continuum dislocation theory: size effects and formation of … · 2012-09-11 · continuum...
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Continuum dislocation theory: size effects andformation of microstructure
K.C. Le
Lehrstuhl fur Mechanik - MaterialtheorieRuhr-Universitat Bochum, 44780 Bochum, Germany
Workshop on ”Multiscale Material Modeling”, Bad Herrenalb 2012
Collaborators: V. Berdichevsky, D. Kochmann, Q.S. Nguyen,P. Sembiring, B.D. Nguyen
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 1 / 78
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Outline of my lecture
Size effects in crystal plasticity and formation of microstructure
Why continuum dislocation theory?
Thermodynamic framework of CDT
Antiplane constrained shear
Plane constrained shear
Double slip systems
Mechanism of twin formation
Continuum model of deformation twinning
Bending
Polygonization
Conclusion
Further works
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 2 / 78
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Size effects in crystal plasticity and formation ofmicrostructures
Hall-Petch relation
Indentation
Shear, torsion, bending
Deformation twinning
Polygonization
Recrystallization
Grain growth
Texturing
ect.
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 3 / 78
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Hall-Petch relation
σY ∝ σY 0 +λ√d
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 4 / 78
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Indentation test
H =F
A(Hardness)
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 5 / 78
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Twin formation
TEM micrograph of the [111] oriented single crystal under tension. Strain30%, Karaman et al., 2000
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Polygonization
Polygonized state of a bent single crystal beam
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 7 / 78
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View along intersections of slip planes with polygon boundaries in apolygonized zinc crystal, Gilman, 1955
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Why continuum dislocation theory?
Plastic deformations are due to nucleation, multiplication and motionof dislocations
Dislocations appear to reduce energy of crystals
Motion of dislocations produces energy dissipation which causes theresistance to their motion
Under favorable condition the rearrangement of dislocations bydislocation climbing and gliding may reduce further energy of crystals
Deformation twinning and polygonization are low energy dislocationstructures
Continuum description is dictated by high dislocation densities(108-1014m−2). To compare: dislocation density 4×1011m−2 meansthat the total length of dislocation loops in one cubic meter of crystalequals the distance from the earth to the moon
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 9 / 78
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Related literature
Continuum dislocation theory: Kondo,1952; Nye,1953; Bilby etal.,1955; Kroner,1955,1958; Berdichevsky & Sedov,1967; Le &Stumpf,1996a,b,c; Ortiz & Repetto,1999; Ortiz et al.,2000;Acharya,2001; Svendsen,2002; Gurtin,2002,2004; Berdichevsky,2006;Berdichevsky & Le,2007; Le & Sembiring,2008a,b,2009; Kochmann &Le,2008,2009a,b; Le &Nguyen,2010,2011; Le & Nguyen,2012.
Strain gradient plasticity: Fleck et al.,1994; Shu & Fleck,1999; Gao etal.,1999; Acharya & Bassani,2000; Huang et al.,2000,2004; Fleck &Hutchinson,2001; Han et al.,2005; Aifantis & Willis,2005.
Statistical mechanics of dislocations: Le & Berdichevsky,2001,2002;Groma et al.,2003,2005; Berdichevsky,2005,2006.
Discrete dislocation simulations: Needleman & Van der Giessen,2001;Shu et al.,2001; Yefimov et al.,2004.
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 10 / 78
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Thermodynamic framework of CDTKinematics
s
m
ep
ee
e e e= +e p
Small strain theory: unknown functions ui (x), βij(x)Plastic distortion: βij =
∑na=1 βa(x)sai m
aj
Total (compatible) strain: εij = 12 (ui ,j + uj ,i )
Plastic (incompatible) strain: εpij = 12 (βij + βji )
Elastic (incompatible) strain: εeij = εij − εpijK.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 11 / 78
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Dislocation density
da
Bi=αijnjda
1 2 3 4 5
6
7
89101112
13
1 2 3 4 5
6
7
891011
12
13
14 14
b
a) b)
Dislocation density: αij = εjklβil ,k
Geometrical interpretation: Stokes theorem∫Sαijnj da =
∮Lβijτj ds = Bi
Bi is the resultant Burgers vector of all GND (excess dislocations), whosedislocation lines cuts the area S. For single slip the scalar dislocationdensity is ρ = 1
b |εjklβ,kmlnj |.
