n s h i p between m i c r o s t r u c t u r e and p r o p e r t i e s of p o l y c r y s t a l l i n e
This s e c t i o n d i s c u s s e s f e a s l b l e ways how t o endow p o l y c r y s t a l l i n e s o l ~ d s
materials.
w i t h d e s i r a b l e mechanical p r o p e r t i e s by g r a i n boundary d e s i g n (5,36,37).
Here we c o n s i d e r t h e possibility o t improvement i n ductility by s u p p r e s s i o n of i n t e r Our f i r s t approach by g r a i n boundary d e s i g n i s t o i n c r e a s e t h e
granular fracture.
frequency o f low--energy b o u n d a r i e s which have been evidenced t o be r e s i s t a n t t o
f r a c t u r e i n any s t r e s s c o n d i t i o n and environment.
F i g u r e 11 s c h e m a t i c a l l y shows p o s s i b i e f r a c t u r e p r o c e s s e s o c c u r r i n g i n model polyc r y s t a l s w i t n hexagonal network of low-energy b o u n d a r i e s ( denoted by C j and highenergy b o u n d a r i e s ( denoted by R ) w i t h v a r y i n g t r a c t i o n (47).
We assume thdtmonly
high-energy random b o u n d a r i e s can b r e a k and n o t a t a l l on low-energy boundaries.
Under normal strsiss exceeding t h e i n t r i n s i c i n t e r g r a n u l a r f r a c t u r e s t r e s s , c r a c k s a r e
When t h e f r a c t i o n of random boundaries exceeds 2/3,
formed a t random b o u n d a r l e s .
i n t e r g r a n u l a r c r a c k s can p r o p a g a t e t o neighbouring random b o u n d a r i e s r e s u l t i n g i n
t y p i c a l i n t e r g r a n u l a r b r i t t l e f r a c t u r e both a t low t e m p e r a t u r e ( F i g . l l A ) and h i g h
However, when t h e f r a c t i o n o f random b o u n d a r i e s i s reduced
t e m p e r a t u r e ( F i g . l l B ).
t o 1 / 3 , t h e premature c r a c k s formed a t random b o u n d a r i e s p e r p e n d i c u l a r t o s t r e s s a x i s
Accordingly i n t e r cannot p r o p a g a t e f u r t h e r t o n e i g h b o u r i n g boundaries ( F i g . l l C ).
g r a n u l a r f r a c t u r e may b e c o n t r o l l a b l e by lowering t h e f r e q u e n c y of random boundaries
or conversely by increasing the frequency of low-energy boundaries.
From presently
available experimental observations, polycrystals which have the frequency of lowenergy boundaries exceeding about 40% appear to show high ductility.
However this
needs to be confirmed by more quantitative experiments.
Partial digeneration of
periodic arrangement of low-energy boundaries and high-energy boundaries may allow
premature cracks to propagate witnin the region of digeneration as shown in Fig.llD.
In view of this, some regular arrangement of low-energy boundaries and high-energy
boundaries should be considered for suppression of intergranular fracture by controlling GBCD.
5.2
The Control of Grain Boundary Inclination.
As far as mechanical properties 1s concerned, the control of boundary inclination
is considered effective for controlling intergranular deformation and fracture at
Intergranular fracture can
particular boundary geometry to the applled stress.
occur most feasibly at random boundary perpendicular to the stress axis under the
maximum normal stress. High temperature deformation and fracture associated with
sliding will occur most preferably at random boundary lying at 45' to the stress axis.
Thus, the inclination of grain boundary is one of factors controlling mechanical properties of polycrystals.
Let us conslder a special case where grain boundaries are
lying along speciflc direction(s),for instance along rolling direction as often seen
in textured polycrystals.
Ln such case we can expect a strong anisotropy of intergranular deformation or fracture, again depending on GBCD in the materials.
As is shown by Fig.llE , when random boundaries border long sides of elongated grains
and are lying more perpendicular to the stress axis, intergranular fracture would take
place more dominantly at the traction or random boundaries ot 2 1 3 .
However by choosing the stress direction along the axis of eiongated grain, intergranular fracture
may be suppressed ( Fig.llF ) .
Thus it seems possible to suppress Intergranular
fracture by controlling the boundary inclination and GBCD.
Combined effects of GBCD
and the boundary inclination may
P
enhance the anisotropy of inter(A)
ICI
granular deformation and fracture.
The anisotropy of mechanical propert t
ties should be effectively utilized
\ L a
in the deslgn for desirable mechanical properties of polycrystalline
materials.
Random j
-4
M
;PR
6. Conclusion- Recent knowledge of
structure-dependent mechanical properties of grain boundaries can be
utilized for the design and prediction of desirable bulk mechanical
properties of polycrystalline solids
by introducing a new microstructural
parameter,i.e.grain boundary character distribution GBCD.
The GBCD
is considered the most important
microstructural parameter controlline,
- the bulk mechanical m. rover
. ties
of polycrystalline solids.
Acknowledgements- The author wishes
to acknowledge the organizers of the
international conference on Interface Science and Engineering's7 for
financial support for him. He also
wishes to thank the Nippon Glass
Sheet Foundation and The Ministry
of Education,Science and Culture,
Japan tor Grant-in-Aid for Research.
ID)
(01
L
-Me
contlgurmon
R
\ L R
Random
.)
undo*
4
4
0-
0-
u
t
j.
u
Fig.11 Fracture processes in polycrystals with
different GBCDs and boundary configurations.
