ME 518
Lecture 2 - Review of
Material Science
TOPICS
- Metals-
- Materials containing only metallic atoms either as single
elements or in combination (alloys)
- High thermal and electrical conductivity due to unbound valance
electrons
- Polymers -
- Low density structures of non-metallic elements, often in
the form of macromolecules
- Poor thermal and electrical conductors due to the affinity
of the elements to attract or share valence electrons
- Ceramics-
- Compounds that contain both non-metallic and metallic elements
- Includes cement and rocks, glass, and oxide structures
- Hard and brittle materials
- Good thermal and electrical insulator
- Resistant to high temperatures and severe environments
- Combined cation-anion structure results in mechanical and
thermal/electrical properties
- Properties of a material relate ultimately to its structure
- Structure can be viewed at many levels
- Each level of structure in a material effects the overall
properties of that material in different ways
- Figure 1 - Material Structure
- Defined by the mer (polymers) or crystal structure (metals
and ceramics) of the material
- Crystalline Materials (See Figure 2)
- Types of crystal structure
- Body centered cubic (bcc)
- Face centered cubic (fcc)
- Hexagonal close packed (hcp)
- Polymorphs are two or more distinct types of crystals
that have the same composition
- Polymers
- The mer is the basic unit which is repeated to make
up a polymer
- Isomers are two or more distinct molecular structures
for the same composition of material
- Very common in organic materials where the placement of atoms
in the molecular structure greatly affects properties
- Examples:
- The types of atoms present and their proximity to each other
determine the electrical, thermal, and bonding characteristics
of a material
- Metal atoms have free valence electrons which can "travel"
about the material
- Materials consisting of only metal atoms have high electrical
and thermal conductivity due to the metallic bonds in which
electrons are loosely held to the ions
- Metallic bonds are non-directional (isotropic)
- Metallic bonds allow plastic deformation to occur, as the
electrons can rearrange themselves permanently in response to
the applied forces
- Non-metallic atoms tend to attract free electrons and prevent
them from "traveling"
- Covalent bonds are formed between non-metallic atoms
when the valence electrons are shared to fill each outer orbital
- Covalent bonds result in poor electrical and thermal conductivity
as the stable position for the electrons is fixed
- Covalent bonds are highly directional, allowing chains of
mers to be formed
- Combinations of metallic and non-metallic atoms form molecules
with either covalent bonds or ionic bonds
- Ionic bonds occur when the free electron of the metal
atom is attracted to the non-metallic atom, creating a positive
ion (cation) and a negative ion (anion) which then
are bound by coulombic forces
- Positive ions become surrounded by negative ions
- Again, electrons are bound in a stable position which limits
electrical and thermal conductivity
- Ionically bonded materials tend to be brittle
- Describes the overall crystalline structure, including imperfections
- Defects play a major role in determining the physical properties
of a material
- Point defects (See Figure 3):
- Vacancies
- Allow for increased atomic diffusion
- Maintains charge balance
- Substitutions
- Result in atomic distortion if atomic radii differ
- Interstitial defects
- Less frequent in closely packed structures
- Line defects (See Figure 4):
- Plane of atoms is displaced or dislocated from its regular
lattice space
- Act to reduce the strength and stiffness of the solid as there
is already an increase in energy along the dislocation, so less
energy must be added to move the planes or break the bonds at
this location
- Many line defects will act to strengthen a material as they
interfere with the progression of dislocations
- Grain boundaries (See Figure 5):
- Region between crystals of various orientations, even in a
single phase material
- Within a grain, all crystalline unit cells are aligned in
one direction
- At grain boundaries, there is a transition zone where atoms
are not aligned with either grain
- Typically 1 to 2 atomic distances wide
- Less efficient packing occurs at grain boundaries and they
have higher energy
- Allows for surface corrosion at boundaries and dislocation
along the boundary
- However, grain boundaries, which are not aligned, also prevent
slip planes from progressing and resulting in fracture at normal
temperatures
- Grain structure (See Figure 6) can
vary in:
- Grain size
- Preferred orientation
- Grain shape
- Grain size
- A larger grain size number (GS#) by ASTM standards indicates
a higher number of grains and grain boundaries per unit volume
- In general, a fine-grained structure is stronger than a coarse
one for a given material at normal temperatures
- Grain boundaries interfere with the movement of atoms during
deformation
- A fine-grained structure is weaker and softer than a coarse
one for a given material at elevated temperatures
- Grain boundaries are a source of weakness above temperatures
where atoms start to move significantly
- Grain shape
- Equiaxed - approximately equal dimensions in 3 directions
- Plate-like - one dimension smaller than other two
- Columnar - one dimension larger than other two
- Dendritic (tree-like)
- Grain Orientation
- Refers to crystal orientation within grains
- Typically random orientation within metals
- Preferred orientation can be manipulated to obtain improved
material properties (such as magnetic permeability)
- Describes organization of chain molecules within polymer
- 3 general types (See Figure 7)
- Linear
- Branched
- Cross-linked
- Degree of polymerization gives the average number of
mers or repeating units per molecule or chain
- Longer chains result in less mobility of the polymer and therefore
higher strength and greater thermal stability
- Polymeric microstructure is some combination
of
- Extended chains
- Folded chains
- Amorphous structure
- See Figure 8
- "Fringed micelle" model involves coexistence of
all three structures
- Variations in polymer structure