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 12 / 78
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Energetics
State variables: elastic strain εeij and dislocation density αij (plastic strainand its gradient are history dependent and are not the state variables)
Free energy density (Kroner)
ψ(εij − εpij , αij) = ψ0(εij − εpij) + ψm(αij)
Elastic energy of the crystal lattice
ψ0(εeij) =1
2Cijklε
eijε
ekl
Energy of microstructure (single slip system, Berdichevsky, 2006)
ψm(αij) = µk ln1
1− ρ/ρsSmall up to moderate dislocation density
ψm(αij) ' µk(ρ
ρs+
1
2
ρ2
ρ2s
)K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 13 / 78
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Variational principles of CDT
Negligible resistance to dislocation motion: the energy minimization
I (ui , βij) =
∫Ωψ(εij − εpij , αij)dx → min
u,β
Finite resistance to dislocation motion: Variational equation (Sedov,Berdichesky, 1967)
δI +
∫Ω
∂D
∂βijδβij dx = 0
implying the evolution equation (of Biot type) for the plastic distortion
∂D
∂βij= −δεψ
δβij≡ − ∂ψ
∂βij+
∂
∂xk
∂ψ
∂βij ,k
Note that, if the dissipation potential D = D(βij) is a homogeneousfunction of first order (rate-independent theory), the variational equationreduces to the minimization of “relaxed” energy.K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 14 / 78
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Antiplane constrained shear
x
y
a
z
hL
A single crystal strip is placed in a “hard” device with the prescribeddisplacements at the boundary
uz = γy
γ overall shear strain (control parameter)Assumption: a h, a LK.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 15 / 78
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Single slip system
Plastic distortion
βzy = β(x)
Since dislocation cannot reach the boundary x = 0, a, the plastic distortionβ(y) satisfies the constraints:
β(0) = β(a) = 0
The plastic strains are given by
εpyz = εpzy =1
2β(x).
The only non-zero component of tensor of dislocation density is
αzz = β,x .
The free energy density of the crystal with dislocations takes a simple form
ψ =1
2µ(γ − β)2 + µk(
|β,x |ρsb
+β2,x
ρ2sb
2),
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 16 / 78
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Energy minimization
If the resistance to dislocation motion is negligible, the true plasticdistortion minimizes the total energy
I [β(x)] = hL
∫ a
0
[1
2µ(γ − β)2 + µk(
|β,x |ρsb
+β2,x
ρ2sb
2)
]dx
Introducing the dimensionless quantities
x = xbρs , a = abρs , E =bρsµhL
I
to rewrite the energy functional in the form
E [β(x)] =
∫ a
0
[1
2(γ − β)2 + k(|β′|+ 1
2β′2)
]dx
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 17 / 78
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Energetic threshold
x
Β
Ε aa-Ε
Β
E =1
2(γ − β0)2a + 2k |β0|
Minimization of this function shows that there exists the threshold value
γen =2k
abρs
Size effect (typical to all gradient theories): the threshold value is inverselyproportional to the size a of the specimen.
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Minimizer
Ansatz (based on the feature of dislocation pile-up)
β(x) =
β1(x) for x ∈ (0, l),
βm for x ∈ (l , a− l),
β1(a− x) for x ∈ (a− l , a),
Functional
E = 2
∫ l
0
[1
2(γ − β1)2 + k
(β′1 +
1
2β′21
)]dx +
1
2(γ − βm)2(a− 2l).
Function β1(x) is subject to the boundary conditions
β1(0) = 0, β1(l) = βm.
