Structure of Materials Flashcards
To revise the fundamentals of materials for PhD
Why, generally speaking, are ceramics brittle while metals are ductile?
Because the covalent and ionic bonds in ceramics are much stronger than metallic bonds in metals
The two phases of composite materials?
- Reinforcing Phase 2. Matrix phase
Three categories of composite materials based on strengthening mechanism?
- Dispersion Strengthened 2. Particle Reinforced 3. Fiber Reinforced
How many types of atoms are there?
There are 109 different types of atoms, one for each element in the periodic table - periodic elements are substances that scientists found that could not be made any simpler using chemical reactions.
What are the three types of classification of material structures?
- Atomic structure
- Microstructure
- Macrostructure
What is the atomic structure of a material?
The way atoms are arranged, types of bonding between atoms, features that can’t be seen.
What is microstructure?
Features of a material that can be seen using a microscope
What is macrostructure?
Features that can be seen with the naked eye
What properties does the atomic structure have an effect on?
Chemical, physical, thermal, electrical, magnetic and optical properties
Microstructure and macrostructure have a larger effect on what type of properties?
Mechanical properties and the rate of chemical reaction
What is an atom made up of?
A positively charged nucleus (containing protons and neutrons) surrounded by electrons revolving around it
What is the atomic number of an element?
The atomic number indicates the number of protons in the nucleus
What is the atomic weight of an atom?
The atomic weight indicates how many protons and neutrons are in the nucleus
How many electrons are there usually in an atom?
Because atoms like to have balanced charge, there are usually the same number of electrons as there are protons
What is a metallic bond?
In a metallic bond, the electrons leave the outer shells of metal atoms forming a sea of delocalised electrons and positive metal ions - this is because metal atoms have only partially filled outer shells
What is a covalent bond?
Usually these are bonds between non-metals and is when atoms that have nearly a full outer shell share electrons to get a full outer shell
What are ionic bonds?
This is when metal and nonmetal atoms come together. This is when the metal atoms lose electrons to get an empty shell while nonmetal atoms take these lost electrons to gain a full shell, creating negative and positive ions
What are Van der Waal bonds?
These are particularly important in plastics and polymers. These are bonds between molecules that allow sliding and rupture to occur. In plastics the molecules are very long and so the van der Waal forces are very large.
What is an amorphous solid?
A solid substance with its atoms held apart at equilibrium spacing, but with no long-range periodicity in atom location in its structure.
Examples of amorphous solids?
Glass and some plastics.
Are amorphous solids isotropic?
Yes, they have the same physical properties in all directions.
What are crystalline solids?
Solids with regular, repeating arrangement of atoms or molecules.
Examples of crystalline solids?
Minerals, sand, clay, limestone, metals, carbon, salts (over 90% of natural and artificial solids are crystalline)
What is a unit cell?
A way of describing crystal structures, describes the arrangement within the solid of a small representative group of atoms or molecules. By multiplying identical unit cells in three directions, the location of all particles are determined.
How many different lattices are found in nature?
14
What is the simplest unit cell?
Cubic - these are rare in nature though because they easily distort
How are the atoms arranged in a cubic cell?
Atoms are lined up in a square 3D grid - there is an atom in each corner of the box
Two different types of cubic cells?
Body centred cubic (BCC) - a simple cubic cell with an atom in the middle of the cube
Face centred cubic (FCC) - a simple cubic cell with an atom in the middle of each face.
If planes of atoms are closely packed will more or less plastic deformation occur?
Closely packed planes of atoms slide past each other more easily so they will incur more plastic deformation. FCC lattice structures, therefore, have closely packed planes in several directions and therefore are more ductile than non-cubic lattices. BCC are not as closely packed and form strong metals.
What are the three unit cells that describe most metals and many other solids?
BCC, FCC and hexagonal close packed (HCP)
What is the packing factor?
The volume of atoms in a cell/ the total volume of a cell
What is the packing factor for BCC?
0.68
Examples of materials with BCC structure?
lithium, sodium, potassium, chromium, alpha-iron and tungsten.
Typical properties of BCC metals?
Harder and less malleable than FCC or HCP (since these are closer packed structures).
What is the packing factor for FCC?
0.74
Examples of materials with FCC structure?
aluminium, copper, gold, lead, nickel, platinum, silver.
How are atoms in the hexagonal close packed lattice arranged?
The hexagonal structure of alternating layers is shifted so the atoms align with the gaps of the previous layer. Each cell has 3 layers of atoms, 6 atoms in the top and bottom layers making up the hexagon shape with an additional 7th atom in the middle and 3 atoms in the middle layer.
