124 terms

Material Science Engineering Exam 1

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Terms in this set (...)

smallest building blocks
atoms
Atoms are composed of?
nucleus (protons and neutrons) $ electrons
protons
positive charge; corresponds to atomic number "Z" for natural elements
valence electons
electrons that occupy space in the outermost primary shell
*contribute to formation of (primary) chemical bonds*
Noble gas electron configuration
all states belonging to outermost primary shell (valence electrons) are fully occupied
*STABLE*
What will atoms do to achieve noble gas configuration?
Atoms will participate in chemical reactions to achieve noble gas electron configuration
What drives chemical reactions?
*tendency to achieve noble gas electron configuration drives chemical reactions*
Periodic system
ordering scheme of atoms according to their number of valence electrons
All atoms in same column have same number of...?
!!! all atoms in same column of periodic table have same number of *valence electrons* !!!
electronegative
elements that tend to receive electrons during chemical reactions
electropositive
elements that tend to give up an electron
Bonding force=
bonding force= F{attractive} + F{repulsive}
Attractive force
tendency of atoms to achieve noble gas configuration
Repulsive force
inner, fully occupied electron shells cannot overlap (Pauli's priniciple)
Bonding energy is...
energy required to break atoms apart
Bonding Energy = E(r) =
Bonding energy= E(r) = E{attractive} + E{repulsive}
E(r) = (-) + (+)
generic bonding potential
chart with bonding energy; repulsive energy on top, attractive energy on bottom; lowest bonding energy is where the bond will occur in nature
High bonding energy = high E (o)
closer atoms will be to each other, shorter the bond length will be, stronger the bond

ex: solids, hard, high melting temperatures, low thermal expansion coefficient
small r (distance between atoms)
repulsive force is dominating
large r (distance between atoms)
attractive force is dominating
Why is it important to know where energy is minimum?
Everything in nature goes towards minimum energy
electronegativity
tendency to acquire electrons
Primary bonds
strong, determined by valence electrons (main bonds you know)
ionic bonding
TRANSFER of valence electrons; metal + nonmetal; Large difference in electronegativity required; occurs between + and - ions

LARGE bond energy
*NON-DIRECTIONAL*
Covalent bonds
atoms achieve noble gas configuration by SHARING VALENCE ELECTRONS;
involves hybridization (mixing of orbitals)
*DIRECTIONAL*
Metallic Bonds
valence electrons are being shared among all atoms in material; free electron gas= electron and thermal conductivity
*NON-DIRECTIONAL*
Which bonds are directional and which are nondirectional?
Ionic and Metallic= non-directional
Covalent= directional
Secondary Bonds
involve all electrons, not just valence; WEAK
Van-der-Waal interactions
induced dipolar interactions due to fluctuations in electron cloud, ACTIVE FOR ALL MOLECULES/ATOMS
Permanent Dipole
if covalent bonds form between atoms of distinct electronegativity, permanent dipole moments occur (depending on symmetry of molecules)
ex: water (H2O)-> responsible for higher boiling point of water as compared to methane
Z
atomic number
atomic number
corresponds to number of protons in nucleus for natural elements
isotopes
nuclei with same atomic number "Z" but different number of neutrons
amu
atomic mass unit
Atomic mass unit (amu)
a unit of mass used to express atomic and molecular weights; = (1/12)*mass of C-12
Avogadro's number
= 6.02 x 10^23 (1/mol)
Schrodinger's wave-mechanical model of atoms
wave particle duality of electrons
quantum numbers
Qn; characterize the electronic structure of atoms; nlms
primary/principle quantum number (n)
corresponds with primary energy level (Bohr energy level)
n = 1, 2, 3...
n = k, l, m...
secondary quantum number (l)
describes the shape of subshells (orbitals) that exist in the primary energy level
l = (n-1)
l = 0, 1, 2, 3
l = s, p, d, f
tertiary (magnetic) quantum numbers (m)
describes the number of orbitals and their orientation within a subshell
m= -l to +l
(...-2, -1, 0, 1, 2...)
electron spin quantum number (s)
describes the determination of an atom's ability to generate a magnetic field or not;
s= +(1/2) or -(1/2)
electron configuration
describes the distribution of electrons among the available states
Pauli principle
no two electrons can can agree in all four quantum numbers
four components of the disciplines of material science engineering?
processing-->structure-->properties-->performance