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Varying this energy functional with respect to β1(x) we obtain the Eulerequation for β1(x) on the interval (0, l)
γ − β1 + kβ′′1 = 0.
The variation of energy functional with respect to βm and l yields the twoadditional boundary conditions at x = l
β′1(l) = 0, 2k = (γ − βm)(a− 2l).
Solution
β1(x) = γ − γ(coshx√k− tanh
l√k
sinhx√k
), 0 ≤ x ≤ l .
Transcendental equation to determine l
f (l) ≡ 2l + 2k
γcosh
l√k
= a.
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Evolution of plastic distortion
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0.002
0.004
0.006
0.008
x
β
a
b
c
Evolution of β: a)γ = 0.001, b)γ = 0.005, c)γ = 0.01
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Stress strain curve
0.001 0.002 0.003 0.004 0.005
0.0002
0.0004
0.0006
0.0008
σ/μ
γO
A
B
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Non-zero dissipation
For the case with dissipation (β > 0) we have to solve the evolutionequation
γ − β + kβ,xx = γc , β(0) = β(a) = 0,
with γc = K/µ. Introduce the deviation of γ(t) from the critical shear, γc ,γr = γ − γc > 0
γr − β + kβ′′ = 0.
Thus, β = γrβ1. Similarly, for β < 0: in all formulas γ must be replacedby γl = γ + γc . For β = 0 the evolution equation need not be satisfied. Itis replaced by β = 0, so the plastic distortion is frozen.
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Loading program
t
g
g*
-gc
Loading path
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 24 / 78
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Hysteresis and Bauschinger effect
-0.0005 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030
-0.0004
-0.0002
0.0002
0.0004
O
A
B
C
D
σ/μ
γ
Average shear stress versus shear strain curve
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Plane strain constrained shear
0
y
x
h
sm
a
L
z
γh
A single crystal strip is placed in a “hard” device with the prescribeddisplacements at the boundary
u = γy , v = 0 at y = 0, h
γ overall shear strain (control parameter)Assumption: h a LK.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 26 / 78
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Single slip system
The plastic distortion tensor
βij = β(y)simj
where
s = (cosϕ, sinϕ, 0), m = (− sinϕ, cosϕ, 0)
Since dislocation cannot reach the boundary y = 0, h, the plasticdistortion β(y) satisfies the constraints:
β(0) = β(h) = 0
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Strain measures
Total strain
εxx = 0, εxy =1
2u,y , εyy = v,y
Plastic strains
εpxx = −1
2β sin 2ϕ, εpxy =
1
2β cos 2ϕ, εpyy =
1
2β sin 2ϕ
Elastic strain
εexx =1
2β sin 2ϕ, εexy =
1
2(u,y − β cos 2ϕ),
εeyy = v,y −1
2β sin 2ϕ
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Nye’s dislocation density
da
Bi=αijnjda
Non-zero components of dislocation density tensor
αxz = β,y sinϕ cosϕ, αyz = β,y sin2 ϕ
Scalar dislocation density
ρ =1
b
√α2xz + α2
yz =1
b|β,y || sinϕ|
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Energy density
Energy per unit volume of dislocated crystal
ψ(εeij , αij) =1
2λ (εeii )
2 + µεeijεeij + µk ln
1
1− ρρs
λ, µ - Lame constants, b - magnitude of Burgers’ vector, ρs - saturateddislocation density, k - material constant
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Energy functional
E = aL
∫ h
0
[1
2λv2
,y +1
2µ(u,y − β cos 2ϕ)2 +
1
4µβ2 sin2 2ϕ
+ µ(v,y −1
2β sin 2ϕ)2 + µk ln
1
1− |β,y || sinϕ|bρs
]dy
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Reduced energy functional
Minimization with respect to u and v leads to
E (β) =aL
∫ h
0µ
[1
2(1− κ)β2 sin2 2ϕ+
1
2κ〈β〉2 sin2 2ϕ
+1
2(γ − 〈β〉 cos 2ϕ)2 + k ln
1
1− |β,y || sinϕ|bρs
]dy
where 〈β〉 = 1h
∫ h0 β(y) dy
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Energy minimization
If the resistance to dislocation motion is negligible, the true plasticdistortion minimizes the total energy
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Energetic threshold
γen =2k
hbρs
| sinϕ|| cos 2ϕ|
Size effect: the threshold value is inversely proportional to the size h.
