5/6/2002, Monday

Summary: What we learned from this course?

Multi-scales Material Behaviors

nanoscale

microscale

millimeter scale

macroscale

Nanometer Scales

Burgers Vector

Magnitude of Burgers vector ~0.5nm

Bond breakage occurs consecutively rather than simultaneously.

Hydrogen EmbrittlementHydrogen can diffuse rapidly through the lattice because of its small size.

Hydrogen tends to accumulate at the tension part of the dislocation, thus hinders the movement of dislocation.

Environment Assisted Cracking

Dislocation Emissions

Dislocation free zone size is about several nanometers

Slip Systems

Resolved shear stress acting on the slip system

For FCC crystals, slip occurs most often on {111} planes and in <110> directions.

Slip Plane

Monotonic loading gives rise to staircase morphology slip offsets.

Cyclic loading produces sharp peaks (extrusions) and troughs (intrusions).

Micrometer Scales

Strength of metal crystals as a function of dislocation density

The strength of a metal approaches extremely high levels when there are either no dislocations present or when the number of dislocations is extremely high.

Dislocation DensityOn the surface of a thin metal film:

€

1011−1012 dislocations/cm2

€

0.03μm−0.01μmAverage inter-dislocation distance

Cold-worked metal:

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104 −105 dislocations/cm2

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100μm−32μmAverage inter-dislocation distance

Annealed metal:

Inter- or Intra-granular Fracture

Intergranular-fracture Intragranular-fracture

For regular material, crack prefers to follow grain boundaries.

Grain Boundary Strengthening

Grain boundaries serve as effective barriers to the movement of glide dislocations.

Fatigue Striation

Dislocation Creep

Dislocation creep involves the climbing of edge dislocations away from dislocation barriers.

Millimeter Scales

Crack Tip Plastic Zone

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12π

K IC

σ ys

⎛

⎝ ⎜

⎞

⎠ ⎟

2

=1

2π72.5MPa m1340MPa

⎛

⎝ ⎜ ⎞

⎠ ⎟

2

~0.5mm

A typical steel

Ductile Tensile Test

Macro Scales

Specimen Size for Fracture Test

Valid plane-strain fracture toughness conditions.

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t and a≥2.5K IC

σys

⎛

⎝ ⎜

⎞

⎠ ⎟

2

€

t and a≥2.533MPa m490MPa

⎛

⎝ ⎜ ⎞

⎠ ⎟

2

=0.011m

Typical metal alloy:

Simple Torsion Failure

The maximum normal stress failure theory is generally suitable for brittle materials.

The Tresca theory describes failure as taking place when the maximum shear stress exceeds the shear strength.

Necking

With increasing load, a point is reached where the strain-hardening capacity of the material is exhausted and the further plastic deformation is localized in the necked region.