Test 2 Material Science

Jermey Myers

What is a vacancy?

A missing atom or ion from its normal lattice site.

What is an interstitial defect?

An extra atom/ion occupying a normally unoccupied lattice position.

What is a substitutional defect?

A different atom/ion replaces a normal lattice atom/ion.

Why can point defects strengthen a material?

Their strain fields interfere with dislocation motion.

What is a dislocation?

A line defect whose movement allows a crystalline material to deform.

What makes up a slip system?

A slip plane + slip direction.

Where does slip occur most easily?

Along high-density directions on high-density planes.

What is critical resolved shear stress (CRSS)?

The resolved shear stress required to initiate slip.

What is Schmid’s Law?

τᵣ = σ cosφ cosλ

When does slip begin?

When the resolved shear stress reaches the CRSS.

Why do smaller grains strengthen a material?

More grain boundaries are present to obstruct dislocation movement.

What happens during work/strain hardening?

Dislocation density increases, making further dislocation movement harder.

How does work hardening affect strength and ductility?

Strength up, ductility down

What does annealing do after strain hardening?

Decreases dislocation density and increases ductility.

What is solid-solution strengthening?

Guest atoms distort the crystal lattice and impede dislocation movement.

Could a metal with no dislocations deform easily by slip?

No. Dislocation movement is required for easy slip deformation.

How does increasing dislocation density affect ductility?

It generally decreases ductility because further slip becomes more difficult.

What is diffusion?

Net movement of atoms/species through a material.

What is the driving force in Fick’s 1st Law?

The concentration gradient.

What is Fick’s 1st Law?

J = −D(dc/dx)

What does J represent?

Diffusion flux

What does D represent?

Diffusion coefficient/diffusivity.

Why is Fick’s 1st Law negative?

Diffusion occurs from high concentration → low concentration.

What is self-diffusion?

Atoms of the same material move between lattice positions.

What is interdiffusion?

Diffusion of unlike atoms within materials.

How does vacancy diffusion occur?

An atom moves into an adjacent vacancy.

How does interstitial diffusion occur?

A small atom moves from one interstitial site to another.

Which is usually easier: interstitial or vacancy diffusion?

Interstitial diffusion.

Why is interstitial diffusion usually faster?

It does not require vacancies and generally has lower activation energy.

What does low activation energy Q mean?

Diffusion occurs more easily and rapidly

How does increasing temperature affect diffusion?

It increases the diffusion rate.

What equation relates diffusivity to temperature?

D = D₀e^(−Q/RT)

Rank diffusion paths slowest → fastest.

Volume → Grain boundary → Surface

Which diffusion path has the lowest activation energy?

Surface diffusion.

What does Fick’s 2nd Law describe?

Non-steady-state diffusion, where concentration changes with time

What is the basic form of Fick’s 2nd Law?

∂c/∂t = D(∂²c/∂x²)

What factors strongly affect diffusion?

Time, temperature, diffusivity, crystal structure, and concentration gradient.

What is engineering stress?

S = F/A₀

What is engineering strain?

e = ΔL/L₀

What is stress?

Force per unit area

What is strain?

Change in dimension divided by original dimension; it is dimensionless.

What is elastic deformation?

Recoverable deformation that disappears after the stress is removed.

What is plastic deformation?

Permanent deformation remaining after the stress is removed.

What is Young’s modulus, E?

The slope of the linear elastic portion of the stress-strain curve.

What equation gives Young’s modulus?

E = S/e

What does a larger Young’s modulus mean?

The material is stiffer.

If stress = 600 MPa and strain = 0.10, what is E?

E = 600/0.10 = 6,000 MPa = 6 GPa

What is yield strength?

The stress where the material transitions from elastic to plastic deformation.

What is ultimate tensile strength (UTS)?

The maximum engineering stress on the stress-strain curve

What is tensile toughness?

Energy absorbed before fracture; the area under the stress-strain curve.

What is modulus of resilience?

Elastic energy a material can absorb before yielding.

What is ductility?

The ability to undergo permanent deformation without fracturing

What are two measures of ductility?

% elongation and % reduction in area.

What is the percent elongation equation?

% elongation = [(Lᶠ − L₀)/L₀] × 100

What is the percent reduction in area equation?

%RA = [(A₀ − Aᶠ)/A₀] × 100

What is true stress?

σ = F/A, using the instantaneous area.

What is true strain?

ε = ln(L/L₀)

How do you convert engineering stress to true stress?

σ = S(1 + e)

How do you convert engineering strain to true strain?

ε = ln(1 + e)

What is Poisson’s ratio?

ν = −e_lateral/e_longitudinal

If ν = 0.5 and longitudinal strain = 0.02, what is lateral strain?

−0.01

What type of loading causes fatigue failure?

Repeated/cyclic loading.

What is the mean-stress equation?

σₘ = (σmax + σmin)/2

What is the stress-amplitude equation?

σₐ = (σmax − σmin)/2

If σmax = −100 MPa and σmin = −220 MPa, what is mean stress?

−160 MPa

For σmax = −100 MPa and σmin = −220 MPa, what is stress amplitude?

60 MPa

What conditions promote creep?

Sustained stress + elevated temperature + time.

What is creep?

Time-dependent permanent deformation under sustained stress.

What is ductile fracture associated with?

Significant plastic deformation before final fracture.

What are the major stages of ductile fracture?

Void formation → void growth → void coalescence → final fracture.

What is DBTT?

The ductile-to-brittle transition temperature, where fracture behavior changes from ductile to brittle.

Do all metals exhibit a DBTT?

No. The Chapter 6 slides specifically note that FCC metals do not exhibit this behavior.

What are the three types of dislocations?

Edge, screw, and mixed dislocations.

What type of defect is a dislocation?

A line defect.

Shear stress acts parallel or perpendicular to a plane?

Parallel to the plane.

Normal stress acts parallel or perpendicular to an area?

Perpendicular to the area.

What would the properties of a perfect metal crystal with no defects be?

It would have very high strength but very low ductility because without dislocations, slip/plastic deformation would be extremely difficult.

What is the general relationship between strength and ductility?

Generally, as strength increases, ductility decreases, and vice versa.

Why are dislocations important to the mechanical behavior of metals?

Their movement allows slip and plastic deformation; restricting their movement increases strength.