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Material parameter
Material µ (GPa) ν b (A) ρs (µm−2) k
Aluminum 26.3 0.33 2.5 1.834 103 0.000156
Table: Material characteristics
In all simulations h = 1µm
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Evolution of plastic distortion
0.1 0.2 0.3 0.4
-0.0075
-0.005
-0.0025
0.0025
0.005
0.0075
a
b
c
d
e
f
y-=30°=60°
Β
Evolution of β: a,d)γ = 0.0068, b,e)γ = 0.0118, c,f)γ = 0.0168
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Stress strain curve
0.002 0.004 0.006 0.008 0.01
0.002
0.004
0.006
0.008
=30°=60°
O
A
B
A´
B´
Γ
ΤΜ
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Non-zero dissipation
Non-zero resistance to dislocation motion: energy minimization is replacedby the flow rule
∂D
∂β= −δγψ
δβ
for β 6= 0, where
D = K |β|
κ ≡ −δγψδβ
= −∂ψ∂β
+∂
∂y
∂ψ
∂β,y
Plastic distortion may evolve only if the yield condition |κ| = K is fulfilled.If |κ| < K , then the plastic distortion β is frozen
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Yield condition
Differential equation
|k β,yy sin2 ϕ
b2ρ2s
− (1− κ)β sin2 2ϕ
− (cos2 2ϕ + κ sin2 2ϕ)〈β〉+ γ cos 2ϕ| = K/µ = γcr cos 2ϕ
Boundary conditions
β(0) = β(h) = 0
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Evolution of plastic distortion
β(y) = γrβ1(y), where γr = γ − γcr
j=30°j=60°
y
_
b1
Graphs of β1(y)
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Dislocation density
α(y) = γrα1(y)
j=30°j=60°
y_
a1
Graphs of α1(y)
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Size effect for hardening rate
It is interesting to calculate the shear stress τ which is a measurablequantity. During the loading, we have for the normalized shear stress
τ
µ= γcr + γr
(1−
(1−
2 tanh ηh2
ηh
)β1p cos 2ϕ
)
where β1p is calculated from the solution. The second term of thisequation causes the hardening due to the dislocations pile-up anddescribes the size effect in this model.
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Comparison
ΤΤ
h/d=1.15h/d=2.3h/d=80with dissipationenergy minimization
Γ
Shear stress vs. shear strain curve
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Comparison
y/h
h/d=2.3h/d=80energy minimizationwith dissipation
Γ
Γ
Γ
Γ
u,y
Shear strain profile
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Loading program
t
ΓΓ
Γ
O
A
B
C
Loading path
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Bauschinger effect
-0.002 0.002 0.004 0.006 0.008 0.01-0.002
0.002
0.004
0.006
0.008
OA
B
C
DΓ
ΤΜ
Normalized shear stress versus shear strain curve for ϕ = 60
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Double slip system
jrjl
y
h
0
z
xa
L
gh
sl
ml
mr
sr
Tensor of plastic distortion
βij(y) = βl(y)s limlj + βr (y)sri m
rj
with βl(y) and βr (y) satisfying
βl(0) = βl(h) = βr (0) = βr (h) = 0
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Strain measures
Total strain
εxx = 0, εxy =1
2u,y , εyy = v,y
Plastic strains
εpxx = −1
2(βl sin 2ϕl + βr sin 2ϕr ),
εpxy =1
2(βl cos 2ϕl + βr cos 2ϕr ),
εpyy =1
2(βl sin 2ϕl + βr sin 2ϕr )
Elastic strain
εexx =1
2(βl sin 2ϕl + βr sin 2ϕr ),
εexy =1
2(u,y − βl cos 2ϕl − βr cos 2ϕr ),
εeyy = v,y −1
2(βl sin 2ϕl + βr sin 2ϕr )
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Nye’s dislocation density
Non-zero components of Nye’s dislocation density tensor
αxz = βl ,y sinϕl cosϕl + βr ,y sinϕr cosϕr
αyz = βl ,y sin2 ϕl + βr ,y sin2 ϕr
Scalar dislocation density
ρ = ρl + ρr
=1
b|βl ,y sinϕl |+
1
b|βr ,y sinϕr |
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Energetic threshold
γen =2k
hbρs
| sinϕ|| cos 2ϕ|
Size effect: the threshold value is inversely proportional to the size h.Mention that the energetic threshold value for the symmetric double slip isequal to that of the single slip found in (Le & Sembiring, 2007).
