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 Embrittlement Hydrogen 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 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 Density On the surface of a thin metal film: Average inter-dislocation distance Cold-worked metal: Average 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 A typical steel

Ductile Tensile Test

Macro Scales

Specimen Size for Fracture Test Valid plane-strain fracture toughness conditions. 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.