Dislocation dynamics and crystal plasticity

6 problems

• Universality class of dislocation avalanches in microcrystals

  Contested

  Small metal crystals deform in sudden jumps called avalanches, whose sizes follow a power law, much like earthquakes. Whether these jumps obey the same simple theory as other avalanche systems, and what sets the largest size, is unsettled.

• Grain size of maximum strength in nanocrystalline metals

  Contested

  Metals usually get stronger as their crystal grains shrink, but when grains are only a few dozen atoms across they get weaker again. Where the peak lies and which mechanism takes over is debated.

• Origin of the size dependence of micropillar strength

  Partially resolved

  Metal pillars a few hundredths of a millimeter wide or smaller are many times stronger than large crystals, with strength rising as the diameter shrinks by a power law. Several mechanisms are proposed, but none predicts the exponent and its variation between metals.

• How persistent slip bands initiate fatigue cracks

  Open

  Under repeated small loads, metals form thin bands of concentrated slip with a ladder-like dislocation structure, and cracks begin where these bands meet the surface. How the bands push out surface ridges and start cracks has not been derived from dislocation physics.

• Driving force of tin whisker growth

  Open

  Thin tin coatings in electronics spontaneously grow hair-like metal filaments that can short circuits, sometimes years after manufacture. What stress or process pushes the atoms into the whisker is not established.

• First-principles theory of work hardening and dislocation patterning

  Open

  As a metal is deformed it gets harder, and its dislocations arrange themselves into cells and walls whose size shrinks as the stress grows. Neither the hardening rate nor the pattern has been derived from the motion of individual dislocations.