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Comparison
h/d=40
h/d=160h/d=80
h/d=240with dissipationenergy minimization
t/t0
g
Shear stress vs. shear strain curve
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Comparison
g=0.0068
g=0.0118
g=0.0168
g=0.0218
h/d=80energy minimizationwith dissipation
y_
h
u,y
The total shear strain profiles
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Mechanism of twins formation
TT
T
TT
Tg
x
y
s
m
h
ax
y bt g
x
y
hs
h(1-s)
ju
jl
rotation
shear
TT
TT
TT
TT
The twin phase is formed by a twinning shear produced by the movementof pre-existing dislocations to the boundary followed by a rigid rotation.The described mechanism of twin formation is closely related to that ofBullough (1957). The difference is that dislocations need not to glidethrough each lattice plane.
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Continuum model of deformation twinning
Tensor of plastic distortion
βij(y) =
βs lim
lj for 0 < y < h(1− s),
βsui muj + βts
lim
lj + ωij for h(1− s) < y < h,
with β(y) satisfying the constraints:
β(0) = β(h) = β (h(1− s)) = 0
The twinning shear is given by βt = −2 cotϕ
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Plastic strains
The in-plane components of the plastic strain tensor εpij = 12 (βij + βji ) read
εpxx = −1
2β sin 2ϕ− 1
2βT sin 2ϕl ,
εpxy =1
2β cos 2ϕ+
1
2βT cos 2ϕl ,
εpyy =1
2β sin 2ϕ+
1
2βT sin 2ϕl ,
with the following quantities defined in the upper and lower part of thecrystal:
[β(y), ϕ, βT ] =
[βu(y), ϕu, βt ] h(1− s) < y < h,
[βl(y), ϕl , 0] 0 < y < h(1− s).
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Dislocation density
da
Bi=αijnjda
Non-zero components of dislocation density tensor
αxz = β,y sinϕ cosϕ, αyz = β,y sin2 ϕ
Scalar dislocation density
ρ =1
b
√α2xz + α2
yz =1
b|β,y || sinϕ|
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Energy density
Free energy per unit volume of dislocated crystal
ψ(εeij , αij) =1
2λ (εeii )
2 + µεeijεeij + µk ln
1
1− ρρs
λ, µ - Lame constants, b - magnitude of Burgers’ vector, ρs - saturateddislocation density, k - material constant
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Energy functional
E (u, v , β, s) = aL
∫ h
0
[12λv
2,y + 1
4µ(β sin 2ϕ+ βT sin 2ϕl)2
+ 12µ(u,y − β cos 2ϕ− βT cos 2ϕl)
2
+ µ(v,y − 12β sin 2ϕ− 1
2βT sin 2ϕl)2
+ µk ln1
1− |β,y sinϕ|bρs
]dy .
TWIP-alloys have rather low stacking fault energies, so the contribution ofsurface energy to this functional can be neglected.
