Metals and Alloys Flashcards
What must be considered when selecting materials for design?
-Mechanical properties
-Raw material cost
-Transportation costs
-Maintenance costs
Aesthetic factors
What mechanical properties must be considered when selecting materials for design?
-Material stiffness (Young’s modulus)
-Resistance to plastic deformation (yield strength)
-Ductility
Name some physical properties of materials.
-Density
-Mass
-Melting and boiling points
-Electrical and thermal conductivity
Name two examples of mechanical property.
Elastic response (stiffness) and plastic flow (yield stress)
What determines the ability for a material to resist deformation?
Internal structure
For metals and alloys, describe the constitutive relation.
Stress will be a function of strain and material microstructure
What experimental methods can be used to determine microstructure characterisation?
-Optical microscopy
-Scanning electron microscopy
-Transmission electron microscopy
What experimental methods can be used to determine mechanical characterisation?
-Tensile/compressive test (Uniaxial loading)
-Fatigue test (Cyclic loading)
Creep test (time dependent behaviour)
What computational material modelling methods can be used to derive structure-property relations?
-Physics-bases, phenomenological or empirical methods
-Simulate process induced microstructures
-Simulate mechanical properties
What does a tensile test provide?
An experiment approach to studying basic material behaviour
What assumptions are made during a tensile test?
Loading is pure axial and the deformation takes place uniformly, both along the length and cross section of the specimen
How is a tensile test carried out?
By moving one end of the specimen at constant speed, while holding the other end fixed
What are the primary variables recorded during a tensile test?
Load (F) and extension (L)
What is required to determine true stress?
Knowledge of the instantaneous area at all times during deformation
What determines plastic deformation?
When stresses exceed the yield stress
What determines elastic behaviour?
Total strain is entirely recoverable - total strain is equal to the elastic strain
What is plastic strain?
A function of the stress on unloading
In the stress tensor matrix, which components are considered to represent normal stress?
Sigma_11, Sigma_22, Sigma_33
What components are present in the hydrostatic stress tensor?
Pure tensile or compressive stress
What components are present in the deviatoric stress tensor?
Shear stresses of the total stress state
What physical change is caused by hydrostatic strain?
Volume change
What physical change is caused by deviatoric strain?
Change in shape
What are the principle stresses?
Where the stress tensor has zero shear components and non-zero normal stresses
How do you define the stress state?
By using all components of the stress tensor
Name three materials with polycrystalline structures.
Steels, aluminium, titanium alloys
What is a polycrystalline structure?
Collection of crystals called grains that are joined together, in each grain atoms are arranged into long-range patterns of atomic order with a repetitive 3D pattern
Name three materials with hexagonal close packed (HCP) structure?
Alpha-titanium, zinc, zirconium
Name three materials with face-centred cubic (FCC) structure?
Nickel, aluminium, gamma-iron
Name four materials with body-centred cubic (BCC) structure?
Chromium, beta-titanium, alpha-iron, beta-iron
What causes lattice defects?
-Crystals are not perfect
-Disruptions in their lattice arrangements
-Atoms may be positioned in a non-lattice site, or an atom may be missing at a given lattice size
What are the different types of crystal defect?
-Point
-Line
-Surface
-Volume
What are some examples of point defect?
Vacancies, interstitial and substitutional atoms
How are vacancy point defects introduced?
Thermal vibrations of atoms or from imperfect packing during crystallisation
How can a solute atom be incorporated into a host lattice?
Substitutionally, Interstitially
What happens when an interstitial atom is inserted into a crystal lattice?
Neighbouring atoms become displaced radially away from the interstitial solute atom
What are line defects?
Dislocations that are characterised by a chain of atoms that are incorrectly places in the lattice
Name three types of line defect/dislocation.
-Edge
-Screw
-Mixed
How do dislocations interact with each other?
Through their elastic fields, this self induces stress and strains within the crystal
What is glide (in terms of dislocations)?
Applied shear stresses induce a force on the dislocations, causing them to move on close-packed planes
How are plastic deformations induced by dislocations?
Shear stresses lead to a displacement of the dislocation through repeated breaking an formation of bonds
How are slip bands measured experimentally?
Using electron-back-scatter-diffraction (EBSD) and digital image correlation (DIC), where the slip bands are revealed as ‘lines’ of high strain
Name three types of surface defect.
-Twins
-Staking faults
-Grain boundaries
What physical change can alter grain boundaries?
Temperature
Name three types of volume defect.
-Voids
-Cracks
-Foreign inclusions
How are volume defects introduced into a crystal lattice?
Unintentionally during processing or fabrication
How is the orientation of a dislocation specified?
By the direction of the tangent vector
What is a Burgers vector?
The vector needed to close a circuit (around a dislocation), from its final point to the starting point
How does a Burger vector change for two dislocations?
The circuit measure the total Burgers vector inside the contour
Describe screw dislocations.
A ‘cutting-displace-gluing’ mechanism, where the displacement faces are parallel to the edge of the cut
Describe a mixed dislocation.
Tangent vector varies along the dislocation line, the Burgers vector will be expressed in terms of the perpendicular and parallel components
Describe how we know that the Burgers vector of dislocations is conserved.
