Topic 5: Strengthening Mechanisms

What is strength

resistance of a material to load

what are the types of strength

yield, ultimate, failure, tensile

What do inclined planes relative to the axis of tensile load feel

resolved shear and tensile stress

what happens in plastic deformation

planes of atoms slip in shear when critial resolved shear stress is reached

what is needed to slide a plane of atoms past another plane

enough energy (resolved shear stress) to break the number of atoms in the plane(critical resolved shear stress)

what causes plastic deformation

dislocations allowing atoms to slide by one another

what takes less energy incrementally breaking bonds, or breaking bonds all at once.

incrementally

what is dislocation motion

movement of extra half-plane of atoms by breaking and reforming of interatomic bonds

what kind of tensile strength is associated with dislocation motion

lower tensile stregth bc imperfections make sliping easier.

Does dislocation motion cause a permanent change

yes, but with less energy

where does slip occur

allong CPPS

why do CPPs have low atomic energy and the shortest slip distance

they have the lowest sitance between atoms which means less energy required

what is the critical resolved shear stress

the amount of energy required to break all of the bonds on a slip plane

how different is the actual resolved shear stress from the theoretical shear stress

actual is 10 lower than theorectical

why is actual shear stress less than theoretical shear stress

imperfections

What does more slip systems mean

more dictile

Why do more slip systems mean higher ductility

more likely that a slip system is aligned with a resolved shear stress

Which metal structure type would be more brittle

HCP bc less slip planes

What makes a material yield

more slip

How can you strengthen

make slip more difficult

What are strengthening mechanisms

Chemcial - Adding different elements

Physical - application of processing the material

Example of chemical strengthening

Alloying ( solid solution hardening, precipitation hardening)

Example of Physical strengthening

grain refinement, strain hardening (cold working)

what do 1 D defects and dislocations (2d) create in a lattice

stress field

what happens when stress fields interact

takes more energy to move dislocations past foreign atoms

How to stress fields affect eachother

they repel

How do interstitial, vacancy, and substitution defects affect dislocation movement

they act like speed bumps, slowing it down

Why does it take more energy to move dislocations past defects

requires more energy

What does more energy needed to move dislocations mean

more resolved shear stress and more applied tensile stress

How does adding these defects affect material properties

makes it stronger, but more brittle

How could you make gold stronger

add silver or copper

How to make iron stronger

add carbon

What kind of imperfection is precipitation hardening based on

3D imperfection (precipitate)

How do precipitates affect dislocation motion

make it more difficult

waht changes when a dislocation encounters a precipitate

slip planes, bond length, and bond energy

what happens when a dislocation reaches the interface between two phases

it gets stuck

what is required to keep dislocation moving past a precipitate

increased stress (higher yielding)

what is overall effect of precipitation hardening on the material

becomes stronger but more brittle

visually what is the best for precipitation hardening

want the precipitate smaller and more spread out

Why cant dislocations move easily past grain boundaries

the boundary distrups dislocation motion

What is required for a dislocation to pass a grain boundary?

Greater applied tensile stress

What changes at a grain boundary?

Bond length and the angle of the slip plane relative to the load

How does grain size affect dislocation motion?

Smaller grain size → more grain boundaries → less dislocation motion.

What are the effects of smaller grain size on material properties?

Material becomes stronger, harder, and more brittle.

Does grain refinement require alloying elements?

no

How can grain size be controlled?

By cooling rate — faster cooling → smaller grains.

What is strain hardening / cold working

Deforming metals at temperatures much lower than melting

What happens to dislocations during cold working?

Plastic deformation creates more dislocations (dislocation multiplication).

How do dislocations affect each other during cold working

They run into each other, creating "traffic jams" that make movement harder

What is the effect of dislocation traffic jams on the material

The material becomes stronger

Where are more dislocations introduced?

In the plastic deformation region.

What is heat treatment / annealing?

restoring a material's microstructure to its pre-cold-worked state.

How is heat treatment done?

Reheating and slow re-cooling to relieve strain energy.

Why do we do heat treatment?

To increase ductility of the material.

What happens during recovery in annealing?

Internal strain energy is relieved, and the number of dislocations is reduced due to enhanced atomic diffusion.

What happens during recrystallization?

New strain-free grains form with fewer dislocations.

Grains grow and consume surrounding areas.

Mechanical properties from prior processing are restored.

What happens during grain growth?

Strain-free grains at boundaries continue to grow, opposite of grain refinement.

What is the overall effect of annealing on a material?

Makes the material less brittle and restores ductility.

How can chemical strengthening (alloying) be reversed?

By completely re-melting the structure and adding more of the minority element.

What happens during elastic deformation?

Metallic bonds stretch under tensile load, but return to original length if unloaded.

What happens if a brittle material undergoes elastic deformation?

Bonds break or fracture under tension.

What is plastic deformation?

Dislocations move along close-packed planes under resolved shear stress, causing permanent slip.

What is another term for plastic deformation in metals?

Yielding — permanent deformation in ductile materials.

Why does dislocation motion make metals weaker than a perfect crystal?

It takes less energy to move one bond at a time along CPPs than to break all bonds at once.

When will plastic deformation occur in metals?

(a) Unit cells must have close-packed planes

(b) Dislocations must be present (true for all natural materials).

Are all pure metals ductile?

yes

How can we make metals stronger?

Use strengthening strategies that increase stress required for slip, but this reduces ductility (more brittle).

What is resolved shear stress?

The shear stress on a plane at an angle to the applied tensile load.

When will slip occur on a plane?

When τ > τc (shear stress exceeds critical value).

Why does slip occur more easily on CPPs?

Slip distance is shorter and less energy is required.

Which crystal structures are more ductile?

FCC and BCC are more ductile than HCP because they have more slip systems, increasing the chance a slip plane aligns with stress.

why doesn't resolved shear stress cause brittle materials to deform during loading

Brittle materials have few slip systems and strong direction bonds so dislocations cant move. They will just break