Glossary - Lent 2013 Flashcards
Annealing
Holding a sample at an elevated temperature, to bring the microstructure closer to stable equilibrium
Austenite
The cubic close packed allotrope of iron found between 912C and 1394C for pure iron. Also called the gamma phase
Cast iron
Any of a wide range of Fe-C alloys, with a carbon content of 2-4wt%. The carbon content is higher than for steels
Cementite
Iron carbide Fe3C often found in cast irons and steels although it is a metastable phase. (Graphite is actually the stable C-rich phase in the Fe-C system)
Coherent interface
An interface between 2 crystalline phases such that the 2 lattices match perfectly at the interface. Such an interface is typically of low free energy (see Semi-coherent & Incoherent interfaces)
Cooling curve
A plot of a sample temperature vs. time cooling. Changes in the cooling rate indicate phase transformations
Common-tangent construction
The algorithm for calculating the compositions of 2 phases co-existing in equilibrium, based on the relevant free energy vs. composition curves
Component
The different elements or chemical compounds which make up a system. The composition of a phase or the system can be described by giving the relative amount of each component
Coring
Variation of solute content in a phase arising when solidification is too rapid to permit a uniform composition to be achieved by solid-state diffusion
Dendrite
A branched tree-like form of crystal commonly found when metals solidify, often originating because of solute accumulation or depletion around the growing crystal
Diffusion
Transport of atoms in a liquid or solid in which the diffusing atom moves relative to its neighbours
Diffusion coefficient (Diffusivity, D)
A temperature dependent coefficient which describes the rate of diffusion. Has units of m^2s^-1
Displacive Phase Transition
The transition between 2 phases which are related by small atomic displacements (see Reconstructive Phase Transition)
Driving force
The difference in free energy driving a transformation
Enthalpy, H
A thermodynamic extensive variable, defined so that the changes in H are given by the heat input at constant pressure. (H= U + PV)
Entropy, S
A thermodynamic extensive variable, describing the degree of disorder of a system. For natural (spontaneous) changes, total entropy (of the system and its surroundings) always increases. In statistical mechanics, the entropy is related to the number of configurations of the system, Ω by S = klnΩ
Equilibrium
A state of a system in which there is no driving force for infinitesimal change. Different types of equilibrium are distinguished by the stability of the system to small and large perturbations (see Metastable, Neutral, Stable and Unstable equilibrium)
Eutectic
Relating to the transformation in which one liquid phase transforms to 2 solid phases on cooling. Is locally the lowest freezing point in the system
Eutectoid
Relating to the transformation in which one solid phase transforms to 2 solid phases on cooling.
Ferrite
The body centred cubic allotrope of iron - at lower temperature (below 912C in pure iron) labelled as alpha, at higher temperature (above 1394C in pure iron) as delta
First order Transformation
A transformation from one phase to another in which the first derivatives of the free energy (e.g. entropy & volume) are discontinuous
Free energy
Gibbs free energy is used to analyse equilibrium at constant pressure, Helmholtz free energy at constant volume
Freezing range
The temperature over which freezing occurs in solidification of a liquid with more than one component. It is the interval between the liquidus and solidus
Gibbs free energy, G
The thermodynamic potential which is minimised for equilibrium at constant pressure. Defined by G = H - TS
Glass
The phase formed when a liquid is cooled into a state of solid-like viscosity without crystallisation. Can be a material with any of the common bonding types (covalent, ionic, metallic, van der Waals)
Glass-ceramic
A fine-grained, almost totally crystalline product obtained by annealing a glass
Heterogeneous nucleation
Nucleation which occurs because of the presence of heterogeneities (see Homogeneous nucleation)
Homogeneous nucleation
Nucleation which occurs in a uniform phase without any influence of heterogeneities (see Heterogeneous nucleation)
Ideal solution
A solution in which the enthalpy of mixing is zero and the entropy of mixing has its ideal value
Incoherent interface
An interface between 2 crystalline phases at which there is no lattice matching. Such an interface will typically have a high free energy (see Coherent and Semi-coherent interfaces)
Intergrowth
A mixture of 2 phases, usually on a fine scale, formed when they grow cooperatively from a 3rd phase. Typical product of eutectic and eutectoid transformations
Internal energy, U
The potential energy and kinetic energy of a system. Changes in U are given by heat input at constant volume
Isothermal transformation diagram
Also known as a time-temperature-transformation (TTT) diagram. Temperature is plotted vertically, and the isothermal annealing time for a given fraction of transformation is plotted horizontally, usually on a logarithmic scale
