Deformation in Crystalline Materials

What is elastic deformation?

Reversible deformation caused by stretching of atomic bonds

What happens to atoms during elastic deformation?

Atoms move slightly apart and return to original positions when unloaded.

What is plastic deformation?

Permanent deformation caused by slipping of atomic planes.

What causes metals to permanently change shape?

Slip of atomic planes.

What is slip in crystalline materials?

The movement of atomic planes relative to one another.

Why does slip occur most easily on close-packed planes?

Because they require the least energy for atomic movement.

What determines whether a material is ductile or brittle?

The number of available slip systems.

What is a slip system?

A close-packed direction on a close-packed plane.

How is the number of slip systems calculated?

Number of CP directions × number of CP planes.

Why are materials with more slip systems more ductile?

Slip can occur more easily in many directions,

so can deform plastically without cracking suddenly

Which crystal structures are generally ductile?

FCC and BCC

Which crystal structures are generally brittle?

HCP

How many slip systems does HCP have?

3

How many slip systems does FCC have?

12

How many principal slip systems does BCC have?

12

Why is slip along non-close-packed planes difficult?

It requires greater atomic movement and more energy.

What type of stress drives slip?

Shear stress.

What is resolved shear stress?

The component of stress acting in the slip direction on the slip plane.

What is the critical resolved shear stress (CRSS)?

The minimum resolved shear stress required to initiate slip

What happens when resolved shear stress reaches CRSS?

Yielding begins.

In a single crystal, when is yielding easiest?

When ϕ=45^o and λ=45^o

What is the relationship between yield stress and CRSS at optimal orientation?

σy = 2T_CRSS

Why are actual metal strengths much lower than theoretical strengths?

Because dislocations allow slip at lower stresses.

What is work hardening?

Strengthening caused by plastic deformation and increased dislocation interactions.

What is the key difference between elastic and plastic deformation at the atomic level?

Elastic = bond stretching

Plastic = atomic plane slip.

Why does slip tend to occur gradually rather than all at once?

Due to imperfections in the crystal lattice.