03 Defects, Diffusion, and Microstructure Free Online FlashCards

What is a vacancy in a crystal?

A vacancy is a missing atom at a crystal lattice site. Its concentration generally increases as temperature rises.

What is an interstitial defect?

An interstitial is an atom occupying a space in the crystal that is normally unoccupied.

What vacancies form a Schottky defect?

A Schottky defect is a paired set of vacancies: one cation vacancy and one anion vacancy.

How does a Frenkel defect form?

A Frenkel defect forms when an ion leaves its lattice site and occupies an interstitial site.

How do dislocations enable plastic deformation?

A dislocation is a line defect that can move through a crystal, allowing plastic deformation without shifting an entire atomic plane at once.

What separates two grains with different orientations?

A grain boundary is an interface separating crystals, or grains, with different orientations.

Why can pores and cracks weaken a material?

Pores and cracks can reduce load-bearing area and concentrate stress, making them potential sites for fracture initiation.

What is diffusion in a solid?

Diffusion is the net transport of particles down a concentration gradient, caused by random thermally activated motion.

What does Fick’s first law describe?

For a one-dimensional steady concentration profile, Fick’s first law is $J=-D\frac{\partial C}{\partial x}$. The minus sign indicates flux down the concentration gradient.

What does Fick’s second law describe?

For constant diffusion coefficient $D$, Fick’s second law is $\frac{\partial C}{\partial t}=D\frac{\partial^2 C}{\partial x^2}$. It describes how concentration changes with time.

How does temperature generally affect diffusion?

The Arrhenius relation is $D=D_0\exp\left(-\frac{Q}{RT}\right)$. Because diffusion is thermally activated, raising temperature usually increases its rate substantially.

Why is diffusion often faster along grain boundaries?

Short-circuit paths, such as grain boundaries and dislocations, generally allow faster diffusion than the crystal lattice because their structures are less regular.