Lamella(e)
Thin plate(s)
Lever rule
The algorithm for calculating the relative proportions of 2 phases, based on their compositions and overall composition
Line compound
A compound of very limited composition range which appears as a vertical line on a phase diagram of T vs. Composition. Such a compound is stoichiometric
Liquidus
The locus of the equilibrium temperature for solidification to start as a function of composition (see Solidus)
Martensite
A phase formed from austenite by a diffusionless transformation if the eutectoid transformation to pearlite is avoided on cooling. A martensitic transformation also refers, more generally, to any diffusionless shear transformation
Mechanical mixture
A mixture of phases in which each retains its separate identity. The free energy of the mixture is the weighted sum of the free energies of the constituent phases
Metastable equilibrium
Applies to a system which is stable against small fluctuations but not against large (see Equilibrium)
Microstructure
The arrangement of phases and other structural features that make up a solid material
Neutral equilibrium
Applies to a system in which there is no variation of potential with any change (see Equilibrium)
Pearlite
Fine eutectoid intergrowth of ferrite and cementite found in steels and cast irons
Phase
A portion of a system whose structure, properties and composition are homogeneous and which is physically distinct from other parts of the system
Phase separation
As a single phase solution is cooled, it may develop a free energy composition curve which has 2 minima, It will then separate into 2 isostructural phases
Precipitation
Formation of one phase in another of different chemical composition
Pro-eutectoid
Applied to a phase appearing by precipitation in advance of a eutectoid transformation. The precipitate phase is one of the phases in the eutectoid intergrowth
Reconstructive Phase Transition
A complete rearrangement of the structural topology involving the breaking and remaking of bonds.
Quench
A rapid cooling of a sample intended to avoid a phase transformation
Semi-coherent interface
An interface between 2 crystalline phases at which areas of perfect lattice matching are separated by dislocations. The dislocations act to relieve strain energy when lattice spacings in the interface plane are not the same in the 2 phases
Sintering
Particle coalescence in a powder by diffusion, accomplished by thermal treatment
Solidus
The locus of the equilibrium temperature for solidification to finish as function of composition (see Liquidus)
Solution
A mixture of components which is one phase. Contrasted with a mechanical mixture
Solvus
The temperature-composition locus showing the onset of precipitation. Indicates the limits of solubility
Stable equilibrium
Applies to a system which is stable against all fluctuations. It has the lowest possible free energy (see Equilibrium)
Steel
Any of a wide range of Fe-C alloys with a carbon content of ~0.1 - 1.4wt%. The carbon content is less than for cast irons
Stoichiometric
Obeying a strict chemical formula
Supercooling
Below an equilibrium temperature the temperature interval in which the higher temperature phase is preserved in a metastable state (also called undercooling)
System
The matter which is the subject of a thermodynamic analysis. In metallurgy, often an alloy which can exist as one or more phases
Texture
The preferred crystallographic orientation of grains in a material
Tie-line
Horizontal line on a temperature vs composition phase diagram the ends of which indicate phase compositions which co-exist in equilibrium
Transformation/Transition
These terms are used interchangeably here to denote a change in phase
Twins
Regions within a crystal with the same composition and structure, but different crystallographic orientation, and showing a specific orientation relationship between them
Unstable equilibrium
Applies to a system which is unstable against all fluctuations (see Equilibrium)
Widmannstatten
The characteristic pattern of precipitation of one solid phase in another, in which plate like precipitates have specific orientations governed by the symmetry of the matrix phase
Beam stiffness (Σ)
Product of Young’s Modulus and moment of Inertia, characterising the resistance to deflection when subjected to a bending moment
Bending moment (M)
Turning moment generated in a beam by set of applied forces. The bending moment is balanced at each point along the beam by the moment of the internal stresses
Brittle fracture
Fracture not involving gross plastic flow, although some local plastic deformation may occur at the tip of the crack
Burgers vector (b)
Vector giving magnitude and direction of lattice displacement generated by passage of a dislocation. It is a lattice vector for a perfect dislocation
Climb
Movement of an edge dislocation by absorption or emission of a vacancy
Critical resolved shear stress (
Value of resolved shear stress at which slip occurs on a specified slip system
Cross-slip
Movement of a screw dislocation from one permissible slip plane on to another, usually in order to bypass an obstacle
Curvature (