-Consider a dislocation segment with Burgers vector b1
-If the dislocation splits into two other segments with Burgers vectors b2 and b3
-Since the Burgers circuit measure the Burgers vector of dislocations contained within it, b1 = b2 + b3
What parameters determine the energy of a dislocation?
Shear modulus and magnitude of the Burgers vector
What are partial dislocations?
Dislocation which has Burgers vector whose magnitude is a fraction of the lattice spacing, the atoms between the partials form a stacking fault
Why do partial dislocations form?
When there is a reduction in enegry
How do we determine if a dissociation will occur?
If the energy change associated with the reaction at the first dislocation is greater than that associated with the split dislocations
How do we determine if two dislocations will combine to form a new segment?
If the energy associated with with the reaction at the first dislocation is lower than that associated with the split dislocations
What does the Orowan-Bailey relation describe?
Shear strain increment
What are isomorphous alloys?
Characterised by complete solubility of the two components
What can be determined from a phase diagram?
The fraction of phase present at equilibrium
What is the composition and phase fraction of a single phase?
Composition is C_0 and the phase fraction is 1
How is the phase fraction for a 2-phase region determined?
Using the tie-line and Lever rule
What parameter is determined by the lever rule?
Mass fraction
What is a eutectic reaction?
A reaction in which a single liquid changes phase into a 2-phase solid
Describe ferrite.
-At room temperature the structure is BCC
-Low solubility of C atoms in interstitial sites
-Can dissolve Cr, Si, W and Mo by substitution
Describe Austenite.
-At elevated temperature the structure is FCC
-Can dissolve more carbon interstitially than Ferrite (Alpha-iron)
-Can dissolve by substitution large amounts of Ni and Mn
Describe Carbide.
-Iron and carbon form the compound cementite
-Hard and brittle phase
-There can be other alloying elements in the cementite such as Cr, Mn and Mo (carbides)
What is a eutectoid reaction?
Involves the transformation of solid solution to two phases
Describe hypoeutectoid steels.
-At 1000 degrees there is a single austenite phase
-At 800 degrees there are two phases, austenite and proeutectoid ferrite
-At 730 degrees the grain boundaires are plated by proeutectoid ferrite
-Cooling below 730 degrees, forms pearlite (a mixture of ferrite and carbide)
What does the mean melting temperature tell us about Gibbs free energy?
Above the mean melting temperature, the liquid has a lower free energy.
For temperatures below the mean melting temperature, the Gibbs free energy of a solid is higher than the Gibbs free energy for a liquid.
What is the process for solidification in real alloys?
Nucleation and growth of solid particles
What two contributions are associated with the change in free enery?
-Formation of a volume of solid
-Formation of interface separating the solid and liquid phase
How does a large particle radius affect the nucleation energy?
Decreases it
What needs to be done by a system with a lower radius compared to critical radius?
Lower its energy by dissolution of the particle
What happens to a system with a radius is higher than the critical radius?
Free energy decreases by growth of the particle
How can the critical radius and free energy in terms of undercooling an latent heat of fusion be decreased?
Increasing the undercooling
How does particle growth of a nucleated phase occur?
Via diffusion of atoms
What parameter determined the growth rate of particles?
Diffusion coefficient of atomic species making up the phase of the nuclei
What equation (name) can be used to describe the growth rate of particles?
Arrhenius equation
Describe the system of particles at a temperature close to the melting temperature.
Nucleation rate is slow with high growth rates, the resulting microstructure will contain relatively few but coarse particles
Describe a system of transforming particles at lower temperatures (below melting).
Nucleation rate is high and growth rates are slow, resulting in a fine distribution of particles
What three factors induce phase transitions?
-Change in temperature
-Change in composition
-Change in applied pressure
How is Martensite formed?
Steels are rapidly cooled from the austenite phase field to low temperatures
What is the change in crystal structure that takes place when Martensite is formed?
FCC-BCT (Body centered tetragonal)
Describe what the yield stress associated with the obstacles in a dislocation line shows.
Stress needed to be applied in order for the dislocation to by-pass the obstacles
Describe the Frank-Read mechanism.
A dislocation segment is pinned at its ends will blow out under the action of an applied stress (this is an example of a dislocation source)
Describe solid solution strengthening.
-Solute atoms induce distortions, which inside a stress state
-The distortions can result in tensile or compressive stresses
-Interstitial atoms ‘pull’ neighbouring host atoms towards them, this can either induce a tensile stretch or a compressive stress
-The elastic field associated with solute atoms can interact with the dislocations, these can pin the dislocation line and restrict motion
How does yield stress change with the concentration of solute atoms during solid solution strengthening?
As the yield stress of alloys increases, the concentration of solute atoms also increases
What influence do elastic interactions have on dislocations?
They can pin them
What effect does plastic deformation have on the number of dislocations?
Number of dislocations increase, as shown by the Orowan-Bailey relation
How are yield stress and dislocation density related?
Yield stress increases parabolically with the dislocation density
What is told about the yield stress of a material if is is said to work harden?
Yield stress increases with a decreasing positive gradient
Describe the Hall-Petch effect.
Smaller grains have higher yield stress compared to larger grains
