自结晶纳米力学—杨卫

清华大学工程力学系
Surface Self-Nanocrystallization Self-
清华大学工程力学系
清华大学工程力学系
Experiment by Lu and Lu
Sample holder
Sample Shots
NanoNano-grains of 10nm were formed in the surface layer of pure iron with initial grain size of 50µm after 50µ shotshot-peening of 450s。 450s。
Density evolution by dislocation migration Density evolution by dislocation generation
Plastic strain rate by dislocation motion
Change in sample surface profile Exit iteration
Columb contact
Flow Chart
Data input, time step selection Iteration starts
Pressure & friction force of shot-peens Total change in dislocation density
FEM computes velocity and stress fields Density evolution by dislocation interaction
清华大学工程力学系
Dislocation Injection by Shot-peening ShotPlane strain simplification
Specimen dimension 30×35 mm2 30× Shot diameter 2 mm Number of shots 300 Peening speed 5~20 5~20 m/s Peening flux 9~90 /(smm2) /(smm
& ρT = −∫
2π
0
r r eφ • ∇(ρf (φ )V )dφ
Ω1, Γ1 Ω, Γ
Orientation evolution
r dL
r v
r n
d( f (φ ) ) 1 r r & = − eφ • ∇(ρf (φ )V ) + ρ T f (φ ) dt ρ
[
]
清华大学工程力学系
Steady State Dislocation Emission from a Source
Dislocation density across the depth N＝800 /mm V/R=20000 s-1，ξ＝0, x = 0 V/R=20000
Oblique Impact
N＝500 /mm
Dislocation density across the depth V/R=20000 s-1， ξ＝20°, x = 0 V/R=20000 20° N＝800 /mm
Surface grain size ranges from 10 to 20nm.
Sample Vacuum Vacuum Sample
v
f
f
清华大学工程力学系
清华大学工程力学系
Nanostructured layer
Dislocations
Slip direction
表面纳米层阻滞内部 位错的逸出， 位错的逸出，形成位 错塞积， 错塞积，导致加工硬 化和内部孔洞形成。 化和内部孔洞形成。
& & & & ρ = ρT + ρ G − ρ D
Distribution of dislocation orientations is described by a normalized density f(φ)
清华大学工程力学系
Dislocation Migration
Dislocations entering and exiting a prescribed region
Dynamic FEM Simulation
Iteration Scheme
1 1 M+ C a (t + ∆t ) = Q (t ) + Ka 0 (t ) 2 2∆t ∆t 2 1 1 − K − 2 M a (t ) − 2 M − C a (t − ∆t ) 2∆t ∆t ∆t
清华大学工程力学系
Plastic Strain Rate
Plane Strain Incompressible Dislocation Glide Orowan Kinematic Law
& ε P = bκρ ∫
2π
0
f (φ )V (φ )e
i (π / 2 + 2φ )
dφ
清华大学工程力学系
清华大学工程力学系
Impact Angles
As impact angle increases, the indent depth declines, while the maximum dislocation density peaks at 20º.
Normal Impact
N＝500 /mm
τ Y = τ 0 + α 0 µbρ ALL
−1 / 2
清华大学工程力学系
Dislocation Density Evolution
Evolution of mobile dislocations consists of their migration, generation and annihilation
MD Simulation
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2000 MD steps
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4000 MD steps
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A statistical approach based on mutual orientations
& ρ D = ρ ∫ PB f (φ )dφ
0 2π
PB = V ∑ H (α i )ρ (α i )
i= i =1
K
Orientation evolution
d( f (φ ) ) 1 2π & = ρ ∫ PB f (φ )dφ + ρ D f (φ ) dt ρ 0
High frequency displacement generator
清华大学工程力学系
NanoNano-grains formed in a surface layer of 10 microns
Surface Self-Nanocrystallization Selfby Muliti-direction Shot-peening MulitiShot-
清华大学工程力学系
Dislocation Motion
Gliding speed of dislocations under Schmid stress
(τ − τ Y ) V = V S 1 − exp − B
TaylorTaylor-type hardening law
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6000 MD steps
____ 10nm
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10
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8300 MD steps
Tail dislocation from a source
2 µb η= π (1 − ν )τ
Dislocations generated in time dt dt
mS dnG = 2
m S V dt
η
清华大学工程力学系
Dislocation Generation
Density change by dislocation generation
o
Grain boundary of larger angle is energetically favored
θ = 20°，D = 13 nm 20°
A
o
清华大学工程力学系
表面纳米化
二维模型 1万多个原子 FS势 NVT系综(313K) 周期性边界条件 0.5fs时间步长 引入位错密度 2.17*1011mm-2 最后形成约10nm的晶粒
µb 2

Low angle GB forms if θD > 2b
清华大学工程力学系
Forming Nano-grains NanoLow angle grain boundary
ρ = 2θ /(bD)
A θ = 10°，ρ = 2.17 × 1011 mm-2，D = 6.5 nm 10°
& ρG =Orintation evolution