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Condensed energy
0.05 0.1 0.15 0.2 0.25
0.0001
0.0002
0.0003
0.0004
0.0005
0.002 0.004 0.006 0.008 0.01
0.00001
0.000012
s = 0
s = 1
s = 0.9
s = 0.7
s = 0.6
s = 0.5
s = 0.8
s = 0.4
s = 0.3
s = 0.2s = 0.1
condE
g
8 -6·10
6 -6·10
4 -6·10
2-6
·10
condE
g
s = 0
s
Condensed energy E cond(s, γ) = minβ E (β, s, γ) versus overall shear strainγ for various volume fractions s (left) with a magnification (right) forsmall values of s and small strains (clearly indicating that s = 0 minimizesthe energy in that region). The actual energy with evolving s follows thepath of least energy
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0.05 0.1 0.15 0.2 0.25 0.3
0.00005
0.0001
0.00015
0.0002
0.00025
0.0003
0.05 0.1 0.15 0.2 0.25 0.3
0.2
0.4
0.6
0.8
1
0.05 0.1 0.15 0.2 0.25 0.3
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0.05 0.1 0.15 0.2 0.25 0.3
-0.2
-0.15
-0.1
-0.05
0.05 0.1 0.15 0.2 0.25 0.3
0.2
0.4
0.6
0.8
1
0.05 0.1 0.15 0.2 0.25 0.3
0.1
0.2
0.3
0.4
0.5
E
g
s
g
blp
gbup
g
ll
g g
lu
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Stress-strain curve
0.05 0.1 0.15 0.2 0.25 0.3
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
t/m
g
A
B
C
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Bending
x
y
r
ja
h sm
A single crystal beam is bent along a rigid cylinder. The displacements ofthe lower face are prescribed
ux(x , 0) = r sin(x/r)− x , uy (x , 0) = r cos(x/r)− r
Assumption: h a
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Single slip system
Tensor of plastic distortion
βij = β(x , y)simj , s = (cosϕ, sinϕ), m = (− sinϕ, cosϕ)
Plastic strains
εpxx = −1
2β sin 2ϕ, εpyy =
1
2β sin 2ϕ, εpxy =
1
2β cos 2ϕ
Dislocation density
αxz = β,x cos2 ϕ+ β,y cosϕ sinϕ,
αyz = β,x cosϕ sinϕ+ β,y sin2 ϕ
Scalar dislocation density
ρ =1
b
√(αxz)2 + (αyz)2 =
1
b|β,x cosϕ+ β,y sinϕ|
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Energy density
Free energy per unit volume of dislocated crystal
ψ(εeij , αij) =1
2λ (εeii )
2 + µεeijεeij + µk ln
1
1− ρρs
λ, µ - Lame constants, b - magnitude of Burgers’ vector, ρs - saturateddislocation density, k - material constant
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Energy functional of the bent beam
I =
∫ a
0
∫ h
0[1
2λ(ux ,x + uy ,y )2 + µ(ux ,x +
1
2β sin 2ϕ)2
+ µ(uy ,y −1
2β sin 2ϕ)2 +
1
2µ(ux ,y + uy ,x − β cos 2ϕ)2
+ µk ln1
1− |β,x cosϕ+β,y sinϕ|bρs
] dxdy
Because of the prescribed displacements at y = 0 dislocations cannotreach the lower face of the beam which is in contact with the bending jigin the deformed state, therefore
β(x , 0) = 0
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Reduced energy functional
Variational-asymptotic analysis reduces the energy functional containingthe small parameter h/a to
E1 =
∫ a
0
∫ h
0[κ(cos
x
r− 1 +
1
2β′ sin 2ϕ)2 + k|β′,y sinϕ|
+1
2k(β′,y sinϕ)2] dxdy
where κ is given by
κ =1
1− ν,
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Energy minimization
In the equilibrium state the true plastic distortion minimizes the totalenergy
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Smooth minimizer
The plastic distortion
β′(x , y) =
β1(x , y) for y ∈ (0, l(x)),
β0(x) for y ∈ (l(x), h)
where
β1(x , y) = β1p(1− coshχy + tanhχl(x) sinhχy)
β1p = − 2
sin 2ϕ(cos
x
r− 1), χ =
√2κ
kcosϕ
Transcendental equation to determine l(x)
k sinϕ+ κ[(cosx
r− 1)
+1
2β1p(1− 1
coshχl(x))] sin 2ϕ) sin 2ϕ(h − l(x)) = 0.
