Introduction and point groups

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Introduction and point groups Stereographic
projections Low symmetry systems Space
groups Deformation and texture Interfaces,
orientation relationships Martensitic
transformations
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Materials, transformation temperatures strength
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Diffusionless transformation?
Martensite can form at very low
temperatures. Martensite can grow very
rapidly. No composition change during
transformation.
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Shape of martensite
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Irrational why?
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Orientation relationships irrational
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Creation of a bi-crystal
cut and rotate by angle q about axis normal to
diagram
q
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Glissile interface
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Glissile interface cannot contain more than one
set of dislocations. Martensitic transformation
only possible if the deformation which changes
the parent into the product leaves one line
undistorted and unrotated, i.e. an
invariant-line. Deformation is an invariant-line
strain.
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s
d
c
r
1
Christian, 1957
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body-centred cubic
cubic close-packed
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b
a
3
3
a
2
b
b
2
1
a
1
(a)
(b)
BAIN
STRAIN
(d)
(c)
Body-centered
Body-centered
cubic martensite
tetragonal
austenite
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001
b
b'
100
o
a'
a
b
(a)
o
b'
(b)
a,a'
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transformation twins (Wayman)
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Two non-coplanar invariant-plane strains
p
d
e
q
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The Bain Strain
(F B F)
where F is an orthonormal basis parallel to the
unit cell of austenite
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Assume that the lattice invariant deformation is
on the system
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u deformed to x by (F B F)
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lattice invariant deformation is
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h deformed to l by (F B F)
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