processing can change structure, properties depend on structure
Structure of a material is based off...?
how it was processed
function of a material's properties tells us its...?
performance
Materials Science
investigation of the relationship between structures
and properties of materials
Materials Engineering
designing/engineering of material structures
to meet certain property requirements
(material selection/processing)
metals
moderate melting temperature
moderate bond energy
moderate coefficient of thermal expansion
**METALLIC BONDS**
ceramics
high melting temperature
LARGE BONDING ENERGY
small coefficient of thermal expansion
**IONIC & COVALENT BONDS**
polymers
**Directional Properties
***Secondary bonding dominates***
small melting temperature
small bond energy
large coefficient of thermal expansion
**COVALENT & SECONDARY BONDS**
crystal structure
regular, periodic arrangement of atoms
crystalline solids represent atoms/ions as...?
represents atoms/ions as "hard spheres"
Unit Cell (UC)
smallest repeat unit that allows to reproduce the positions of all atoms by translating UC integer multiples along its edges;
chosen to represent the symmetry of the LATTICE
lattice
a discrete but infinite regular arrangement of points (lattice sites) in a vector space
3 types of crystal structures of metals
1. face-centered cubic
2. body-centered cubic
3. hexagonal close-packed
Coordinate number (CN) of crystal structure
number of nearest neighbors
Atomic Packing fraction (APF)
(volume of atoms/UC) / (volume of UC)
Face-centered cubic (FCC)
atoms are located at corners and face-center of cubical UC
*reference notebook for diagram*
ex: Al, Au, Cu, Pb
atomic radius of FCC (a)
a= (4R) / (sqrt 2)
# of atoms in UC of FCC
4
Coordinate # (CN) of FCC
12
Atomic Packing Fraction (APF) of FCC
0.74
Body-Centered Cubic (BCC)
cubic UC; atoms at corners & center of cube; atoms connect along body diagonal
*reference notebook for diagram*
ex: W, Fe, Cr
atomic radius of BCC (a=)
a= (4R) / (sqrt 3)
# of atoms per UC of BCC
2
Coordinate number (CN) of BCC
8
atomic packing fraction (APF) of BCC
0.68
Hexagonal Close Packed (HCP)
UC has hexagonal symmetry; atoms are in each corner, center of top & bottom face, plus 3 atoms inside UC
*reference notebook for diagram*
ex: Co, Ti, Mg
c/a pf HCP
c/a= 1.633
# of atoms in UC of HCP
6
Coordinate Number (CN) of HCP
12
Atomic Packing Fraction (APF) of HCP
0.74
density (p) =
=Mass/volume
= [(# of atoms/UC)(atomic weight)]/[(volume of UC)(Avogadro's #)]
= (mA)/(VucNav)
polymorphism
metals/nonmetals having more than one crystal structure
allotropy
polymorphism in elemental metals
crystal structures
grouping scheme of possible UC structures depending on symmetry
There are __ combinations of lattice parameters that can form a periodic structure
(BRAVAIS systems)
There are SEVEN combinations of lattice parameters that can form a periodic structure
Bravais systems
7 systems, in order from most symmetrical/ordered to least
(cubic--> tricluic)
high symmetry constituents (spheres) will often form...
high symmetry constituents will often form high symmetry structures
metals have the crystal structure of:
cubic, hexagonal
proteins/ polymers have the crystal structure of:
monoclinic, triclinic
crystallographic point coordinates
identifies coordinates of a point within the coordinate system that is spanned by UC edges
**presented in parenthesis WITH commas (1, 1, 0)
Family of crystallographic equivalent directions
<u v w>
set of directions that are symmetry equivalent
***Which direction constitute a family depends on the crystal system!!
Orientation of crystallographic plane is indicated by ______
MILLER indeces (h k l) **NO COMMAS
for crystallographic planes, assume:
assume the plane passes through one corner of UC that is NOT the origin
family of equivalent planes
set of planes with equivalent atomic packing {h k l}
linear density (LD)
(# of atoms along direction vector)/(length of vector)

[LD]= 1/length
planar density (PD)
(# atoms centered on plane)/(area of plane)

[PD]=1/area
polycrystallinity
material made up of distinct grains with equal crystal structure but different orientation
defect
lattice imperfection with at least one dimension on atomic scale
3 types of defects
point defect, one-dimensional, two-dimensional
point defect
vacancy, self-interstitial, & impurity atoms
one-dimensional
dislocations
two-dimensional
grain boundaries, external surfaces, (phase boundary)
vacancy
empty lattice site
driving force for vacancies
increase in entropy
There is an equilibrium number of ________ that exists in materials
There is an equilibrium number of defects that exists in materials
Boltzman Equations describes number of _______
Boltzman equation= number of vacancies
Boltzman equation
N(vacancies)=N(total)e^-[delta Q/[k(B)T]]
Boltzman equation: delta Q=
energy penalty associated with creating the defect
Boltzman equation: k(B)
Boltzman constant= 1.38 * 10^-23 J/K
Boltzman Equation: T
T=absolute temperature
Boltzman Equation: k(B)T
k(B)T= thermal energy (vibrations)
Self-Interstitial Defect
when an atom is displaced into interstitial space
Self-Interstitial defects cause large _____________ for disturbing lattice
Self-interstitial defects cause large ENERGY PENALTY for distorting lattice
***energy penalty for self-inter>>energy penalty for vacancy
Impurity atoms
always present, distribute throughout a material
Two possibilities for how impurity atoms can distribute in a material:
solid solution or phase separation
solid solution
uniform distribution of 2nd component atoms in host lattice
(in the solution, solvent is majority component, solute is minority)
phase separation
clustering of 2nd component atoms to form a new phase
miscibility
ability for two solids to be partially or completely soluble in each other
Substitutional Solid Solution
Solvent atoms are replaced by solute atoms; Perfect miscibility in substitutional solid solutions
**HUME ROTHERY RULES**
What do Hume Rothery Rules do?
describe the conditions under which an element could dissolve in a metal, forming a solid solution
What are the Hume Rothery Rules?
1.) similar atomic size (size must differ by <15%)
2.) equal crystal structure in element pure state
3.) similar electronegativity
4.) better solubility with increasing atomic number
Interstitial Solid Solutions
solid fills interstitial regions in solvent lattice; only when r(solute) << r(solvent);
typically limited to small solute concentrations due to lattice distortions
Line defects-- dislocations
1D defects around which the lattice is distorted
2 types of dislocation:
edge dislocation & screw dislocation
edge dislocation
line defect that centers along the edge of an extra half-plane of atoms that has been inserted into the crystal
screw dislocation
line along the center of a helical path that is traced by atomic planes about the dislocation direction
general properties of dislocations
in reality often mixed type, lattice around dislocations is distorted (gives rise to stress/strain fields), amplitude of distortion decreases with increasing distance from dislocation
Burger's Vector
describes the magnitude and direction of lattice distortion associated with dislocation
edge dislocation is ____ to dislocation direction
edge location is NORMAL to dislocation direction
screw dislocation is _______ to dislocation direction
screw dislocation is PARALLEL to dislocation direction