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Energetic threshold
Size effect: the threshold value depends on a and h
rcr =a
arccos(1− k2κh cosϕ)
Thus, if the radius of the bending jig r > rcr , then l(x) = 0 and β = 0everywhere yielding the purely elastic deformation without dislocations.For r < rcr the dislocations are nucleated and pile-up against the lowerboundary y = 0 with x > x∗ forming there the boundary layer.
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Material parameter
Material µ (GPa) ν b (A) ρs (µm−2) k
Zinc 43 0.25 2.68 145.4 0.000156
Table: Material characteristics
In all simulations a = 10mm, h = 1.3mm, ϕ = 35.
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 70 / 78
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2 4 6 8 10
0.02
0.04
0.06
0.08
x
l
Function l(x).
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2 4 6 8 10
0.5
1.0
1.5
2.0
2.5
3.0
x
β0
Function β0(x).
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Polygonization
During annealing dislocations may climb in the transversal direction andthen glide along the slip direction and be rearranged as shown in thisFigure.K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 73 / 78
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After annealingIn the final polygonized relaxed state the dislocations form low angle tiltboundaries between polygons which are perpendicular to the slip direction,while inside the polygons there are no dislocations. We want to show thatthis rearrangement of dislocations correspond to a sequence of piecewiseconstant β(x , y) reducing energy of the beam. Here and below check isused to denote the polygonized relaxed state after annealing. The jump ofβ means the dislocations concentrated at the surface, therefore we ascribeto each jump point the normalized Read-Shockley surface energy density
γ([[β]]) = γ∗|[[β]]| ln eβ∗
|[[β]]|,
with [[β]](xi ) = β(xi + 0)− β(xi − 0) denoting the jump of β(x),γ∗ = b
4π(1−ν) , and β∗ the saturated misorientation angle.
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Energy reducing sequence
2 4 6 8 10
0.5
1.0
1.5
2.0
2.5
3.0
x
β0
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Number of polygonsThe number of polygons can be estimated from above by requiring that theincrease of the surface energy is less than the reduction of the bulk energyin gradient terms. For the material parameters of zinc, the estimatedaverage polygon distance (taken as the length of the beam divided by thenumber of polygons) is equal to around 2.7× 10−7 m which is in excellentagreement with the experimental result obtained by Gilman in 1955.
0 2 4 6 8 100
20
40
60
80
100
r
lnN
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 76 / 78
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Conclusions
CDT enables ones to model dislocation pile-up and size effects
There exists a threshold value for the dislocation nucleationdepending on the grain size
Work hardening and Bauschinger effect can properly be described
Deformation twins exhibit another type of non-convexity
Existence of distinct thresholds for the onset of deformation twinning
The stress-strain response exhibits a sharp load drop (followed by astress plateau) upon the onset of twinning
Polygonization occurs due to the smallness of the Read-Shockleysurface energy as compared with the bulk energy of distributeddislocations
The rearrangement of dislocations is realized by the dislocation climbwith the subsequent dislocation glide.
High-temperature dislocation climb during annealing is crucial for thekinetics of polygonization
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 77 / 78
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Further works
Indentation
Bending and torsion
Particle strengthening
Multiple slip
CDT for polycrystals
Hall-Petch relation
Plastic zone near the crack tip and ductile fracture
Finite twinning shear and finite rotation
Parameter identification and applications to TWIP-alloys
3-D model
Dislocation climb taking into account the interaction with vacancies
Kinetics of polygonization
Formation of dislocation cell structure
K.C. Le (Mechanik - Materialtheorie) CDT: size effects, microstructure Bad Herrenalb workshop 2012 78 / 78