Reciprocal of the radius of curvature adopted by a beam subject to a bending moment. Also equal to the through-thickness gradient of strain in the beam
Deformation twinning
Mode of plastic deformation involving the co-operative shear of atomic arrays into a new orientation of the same crystal structure (reflecting the parent orientation across the twin plane). Also termed mechanical twinning
Dislocation line
Boundary between slipped and unslipped regions of a crystal
Dispersion Strengthening
Raising of yield stress via obstacles to dislocation glide that are thermally stable
Ductile fracture
Fracture involving gross plastic flow
Edge dislocation
Dislocation with a Burgers vector normal to the dislocation line
Engineering strain (ε)
Normal strain, given by ratio of change in length to original length. Also called nominal strain. (See true strain)
Engineering stress (σ)
Normal stress, given by the ratio of applied force to original sectional area. Also called nominal stress. (see true stress)
Fracture energy (G[subscript c])
Energy per unit area required to extend a crack. Also termed the critical strain energy release rate
Fracture toughness (K[subscript c])
Critical value of the stress intensity factor, characterising the resistance of a material to crack propagation
Frank’s rule
A dislocation reaction will occur only if the energy of the product dislocation(s) is less than that of the reacting dislocation(s). The energy of a dislocation is proportional to the square of its Burgers vector
Frank-Read source
A mechanism by which dislocations can multiply
Glide
Motion of a dislocation on its slip plane by small cooperative movements of atoms close to the dislocation core
Glissile dislocation
A dislocation which is able to glide
Griffith criterion
Energy based condition for fracture
Hooke’s Law
Stress is proportional to strain during elastic deformation
Jog
A step in a dislocation that does not lie in the slip plane
Kink
A step in a dislocation that lies in the slip plane
Martensitic transformation
Phase transformation occurring via cooperative shear of atomic arrays into a new crystal structure
Mixed dislocation
Dislocation in which the Burgers vector is neither parallel nor perpendicular to the dislocation line
Neutral axis (of a beam)
Axis (strictly, a plane) parallel to the length of a beam, along which there is no change in length on bending
Normal strain (ε)
Deformation in which change in length is parallel to original length
Normal stress (σ)
Stress induced by a force acting normal to the sectional area to which it is applied
OILS rule
Non-graphical method to find the slip system with the highest Schmid factor in a cubic crystal
Orowan bowing
The bowing of dislocations between precipitates that cannot be cut
Partial dislocation
A dislocation for which the Burgers vector is smaller than a lattice vector
Perfect dislocation
A dislocation for which the Burgers vector is a lattice vector
Primary slip system
Slip system which first becomes active. Normally the one with the highest Schmid factor
Resolved shear stress (tau subscript r)
Component of shear stress acting on a slip plane and parallel to a slip direction in that plane
Schmid factor (cos theta cos fi)
Geometrical factor relating resolved stress to normal stress along the tensile axis in a single crystal under tension or compression
Schmid’s law
Slip initiates at a critical value of resolved shear stress
Screw dislocation
Dislocation with a Burgers vector parallel to its dislocation line
Second moment of area (I)
Parameter dependent on sectional shape, characterising the resistance a beam offers to deflection under an applied bending moment. Also called the moment of inertia
Sessile dislocation
A dislocation which is unable to glide
Shear modulus (G)
Constant of proportionality between shear stress and shear strain
Beam stiffness (Σ)
Product of Young’s Modulus and moment of Inertia, characterising the resistance to deflection when subjected to a bending moment
Bending moment (M)
Turning moment generated in a beam by set of applied forces. The bending moment is balanced at each point along the beam by the moment of the internal stresses
Brittle fracture
Fracture not involving gross plastic flow, although some local plastic deformation may occur at the tip of the crack
Burgers vector (b)
Vector giving magnitude and direction of lattice displacement generated by passage of a dislocation. It is a lattice vector for a perfect dislocation
Climb
Movement of an edge dislocation by absorption or emission of a vacancy
Critical resolved shear stress (tau subscript c)
Value of resolved shear stress at which slip occurs on a specified slip system
Cross-slip
Movement of a screw dislocation from one permissible slip plane on to another, usually in order to bypass an obstacle
Curvature (kappa)
Reciprocal of the radius of curvature adopted by a beam subject to a bending moment. Also equal to the through-thickness gradient of strain in the beam
Deformation twinning
Mode of plastic deformation involving the co-operative shear of atomic arrays into a new orientation of the same crystal structure (reflecting the parent orientation across the twin plane). Also termed mechanical twinning
Dislocation line
Boundary between slipped and unslipped regions of a crystal