JOURNAL DE PHYSIQUE
C5-516
References
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
(2~)
(23)
(24)
(~5)
(26)
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(36)
(37)
(38)
(39)
(40)
(41)
(42)
(43)
(44)
(45)
(46)
(47)
Turnbull,I).ed.,Relation of Properties to Microstructure, ASM.(1957),Cleveland.
McLean,D.,Mater.bci.Engr.,
26(197b),141.
Tien,J.K. and Ansell,G.S.ed., Alloy and Microstructural Design,Academic Press
(1976), New York.
Walter,J.L.,Westbrook,J.H.
and Woodford,D.A.ed.,Grain Boundaries in Engineering
Materials, ClaitorisPub.Div.,(l975),8aton
Rouge.
Watanabe,l'., Res Mechanica, 11(1984),Nol, 47.
Chadwick,G.A.and Smith,D.A.ed.,Grain Boundary Structure and Properties, Academic
Press (1976) ,London.
Ballufti,K.W.ed., Grain Boundary Structure and Kinetics, ASM.(1980).
Ruhle,M.,Ilalluffi,K.W.,Fishmeister,H.
and Sass,S.L.ed., Structure and Properties
of Internal interfaces, J.de Physique,46(1985),Colloque C4,No4.
Proc.4th Japan 1nst.Metals Intern.Symp.(JIMlS-4) on Grain Boundary Structure and
Related Phenomena, Suppl.to JIM.,27(1986).
tiansen,N.,Horsewell,A.,Leffer,T.and Linholt,H.ed.,Deformation of Polycrystals:
Mechanisms and Microstructures, Riso National Laboratory (1981), Koskilde.
Baker,T.N.ed.,Yield,Flow and Fracture of Polycrystals, Applied Science Pub.,
(1983) ,London.
Miura,S. and Saeki,Y.,Acta Met.,26(1978),93.
Kurzydlowr;ki,K.J.,Varin,R.A. and Zielinski,W.,Acta Met.,32(1984),71.
Pond,R.C. and Smith,D.A., Phil.Mag.,36(1977),353.
Lim,L.C. and Raj,R., Acta Met., 33(1985),1577
Biscondi,M., J.de Physique, 43(198L),c6-293.
Watanabe,T. and Davies,P.W., Phil.Mag.,A37(1978),649.
Kokawa,H.,Watanabe,T. and Karashima,S., Phil.Mag.,~44(1981),1239.
Watanabe,'C., Met.Trans., 14A(1983),531.
Kobylanski,A. and Goux,C., Comptes Rendus, 277(1971),1937.
Brosse,J.B.,Fillit,R. and Biscondi,M., Scripta Met., 15(1981),b19.
Kurishita,B.,Kuba,S.,Kubo,H. and Yoshinaga,H., J.Japan.lnst.Metals,47(1983),539.
Watanabe,'P.,Shima,S. and Karashima,S., Embrittlement by Liquid and Solid Metals,
ed.by Kamdar,M.H/, Met.Soc.AiME.(1984),
p.173.
Otuki,A. and Mxzuno,M., ref.(9),p.789.
Lagarde,P, and Biscondi,M., Mem.Sci.Kev.Metallurg.,71(1974),121.
Don,J. and Majumdar,S., Acta Met., 34(1Y86) ,9b1.
Afshar,S. and Biscondi,M., Scripta Met., 20(1986),701.
Watanabe,'T., Creep and Fracture of hngineering Materials and Structures, ed.by
Wilshire,B. and Evans,R.W., 1nst.Metals (1987),p.155.
Lim,L.C. .md K a j , R . , Acta Met., 32(1984),1183.
Watanabe,T.,Kimura,S. and Karashima,S., Phil.Mag., A49(1984),845.
Shvindlennan,L.S. and Straumal,B.B., Acta Met.,33(1985),1735.
Eastman,J.A.,Sickafus,K.E. and Sass,S.L., ref.(9),p.145.
Hash~moto,M.,Ish~da,Y.,Yamamoto,R.,Doyama,M.and Fujiwara,T.,Scripta Met.,l6
(1982),26 7.
Varin,R.A. and Kurzydlowski,K.J., Z.Metallkde.,74(1983),1/7.
Kokawa,H.,Watanabe,T. and Karashima,S., Scripta Met., 1/(1983),1155.
Watanabe,T., ref. (9), p. 73.
Watanabe,T., Materials Forum, Anniversary Volume, (1988).
Watanabe,T.,Yoshikawa,N. and Karash~ma,S.,Proc.bth 1ntern.Conf.on Textures of
Materials (ICOTOM-6), vo1.1(1981), Japan.Iron.Stee1 lnst., p.609.
Rybin,V.V.,Titovetz,Yu,F.,Teplitskiy,D.M.
and Zolotorevskiy,N.Yu, Phys.Metals
of Metallography, 53(1982),No.3,
122.
Watanabe,T. ,Kawamata,Y. and Karashima,S., ref. (9), p. 601.
Watanabe,T., Scripta Met., 21(1982),427.
Arai,K. and Ohmori,K., Met.Trans., 17A(lY86),1295.
Lim,L.C. and Raj.R., Acta Met., 32(1984),1177.
Grabski,M.W., ref.(8),c4-567.
Wyrzykowski,J.W. and Grabski,M. W. , Phil.Mag., ~53(1986), 505.
Watanabe,T., ref.(8),c4-555.
Watanabe,T., Proc.NAT0 Advanced Research Workshop on Chemistry and ~hyslcsof
Fracture, (1987).