Engg204 - Midterm prep

1 amu/atom

1 g/mol

1 mol

6.022 x 10^23 atoms/mol or number of atoms

Enet

-A/r + B/r^n

Bond Hybridization

electron pairs break into unshared electrons

crystal structure value of c

4rsq(6)/3

crystalline

arranged in repeating array

polycrystalline

large array of small crystals (grain), pieced together

amorphous solids

non-crystalline, lack systematic arrangement of atoms

anisotrpy

property value depends on crystallographic direction of measurement (like Young's modulus)

isotropy

property value is identical in all directions

if grain is randomly oriented

isotropic

if grain is textured (mechanically deformed)

anisotropy

allotropy

one element has two or more distinct crystal structure dependent on external factors (ex. temp or pressure)

polymorphism

one element or a molecule has two or more distinct crystal structures dependent on external factors (ex. temp or pressure)

How to determine crystal structure?

a narrow x-ray beam hits crystal, creating a pattern of spots. different atomic planes refract beam in specific directions

unit cell

basic structural unit

Atomic Packing Factor

volume of atoms / volume of unit cell

volume of atoms

number of atoms x ((4/3) (pi) r^3)

number of defects

Number of vacancies ^[-Qv/kT]

Number of vacancies

(Avagadro's Number x density) / Atomic mass

Types of Imperfections

1. vacancies

2. self - interstitial

3. interstitial

4. substitutional

solid solution

when an interstitial atom does not cause strain in lattice structure. there is a solute and it distribute in lattice structure evenly

Hume Rothery conditions

1. Δr < 15%

2. for metals: same crystal structure

3. solute needs high valency

4. must have similar electronegativity

FCC interstitial sites

Octahedral: 1 site in the center, 12 sties centered on the edges, (1 + 12/4) = 4 atoms total

Tetrahedral: 8 sites in the cell, between corner atoms and face atoms, 8 atoms total

BCC interstitial sites

Octrahedral: 12 sites centered on the edges, 6 centered on the faces, (12/4 + 6/2) = 6 atoms total

Tetrahedral: 4 sites off centered on each face, 24 sites total, (24/2) = 12 atoms total

Interstitial composition (things to keep in mind)

1. any total value of number of atoms/moles x at% = number of atoms/moles of that element

2. any total value of mass x wt% = mass of that element

3. (moles i/moles j ) x Number of atoms j per cell = average atoms per unit cells

Alloy

impurity atoms have been added intentionally to impact specific characteristics, like improving mechanical strength and corrosion resistance

engineering stress

how force is distributed over materials area

engineering strain

when stress is applied how the material changes shape or length

shear stress and strain

sliding of one layer of atoms or material over another

angular difference (y = tanθ)

Young's Modulus

slope of linear region on graph

what does young's modulus indicate

a materials resistance to elastic deformation (stiffness)

how much it moves

steeper slope = stiffer

correlation between force-interatomic bond and elastic modulus

the slope of F/r curve at ro is proportional to elastic modulus. Stronger bonds = bigger E

Yield strength

stress when plastic deformation begins

what does yield strength indicate

how difficult it is to permanently deform a material

Tensile Strength

Maximum stress before necking or fracturing, small constriction forms

what does tensile strength indicate

under how much load a material will break

ductility

degree of plastic deformation sustained at fracture, larger strain = more ductile.

it is a percentage value

what does ductility indicate

a brittle (non-ductile) material is prone to snapping, a non-brittle (ductile) material bends

toughness

a materials ability to absorb energy and plastically deform before fracturing

it is the area under a stress-strain curve

what does toughness indicate

the possible energy absorption before fracture

rescillience

a materials ability to absorb energy when deformed elastically and then upon unloading recover

area under curve until yielding point

what does resilience indicate

the amount of energy/times something can be deformed and then spring back to original shape.

hardness

measure of how resistant a material is to localised plastic deformation

what does hardness indicate

how likely a material is to get scratches, denting, or punctures

Difference between engineering and true (stress and strain)

Engineering - based on original

True - 'real time' based on current size

Strength coefficient (K)

similar to stiffness but for plastic region. The higher K the more stress needed to continue plastic deformation

Strain hardening exponent (n)

As material is deformed there is dislocation in the lattice structure, these dislocations block each others motions making it hard for material to deform further

Elastic-strain recovery

When load is released before necking occurs, some fraction of total deformation is recovered. (Parallel to elastic stress-strain curve)