Dispersion Strengthening
Raising of yield stress via obstacles to dislocation glide that are thermally stable
Ductile fracture
Fracture involving gross plastic flow
Edge dislocation
Dislocation with a Burgers vector normal to the dislocation line
Engineering strain (ε)
Normal strain, given by ratio of change in length to original length. Also called nominal strain. (See true strain)
Engineering stress (σ)
Normal stress, given by the ratio of applied force to original sectional area. Also called nominal stress. (see true stress)
Fracture energy (G[subscript c])
Energy per unit area required to extend a crack. Also termed the critical strain energy release rate
Fracture toughness (K[subscript c])
Critical value of the stress intensity factor, characterising the resistance of a material to crack propagation
Frank’s rule
A dislocation reaction will occur only if the energy of the product dislocation(s) is less than that of the reacting dislocation(s). The energy of a dislocation is proportional to the square of its Burgers vector
Frank-Read source
A mechanism by which dislocations can multiply
Glide
Motion of a dislocation on its slip plane by small cooperative movements of atoms close to the dislocation core
Glissile dislocation
A dislocation which is able to glide
Griffith criterion
Energy based condition for fracture
Hooke’s Law
Stress is proportional to strain during elastic deformation
Jog
A step in a dislocation that does not lie in the slip plane
Kink
A step in a dislocation that lies in the slip plane
Martensitic transformation
Phase transformation occurring via cooperative shear of atomic arrays into a new crystal structure
Mechanical twinning
see deformation twinning
Mixed dislocation
Dislocation in which the Burgers vector is neither parallel nor perpendicular to the dislocation line
Neutral axis (of a beam)
Axis (strictly, a plane) parallel to the length of a beam, along which there is no change in length on bending
Nominal strain (ε)
see engineering strain
Nominal stress (σ)
see engineering stress
Normal strain (ε)
Deformation in which change in length is parallel to original length
Normal stress (σ)
Stress induced by a force acting normal to the sectional area to which it is applied
OILS rule
Non-graphical method to find the slip system with the highest Schmid factor in a cubic crystal
Orowan bowing
The bowing of dislocations between precipitates that cannot be cut
Partial dislocation
A dislocation for which the Burgers vector is smaller than a lattice vector
Perfect dislocation
A dislocation for which the Burgers vector is a lattice vector
Primary slip system
Slip system which first becomes active. Normally the one with the highest Schmid factor
Resolved shear stress (
Component of shear stress acting on a slip plane and parallel to a slip direction in that plane
Schmid factor (cos(theta)cos(fi))
Geometrical factor relating resolved stress to normal stress along the tensile axis in a single crystal under tension or compression
Schmid’s law
Slip initiates at a critical value of resolved shear stress
Screw dislocation
Dislocation with a Burgers vector parallel to its dislocation line
Second moment of area (I)
Parameter dependent on sectional shape, characterising the resistance a beam offers to deflection under an applied bending moment. Also called the moment of inertia
Sessile dislocation
A dislocation which is unable to glide
Shear modulus (G)
Constant of proportionality
Shear strain (
Distortional deformation (an angle) arising from a shear displacement
Shear stress (
Stress induced by a force acting parallel to the sectional area to which it is acting
Slip system UVW
Combination of slip plane (hkl) and slip direction [UVW], which lies in the (hkl) plane
Solution strengthening
Increase in yield stress of a material through the addition of solute atoms, caused by the interaction of these solute atoms with dislocations
Stacking fault
A 2d defect in which the sequence of stacking of atomic planes is interrupted
Strain energy release rate (G)
Elastic strain energy released per unit of created crack area
Strain hardening
see work hardening
Stress intensity factor (K)
Parameter characterising the crack driving force, in terms of applied stress level and crack length
Surface energy (
Energy per unit area associated with a free surface
Thermal expansivity (α)
Ratio of change in length to original length, per unit change in temperature. Also known as Coefficient of Thermal Expansion
True strain (ε)
Ratio of change in length to current length. May differ from the engineering strain
True stress (σ)
Ratio of force to current sectional area over which it is acting. May differ from engineering stress
Twinning plane
Plane within which shear takes place during deformation twinning, which also forms a mirror plane between parent and twin structures
Work hardening
Increase in plastic flow stress with strain. Associated with an increase in dislocation density, formation of entanglements, locks etc. Also known as strain hardening
Yield point
Point (stress level = σ[subscript y]) at which plastic deformation first occurs
Young’s modulus (E)
Constant of proportionality between normal stress and normal strain