What is the packing factor for HCP?
0.74
Examples of materials with HCP structure?
cadmium, magnesium, titanium, zinc and zirconium.
What is the difference between the HCP and FCC structures?
HCP has two types of planes (ABAB) while in the FCC structure there are three types of planes (ABCABC) where layers A and C are rotated versions of each other.
How is a single crystal formed?
A single crystal is formed under carefully controlled conditions as it requires crystallization of a large amount of material from one single point of nucleation.
What is a polycrystalline solid?
When a material solidifies it usually involves the growth of multiple crystals in the liquid, since there is more than one crystal, it is known as polycrystalline.
What does the final size of the individual crystals depend on?
The number of nucleation points.
How do crystals increase in size?
By the progressive addition/bonding of atoms.
What is a grain?
In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal without smooth faces because its growth was impeded by contact with another grain or a boundary surface.
What is a grain boundary?
The interface formed between grains.
Do atoms at the grain boundaries have a crystalline structure?
No, they are said to be disordered.
Is a single crystal usually isotropic or anisotropic?
Anisotropic
Are polycrystalline materials usually isotropic or anisotropic?
Isotropic because even although the individual grains are anisotropic, the properties tend to average out.
What are the 3 basic classes of crystal defects?
Point defects, linear defects, planar defects.
What are point defects?
places where an atom is missing or irregularly placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms
What are linear defects?
groups of atoms in irregular positions. Linear defects are commonly called dislocations.
What are planar defects?
interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.
How does plastic deformation occur in a materials?
Due to the movement of dislocations.
What happens when dislocations run into each other?
Often impedes movement of the dislocations and thus strengthens the material.
What is a self-interstitial atom?
An extra atom that has crowded its way into an interstitial void in the crystal structure (occur in low concentrations in metals)
What is a substitutional impurity atom?
It is an atom of a different type than the bulk atoms which has replaced one of the bulk atoms in the lattice (usually close in size to the bulk atom)
What is an interstitial impurity atom?
Much smaller atoms than the atoms in the bulk matrix that fit into the open space between the bulk atoms of the lattice.
What are vacancies?
Empty spaces where an atom should be but it missing, they are common, especially at high temperatures.
What are dislocations?
Areas where the atoms are out of position in the crystal structure.
How are dislocations generated and what causes them to move?
When a (shear) stress is applied
What are edge dislocations?
An extra half-plane of atoms in a lattice
How do dislocations move?
They move along one plane at a time propagating across the crystal.
What are screw dislocations?
When a shear stress is applied across one end and the material begins to rip so that a portion of a plane of atoms in the material is unmoved, a portion is moved with the metallic bonds broken and another part is moved to a new position where the metallic bonds have reformed. The edge dislocation moves parallel to the applied stress while the screw dislocation moves perpendicular to the applied stress.
Why do FCC and BCC metals allow dislocations to move relatively easily?
Because FCC and BCC have many dense planes so less stress is needed to move dislocations between planes.
What is a stacking fault?
A planar defect, a one or two layer interruption in the stacking sequence of atom planes.
What is a twin plane/twin boundary?
When a stacking fault does not correct itself immediately and continues over some atomic spacings, a second stacking fault that is the twin of the first will be produced. The region containing the stacking faults is known as the twin plane and the planes at the edge of this region are known as the twin boundaries.
Why does having smaller grains (more grain boundaries) strengthen a material?
Because grain boundaries limit the movement and size of dislocations.
What are voids?
A type of bulk defect where there are a large number of atoms are missing from the lattice.
What are precipitates?
Another type of bulk defect where impurity atoms cluster together to form small regions of a different phase.
What is elastic deformation?
A temporary shape change that is self-reversing after the force is removed. This involves stretching of the bonds but the atoms do not slip past each other.
What is plastic deformation?
When the stress is sufficient to permanently deform the metal, involving the breaking of metallic bonds by the movement of dislocations.
Why does slip (dislocation movement) occur in parallel planes in the grain?
Because dislocations prefer to move in the direction where the plane density is greatest.
What are slip bands?
When parallel slip planes group together.
What role do dislocations play in fatigue cracking?
Play a major role in the fatigue crack initiation phase - after a large number of loading cycles, dislocations pile up and form structures called persistent slip bands (PSB). These are areas that rise above (extrusion) or fall below (intrusion) the surface of the component. These are stress risers and allow initiation of fatigue cracks.
