summer school generalized continua and dislocation theory
TRANSCRIPT
summer school
Generalized Continua
and Dislocation Theory
Theoretical Concepts,
Computational Methods
And Experimental Verification
July 9-13, 2007
International Centre for Mechanical Science
Udine, Italy
Lectures on:
Introduction to and fundamentals of
discrete dislocations and dislocation
dynamics. Theoretical concepts and
computational methods
Hussein M. ZbibSchool of Mechanical and Materials Engineering
Washington State University
Pullman, WA
Contents
Lecture 1: The Theory of Straight Dislocations – Zbib
Lecture 2: The Theory of Curved Dislocations –Zbib
Lecture 3: Dislocation-Dislocation & Dislocation-Defect Interactions -Zbib
Lecture 4: Dislocations in Crystal Structures - Zbib
Lecture 5: Dislocation Dynamics - I: Equation of Motion, effective mass - Zbib
Lecture 6: Dislocation Dynamics - II: Computational Methods - Zbib
Lecture 7 : Dislocation Dynamics - Classes of Problems – Zbib
Dislocation Dynamics
Crystals is a solid
Closed Packed Crystal structure
Slip systems in F.C.C. Crystal
Slip systems in B.C.C. Crystal
Basic Geometry (bcc)
Microscopic strain and its relation to dislocation
motion
Peierl’s stress
Lecture 4: Dislocations in Crystal Structures
Macroscopic experiment
“Macroscopic Scale”
representative
“homogeneous” element
Continuum Plasticity
“Mesoscopic Scale”
Polycrystalline
plasticity
“Microscopic Scale”
dislocations in single
crystal
,
DD from In-situ Exp.
Dislocation Dynamics
1m
Dislocation structure in a high purity copper
single crystal deformed in tension (Hughes)
Shock recovery experiments M. Schneider et al., Acta Mater. (2003)
TEM:
0.25 mm slices are
cut from the
recovered sample
~50-25 ~25-15 ~8-2 ~2-0Pmax (GPa)
Dislocation density increases with pressure
<134> Cu
205 J pulse
Dislocation dynamics
Shock in Cu, PH=50 GPa
2.5 μm
MD by E. M. Bringa (LLNL)
Crystals is a solid in which atoms are periodically arranged in a regular pattern.
The periodicity can be described by a space lattice which is a regular, 3-dimensional
arrangement of lattice points.
The space lattice can be generated by a simple transformation of a unit cell. A unit cell is
completely characterized by 3 lattice vectors a, b, c (or a, b, c, and three angles α, β,γ).
When a unit cell contains only one atom within it, it is called a primitive cell. There are only
7 possible (geometrically) crystal system and 14 Bravais Lattice (space lattice ) to
characterize the crystal structures,
αβγ
ab
c
1. Cubic: a=b=c, α=β=γ= 90o
P (primitive), I (Body centered), F (Face centered),
4-3-fold symmetry
2. Tetragonal: a=b#c, α=β=γ= 900
P , I. 1-4 fold symmetry
3. Orthorhombic: a#b#c, α=β=γ#900
P, I, F, B (Base centered), 3-2 fold symmetry
4. Rhombohedral: a=b=c, α=β=γ#900, P, 1-3 fold symmetry
5. Hexagonal: a=b#c, α=β γ= 1200, P ,1-6 fold symmetry
6. Monoclinic: a=b=c, α=γ= 900 #β, P. B, 1-2 Fold symmetry
7. Triclinic: a#b#c, α#β#γ, P, NONE
Miller indices
Represents the orientation of a plane or a direction in crystal in relation to the unit cell axes.
i) Direction; [h, k, l]
h, k, l are the smallest integers in x, y, z axes
<h, k, l> - family of [h, k, l]
ii) Plane; (h, , l)
The intercept distance of plane with x, y, a, axes are a, b, c. Take the reciprocal of a, b, c, then make them the smallest integer numbers taking out a common factor (i.e vector normal to plane)
y
k
hl
x
z
[h, k, l]
x
z
ab
c
y
),,(1
),,( abacbcabc
lkh
Closed Packed Crystal structure
1) Closed-Packed plane has the largest possible number of atoms per area or the highest atomic density .