- Crystalline polymer structures have ordered chains
- Folded or extended portions in alignment
- Amorphous polymer structures have uncoordinated chains
- Semi-crystalline structures are the most common for
linear polymers
- Chains are held together either by secondary forces (hydrogen
bonds, van der Waals forces) or covalent bonds through cross-linking
- Chains can tangle easily and side groups, branches, or copolymeric
chains can interfere with long-range ordering of the back-bone
chains
- The polymer ultrastructure also affects microstructural properties
- Linear: no branches, most easily crystallizes, but
not 100%
- Branched: single back-bone allows minimal crystallization
- Cross-linked or 3-dimensional network: forms only amorphous
structures
- Effect of the degree of polymerization:
- Longer polymers (higher degree of polymerization) have a harder
time forming a crystalline structure as they become tangled
- Other factors reducing degree of crystallization
- Atactic molecules -- "rib" elements not all
facing the same way (vs. isotactic molecules, when rib
elements are facing same direction) (See Figure 9)
- Mers with "side lumps" - large rib elements such
as benzene
- Arced mers
- Other non-crystalline structures include glass
- Amorphous solid
- Short range order without long range order
- Geometric properties which influence parameters such as
- Force at failure
- Stiffness
- Bending or buckling
- Stress distribution
- Weight
- Material parameters are independent of macrostructure
- Strength
- Elastic modulus
- Poisson's ratio
- Density
- Materials can consist of one or more phases, or atomic structures,
of material
- Examples of single phase materials
- Single element metals
- Solid solution alloys
- Brass -- zinc in copper solution
- Bronze -- tin in copper solution
- Copper-nickel alloys -- nickel in copper solution
- Ceramics
- Aluminum oxide
- Hydroxyapatite
- Polymers
- Polymethylmethacrylate
- Polyethylene
- Examples of multi-phase materials
- Metal mixtures
- Composites
- Reinforced concrete
- Glass-reinforced plastics
- Portland cement -- clay and limestone
- A solution of two materials involves one material (solute)
directly combined with another material (solvent) without changing
the structural pattern of the solvent
- Can involve substitution of atoms into the structure or placement
of interstitial atoms in the structure (See Figure 10)
- Many solutions have a solubility limit beyond which more solute
cannot be introduced into the solution while maintaining the single
phase requirement
- Add sugar to water, eventually solubility limit reached and
remainder of sugar does not dissolve
- Same effect with solid solutions
- Once the solubility limit is reached, a mixture is formed
between the solution and the solute
- Some metal solutions have a continuum of ratios between the
solute and solvent so that materials can consist of between 0
and 100 wt% of either
- Solution based alloys are typically stronger than pure materials
as the solute atoms interfere with the movement of dislocations
(solution hardening)
- Co-polymers can be seen as a solution of different mers (See
Figure 10)
- Contain two or more types of mers in the macromolecule structure
- Typically have improved physical and mechanical properties
over simple polymers
- A mixture of two materials involves the combination
of two different structures of material without affecting either
individual structure (See Figure 10)
- Involves placing one phase within another phase of the material
either physically or by some chemical process
- Stones in cement to form concrete
- Graphite in many forms of cast iron
- Transition between solid solutions, various mixtures, and
other phases (liquids or gases) can be determined from phase diagrams
- Define phases and combinations as a function of temperature
and % composition
- Phase distribution and shape in a mixture also form part of
the microstructure
- Describe the coexistence of phases of a material over a range
of temperatures and compositions
- Can be used to determine (for a given temperature and composition)
- The stable phases of the material
- The chemical compositions of the phases
- The quantity of each phase present under equilibrium conditions
- Each phase will have different properties at different compositions
- Each mixture will have different properties at different quantities
of each phase
- The properties of a phase or mixture will also vary with temperature
- Example 1:
Salt Water
- Example 2:
Lead-Tin System (Pb-Sn Solder)
- To determine the phases present at a given temperature/composition:
- Find point on diagram corresponding to temperature/overall
composition
- Look at region in which point falls
- Will list phases (ie. salt + water, or alpha + beta)
- A single listed phase indicates a solution (ie. brine)
- Two or more listed phases indicates a mixture (ie. salt +
water)
- If temperature/composition point lies on a point of
intersection between three regions, this is termed a eutectic
point and all three adjoining phases are present
- To determine the composition of the phases present:
- For a solution
- Composition of a solution is the same as the overall composition
of the material
- For a mixture
- Trace horizontal line across isotherm (temperature line) to
each edge of region
- Drop vertical line down to determine the composition of the
phase (in terms of component materials) at that temperature
- Composition of each phase of mixture is that of phase alone
at the that temperature where it transitions between existing
as a pure solution and existing as part of a mixture
- To determine the amount of each phase present (mass fraction):
- Solutions, by definition, are composed of 100 percent of the
single phse
- For mixtures:
- Perform a materials balance, treating the isotherm like a
see-saw
- Percent alpha = (C-beta - C-total)/(C-beta - C-alpha)
- C-x is the composition of the phase or mixture (x) in terms
of one of the two component materials
- ie. Amount of lead in alpha, beta, and the total material
- Percent beta = 1 - percent alpha
- This is the inverse lever rule
References
Van Vlack, L.H., Elements of Materials Science and Engineering,
5th Ed., Addison-Wesley Publishing Co., Reading, MA, 1985.