What is diffusion?
The migration of atoms from a region of high concentration to a region of low concentration.
What is diffusion like in homogeneous materials?
Atoms are routinely moving around but the movement is random - there is always an equal number of atoms moving in all directions.
What is diffusion like in an inhomogeneous material?
All atoms are moving near randomly, but there is a migration of atoms to areas where their concentrations are lower, there is a net diffusion.
What types of atoms can diffuse and how?
Host or substitutional atoms can diffuse to vacancies. Interstitial impurity atoms can diffuse to different interstitial positions.
In terms of dislocations, how can materials be strengthened?
By impeding the movement of dislocations, which can be done by; controlling the grain size, strain hardening, alloying.
How is grain size controlled?
By controlling the rate of solidification from the liquid phase.
What is strain hardening? (Also called work-hardening or cold-working)
The process of making a metal harder and stronger through plastic deformation. Since deforming a metal plastically causes increased dislocation movement and entanglement so they can’t move as much, thus strengthening occurs.
Why must strain-hardening be done at low temperatures?
Because the plastic deformation must occur at a temperature low enough so that atoms cannot rearrange themselves.
What does strain hardening do to ductility?
It reduces ductility significantly.
What effect does elevated temperature have on strain hardened materials?
The strengthening that resulted from plastic deformation can be lost due to recovery, then recrystallisation followed by grain growth.
What is alloying?
The addition of other elements into a metal, the resulting metal is called an alloy.
Examples of common alloys?
Brass, bronze, pewter, cast iron, steel, stainless steel.
What is solid solution strengthening?
Involves the addition of other metallic elements that will dissolve in the parent lattice and cause distortions because of the difference in atom size between the parent metal and the solute metal. This disrupts the regular arrangement of ions and makes it more difficult for the layers to slide over each other.
What are multiphase metals?
Another method of strengthening is by adding elements that have no or partial solubility in the parent metal. This results in the appearance of a secondary phase throughout the crystal or between crystals. This can increase or reduce the strength of the alloy depending on the composition.
What is a phase diagram?
A map showing the structure of phases present as the temperature and overall composition of the alloy is varied.
Difference between binary and ternary phase diagrams?
Binary diagrams are for two element alloys while ternary diagrams are for three-phase alloys.
Explain the melting points of alloys?
Generally, alloys don’t have a single melting point but instead melt (or alternatively solidify) over a range of temperatures.
What is the eutectic point?
When there is a mixture of the constituent elements that produce a single melting temperature, this is the eutectic point.
What is solid solubility?
When one element remains dissolved in another while both are in the solid state, usually possibly up to a few percent composition. The limit for solid solubility usually changes with temperature. The solid solubility region can be plotted on the phase diagram.
Outwith the solid solution regions, when will the alloy become fully solid?
At the eutectic temperature. The eutectic line indicates where this transformation occurs over the range of compositions in the phase diagram.
What is a tie line?
An isothermal (constant temperature) line drawn through the alloy’s position on the phase diagram when it is in a two phase field. Where the ends of the tie line intersect the two adjacent solubility curves indicate the compositions of the two phases that exist in equilibrium at this temperature.
What is a eutectic alloy?
An alloy with composition right at the eutectic point.
What is a hypoeutectic alloy?
An alloy with a composition to the left of the eutectic point.
What is a hypereutectic alloy?
An alloy with a composition to the right of the eutectic point.
What are the major objectives of the different kinds of heat treatments?
- To soften the material for improved workability
- To increase the strength or hardness
- To increase the toughness or resistance to fracture of the material
- To stabilize the mechanical or physical properties against changes that might occur during exposure to service environments
- To ensure part dimensional stability
- To relieve undesirable residual stresses induced during part fabrication
Name some common heat treating processes?
Age-hardening, annealing, normalizing, precipitation heat treatment, solution heat treatment, stress relieving, tempering, quenching.
What is age hardening?
A relatively low-temperature process that strengthens a material by causing the precipitation of components or phases of alloy from a super-saturated solid solution condition.
What is annealing?
A softening process in which metals are heated and then allowed to cool slowly.
What is normalizing?
Similar to annealing, but the cooling process is much faster. Results in increased strength but less ductility.
What is precipitation heat treatment?
A three-step process of solution treating, quenching and age hardening to increase strength and hardness of alloys.
What is solution heat treatment?
Heating the material to a temperature that puts all the elements in solid solution and then cooling very rapidly to freeze the atoms in place.