There are only two ways to accomplish this.
A B C A BC ….. ; (F.C. C.)
the closed-packed plane is {111}
A B A B …………; (H.C. P.)
the closed packed plane is {0001}
Since the Burger’s vectors are restricted to a perfect lattice vector and the energy of a dislocation line is proportional to b2, the slip planes are naturally defined as these close-packed plane.
Slip system in F.C.C. Crystal
In F.C.C crystal, the shortest lattice vector in {111} is <011> Therefore
0112
ab
Hence the slip system of F.C.C crystal is given as {111} - <011>
and there are twelve combinations
Designation of Slip Systems in FCC
)111( Critical plane A
)111( Primary plane B
)111( conjugate plane C
)111( Cross slip plane D
Six Slip Directions
]011[I ]011[V
II ]110[
III ]101[
IV ]011[
]110[VI
Each slip plane contains three slip directions
Slip system in F.C.C. Crystal& Cross-slip planes
I. Basic Geometry (bcc)
Simulation Cell
(5-20 )
[100][010]
[001]
Slip plane
(101)
m b
Initial Condition: Expected outcome!
*Random distribution Mechanical properties (yield stress,
(dislocation, Frank-Read hardening, etc..).
Pinning points (particles) Evolution of dislocation structures
*Dislocation structures Strength, model parameters, etc..
Discrete segments
of mixed character
“Continuum” crystal
(Hull and Bacon 1984)
Microscopic strain and its relation to dislocation motion
3D Discrete Dislocation Dynamics
)nbbn( iii
p
i
N
i
gii
V
vl
1 2
)nbbn(W iii
p
i
N
i
gii
V
vl
1 2
ii vm *F
t
v
dv
dW
vm
1*
Equation of Motion
Effective Mass
Dislocation
velocity?
Dislocation
length?
Dislocation
Burgers
Vector?
The macroscopic strain and its relation to dislocation motion
Can also be derived from energy argument
Work done by externally
applied shear stressdu AduWe
work by dislocation motion: LdxbWd )( dx
We=Wd
vb
dxbd
dxbV
bldx
Ah
bldx
h
du
bldxAdu
Dislocation density:
ρ = l/V
Dislocation length per
unit volume
Peierl’s stressThe stress field was determiend by treating the material as an elastic
continuum, yielding a stress singularity at the dislocation core. The
Peirel’s stress introduces the effect of Lattice periodicity.
b
ux
4sin
2 d
bxy
x1tan2
bux
ν)2(1
d
The width of the dislocation
ν)(1
d
2
The Peierls Stress field for edge dislocation:
)(
)1(2
)1(2
)1(2
yyxxzz
yy
xx
xy
b
b
b
22
2
22
22
2
22
2222
ζ)(yx
y2x
ζ)(yx
y
ζ)(yx
ζ)2y(y
ζ)(yx
)2(3y
ζ)(yx
ζ)2xy(y
ζ)(yx
x
The Peierls Stress field for edge dislocation reduces to the Volterra dislocation for
1/222 )y(xr
The parameter δ removes the singularity at the origin r=0 that is present for the Volterra dislocation
Elastic Energy of Peierls dislocation
Consists of two parts:
1) Elastic strain energy stored in two half crystal
2) Misfit energy due to the distorted bonds
Elastic strain energy stored in two half crystal
2ζ
Rln
ν)(14π
μbE
2El
2
Misfit energy due to the distorted bonds
)-(14
μbE
2
M
Peierls energy
During the dislocation glide, the misfit energy changes periodically, but the elastic energy (Volterrs’s dislocation) does not change. However, the equation given in previous slide does not contain periodic form. This is because we assumed a continuous displacement field. It is shown that when one accounts for atomic periodicity one gets:
)4cos(4
exp
b)-(12
μb
)-(14
μbE
22
M
Then, the Peierls force
b
EM
4expmax
-1
b2 2
Then, the Peierls stress
bp
4exp
-1
2
Note: as p (very sensitive)
In general 24 10~10 p
Dislocation dynamics in BCC system