Mechanical Behavior of Materials Lecture Note (6) 10/3/2010 Nahid Page 1 In order to understand why some materials bend easily while others simply break one must think about the crystal structure, and how a stress being applied to the material will influence the atoms. In all cases, one must think about how easy is it to get atoms to move past one another. The easier this process is, the more “bendable” or ductile a material is. The more difficult, the more brittle that material is. The motion of atoms past one another in a metal is known as slip. The easier it is to induce slip in a material, the more ductile the material is. This is a function of crystallography in the material. There are certain preferred directions that atoms can move called slip directions, and certain preferred planes of atoms that will moved when a stress is applied called slip planes. The combination of the slip directions and slip planes (multiplied together), give much information about the mechanical response of a material called the slip system. The larger the number of the slip system, the more ductile a material will be. Deformation of materials occurs when a line defect (dislocation) moves (slip) through the material.
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Mechanical Behavior of Materials Lecture Note (6) 10/3/2010
Nahid Page 1
In order to understand why some materials bend easily while
others simply break one must think about the crystal structure,
and how a stress being applied to the material will influence
the atoms.
In all cases, one must think about how easy is it to get atoms to
move past one another. The easier this process is, the more
“bendable” or ductile a material is. The more difficult, the
more brittle that material is.
The motion of atoms past one another in a metal is known as slip.
The easier it is to induce slip in a material, the more ductile the material is. This is a
function of crystallography in the material.
There are certain preferred directions that atoms can move called slip directions, and
certain preferred planes of atoms that will moved when a stress is applied called slip
planes.
The combination of the slip directions and slip planes (multiplied together), give
much information about the mechanical response of a material called the slip
system. The larger the number of the slip system, the more ductile a material will
be.
D e f o r m a t i o n o f m a t e r i a l s o c c u r s
w h e n a l i n e d e f e c t ( d i s l o c a t i o n )
m o v e s ( s l i p ) t h r o u g h t h e m a t e r i a l .
Mechanical Behavior of Materials Lecture Note (6) 10/3/2010
Nahid Page 2
P l a s t i c d e f o r m a t i o n i s d u e t o t h e
m o t i o n o f a l a r g e n u m b e r o f
d i s l o c a t i o n s .
W h e n a s h e a r f o r c e i s a p p l i e d t o a
m a t e r i a l , t h e d i s l o c a t i o n s m o v e
R e a l m a t e r i a l s h a v e l o t s o f
d i s l o c a t i o n s , t h e r e f o r e t h e
s t r e n g t h o f t h e m a t e r i a l d e p e n d s
o n t h e f o r c e r e q u i r e d t o m a k e t h e
d i s l o c a t i o n m o v e , n o t t h e b o n d i n g
e n e r g y . ( U n l i k e i n e l a s t i c z o n e )
D i s l o c a t i o n s a l l o w d e f o r m a t i o n a t
m u c h l o w e r s t r e s s t h a n i n a
p e r f e c t c r y s t a l
If the top half of the crystal is slipping one plane at a time then only a small fraction of
the bonds are broken at any given time and this would require a much smaller force.
The propagation of one dislocation across the plane causes the top half of the crystal to
move (to slip) with respect to the bottom half but we do not have to break all the bonds
across the middle plane simultaneously (which would require a very large force).
T h e e a s e w i t h w h i c h d i s l o c a t i o n s
m o v e t h r o u g h a m e t a l c r y s t a l i s