ENGN1215 Study Guide - Final Guide: Thermal Expansion, Compressive Strength, Thermal Shock
Mechanical Properties of Materials
Stiffness: ability to resist deformation = Youngs modulus of elasticity
Ductility: ability to plastically deform
Resilience: capacity of a material to absorb energy (area under stress/strain
curve up until yield stress)
Toughness: ability to resist crack propagation
True stress/strain: takes into account changing area with force (engineering
stress/strain does not) → issue with necking
Hardness: ability to resist localised plastic deformation
Dislocations and Strengthening Mechanisms
Plastic deformation
Edge vs. screw dislocation → same result:
Annihilation
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Slip system will occur so that atomic distortion = minimum (maximum atomic
density)
Resolved shear stress:
σ- applied stress
ϕ- angle between slip plane normal and σ
λ- angle between slip plane direction and σ
Polycrystalline materials
• Theoretically stronger than single crystal equivalents
• Greater stress required to initiate slip → geometric constraints (grains
cannot slip until adjacent grains slip as well)
Single crystals are better in certain directions
Twinning
• Can be fractional displacement
• Occurs in BCC or HCP at low T/ high loading rate (slip is limited)
• Good → introduces new slip system → more slip possible
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Strengthening mechanisms in metals
Grain size reduction
• More grain boundaries → difficult for dislocation to cross boundary
Solid solution strengthening
• Alloying with impurity atoms → substitutional or interstitial dislocations
→ restrict dislocation movement
• Can cancel dislocations out
Strain hardening
• Hardening by plastic deformation
• Increases dislocation density → interacts with strain fields → dislocations
hindered
• Often reversible by heat treatment
Recovery
• Some internal stored energy relieved by dislocation movement
Recrystallization
• Equiaxed, strain free, larger grains
• Weaker, softer + more ductile
• Heat treatment at recrystallization T for 1 hour → completion
Phase Diagrams
• Free energy- function of internal energy of the system (enthalpy) +
degree of randomness of atoms (entropy)
• Equilibrium → internal energy minimised
• Metastable system: having a state of apparent equilibrium although
capable of changing to a more stable state
• Binary Isomorphous: two structures is solid phase
• If α, α+L, L → three phases
• When cooling occurs slowly atoms have time to diffuse and homogenize
• Non equilibrium cooling → cored grains (more of the high temperature
element is retained at the centre grains)
• Solvus line: separates a homogenous solid solution from several phases
(limit of solubility)
• Eutectic reaction: point at which material will change from liquid to
solid and vice vera (usually lowest melting point for a system)
• Eutectoid reaction: one solid into two different solids
• Peritectic reaction: liquid + solid transforms into liquid + different solid
• Terminal phase or terminal solution: is one that exists in the extremes
of concentration (0 and 100%) of the phase diagram
• Intermediate phase: One that exists in the middle, separated from the
extremes
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Document Summary
Stiffness: ability to resist deformation = young(cid:495)s modulus of elasticity. Resilience: capacity of a material to absorb energy (area under stress/strain curve up until yield stress) True stress/strain: takes into account changing area with force (engineering stress/strain does not) issue with necking. Slip system will occur so that atomic distortion = minimum (maximum atomic density) Angle between slip plane normal and . Angle between slip plane direction and . Polycrystalline materials: theoretically stronger than single crystal equivalents, greater stress required to initiate slip geometric constraints (grains cannot slip until adjacent grains slip as well) Twinning: can be fractional displacement, occurs in bcc or hcp at low t/ high loading rate (slip is limited, good introduces new slip system more slip possible. Grain size reduction: more grain boundaries difficult for dislocation to cross boundary. Solid solution strengthening: alloying with impurity atoms substitutional or interstitial dislocations. Restrict dislocation movement: can cancel dislocations out.