What is stress relieving?
A low-temperature heat treatment process that is used to reduce the level of residual stresses in a material.
What is tempering?
Gently heating a hardened metal and allowing it to cool slowly to produce a metal that is still hard but less brittle.
What is quenching?
The rapid cooling of a hot material. Results in a metal that is very hard but also brittle.
What are the two most common chemical bonds for ceramic materials?
Covalent and ionic, much stronger than metallic bonds
General properties of ceramics?
High hardness, high compressive strength, chemical inertness, low ductility, low tensile strength, poor conductors or electricity and heat. However, due to the many different crystal structures of ceramics, the properties are wide and varied.
What are the main compositional classes of engineering ceramics?
Oxides, nitrides and carbides.
Are ceramics glassy or crystalline?
Ceramics can be either glassy or crystalline or a hybrid of both.
What happens to a glassy material as it is cooled from liquid to solid?
As it is cooled it becomes more and more viscous, there is not sharp melting or freezing point. It goes from liquid to soft plastic solid to hard and brittle.
What do most ceramics usually contain?
Both metallic and nonmetallic elements with ionic or covalent bonds.
What are the building criteria for the ceramic crystal structure?
- Maintain neutrality
- Charge balance dictates chemical formula
- Achieve closest packing
Name some ceramic materials outside the glass family?
Silicate ceramics, cement, nitride ceramics, ferroelectric ceramics.
Describe ferroelectric ceramics?
The centers of the positive and negative charges do not coincide even without the application of external electric field. There exists spontaneous polarization in the crystal. When the polarisation of the dielectric can be altered by an electric field, it is called ferroelectric.
Difference between ceramic and metal defects?
Imperfections include point defects and impurities like in metals. However, in ceramics defect formation is strongly affected by the condition of charge neutrality because the creation of areas of unbalanced charges requires an expenditure of a large amount of energy. In ionic crystals, charge neutrality often results in defects that come in pairs of ions with opposite charge or several nearby point defects in which the sum of all charges is zero.
What are the Frenkel and Schottky defect?
Charge neutral defects. Frenkel defect is when a host atoms moves into a nearby interstitial position to create a vacancy-interstitial pair of cations. Schottky defect is a pair of nearby cation and anion vacancies, occurs when a host atom leaves its position and moves to the surface creating a vacancy-vacancy pair.
What is a mer?
The repeating unit in a polymer chain
What is a monomer?
A single mer unit
What is a polymer?
Many mer-units along a chain
What is the degree of polymerization?
The average number of mer-units in a chain
Types of structures for polymers?
linear, branced or network
What type of bonds are in polymers?
Covalent bonds hold atoms in the polymer molecules together while secondary bonds hold groups of polymer chains together
What are copolymers?
Polymers composed of two or more different types of monomers
What is an organic material?
A material whos main backbone is a chain of carbon atoms
What is the significance of the length of the polymer chain?
As the number of carbon atoms in the chain is increased the material goes from liquid state to a waxy solid to completely solid.
Are the molecules in a thermoplastic polymer straight?
No they are a tangled mass of chains.
In thermoplastics what are the binding forces between molecules?
Van der Waals
What is the molecular arrangement like in thermosetting plastics?
It is more like a single large network than many molecules that are formed when a thermosetting plastic undergoes polymerization. For this type of network structure to occur, the mers must have more than 2 places for boning to occur. These chains form jointed structures and rings and may fold back and forth to take on a partially crystalline structure.
Why are thermosetting plastics unable to be altered once set?
Because they are basically composed of one giant molecule, so no movement between molecules is possible.
What is a composite material?
Basically a combination of two or more materials, each of which retains its own distinctive properties. Multiphase metals are composite materials on a micro scale, but generally the term composite is applied to materials that are created by mechanically bonding two or more different materials together.
Composition of composite materials (phases)?
A bulk phase, which is continuous, called the matrix. A dispersed, non-continuous, phase called the reinforcement.
Three categories of composites?
Fiber reinforced, dispersion strengthened, particle reinforced.
What is the major load bearing component in fiber reinforced composites?
The fiber
What is the major load bearing component in dispersion strengthened composites?
The matrix
What is the major load bearing component in particle reinforced composites?
Load is shared between the matrix and particles
What are dispersion strengthened composites?
Small particles are added to the matrix material.
What are particle reinforced composites?
The particles are larger than in dispersion strengthened composites and thus carry more of the load.
What is the interface in a composite?
A bounding surface or zone where a discontinuity occurs.
