NUCL320 Exam 3 Review

Activation energy: The minimum energy required for atoms to move and initiate diffusion.

Concentration gradient: The change in concentration of a species per unit distance, driving diffusion.

Diffusion: The movement of atoms or molecules from regions of high concentration to low concentration.

Diffusion coefficient (D): A measure of the rate at which atoms or molecules diffuse, dependent on temperature and material properties.

Diffusion flux (J): The amount of material diffusing through a unit area per unit time.

Driving force: The factor causing diffusion, often a concentration gradient or chemical potential difference.

Fick’s first law: Describes steady-state diffusion where diffusion flux is proportional to the concentration gradient.

Fick’s second law: Describes nonsteady-state diffusion, where the rate of change of concentration depends on the second derivative of concentration with respect to position.

Interdiffusion: Diffusion of atoms between different materials or phases.

Interstitial diffusion: Diffusion of smaller atoms (e.g., hydrogen, carbon) through the interstitial spaces between larger atoms in a lattice.

Self-diffusion: Diffusion of atoms within a single pure material.

Steady-state diffusion: Diffusion where the concentration gradient remains constant over time.

Nonsteady-state diffusion: Diffusion where the concentration gradient changes with time.

Temperature dependence of D: The diffusion coefficient increases exponentially with temperature, as described by the Arrhenius equation.

Vacancy diffusion: Diffusion mechanism where atoms move by exchanging places with vacancies in the crystal lattice.

Eutectic phase: A phase that forms at the eutectic composition during a eutectic reaction.

Eutectic reaction: A reaction where a liquid transforms into two solid phases simultaneously upon cooling.

Eutectic structure: A microstructure resulting from the eutectic reaction, typically showing alternating layers or lamellae of the two solid phases.

Eutectoid reaction: A reaction where one solid phase transforms into two different solid phases upon cooling.

Hypereutectic/eutectoid alloy: An alloy with a composition greater than the eutectic or eutectoid composition.

Hypoeutectic/eutectoid alloy: An alloy with a composition less than the eutectic or eutectoid composition.

Intermetallic compound: A solid compound of two or more metals with a distinct stoichiometric ratio and crystal structure.

Isomorphous: Describes a system where two components are completely soluble in each other in both liquid and solid states.

Lever rule: A mathematical tool used to determine the proportions of phases in a two-phase region of a phase diagram.

Liquidus line: The line on a phase diagram above which the material is entirely liquid.

Microconstituent: A distinct phase or mixture of phases in a microstructure, observable under a microscope.

Pearlite: A two-phase microstructure of alternating ferrite and cementite layers formed during the eutectoid reaction in steel.

Peritectic reaction: A reaction where a liquid and a solid phase combine to form a different solid phase upon cooling.

Phase: A homogeneous portion of a material with uniform physical and chemical properties.

Phase diagram: A graphical representation of phases present under different temperature, pressure, and composition conditions.

Phase equilibrium: A state where phases coexist at stable compositions and proportions under given conditions.

Primary phases: The first solid phases to form during solidification of an alloy.

Solidus line: The line on a phase diagram below which the material is entirely solid.

Solubility limit: The maximum concentration of a solute that can dissolve in a solvent to form a single phase.

Solvus line: The line on a phase diagram that separates single-phase and two-phase regions based on solute solubility.

System: A specific material or set of materials under study, defined by its components.

Terminal solid solution: A solid solution that exists at the extreme ends of the composition range in a phase diagram.

Tie line: A horizontal line in a two-phase region of a phase diagram that connects the compositions of the coexisting phases.

Phases and microstructure: Phases are distinct regions within a material with uniform physical and chemical properties, while microstructure refers to the arrangement and distribution of these phases at the microscopic level.

Binary isomorphous systems (complete solid solubility): A two-component system where both components are completely soluble in each other in both liquid and solid states, forming a single phase across all compositions (e.g., Cu-Ni).

Binary eutectic systems (limited solid solubility): A two-component system where components have limited solid solubility and a eutectic composition where the liquid phase transforms into two solid phases simultaneously upon cooling (e.g., Pb-Sn).

Binary systems with intermediate phases/compounds: Systems where the components form additional solid phases or compounds at specific compositions, often with distinct stoichiometries and properties (e.g., MgZn2 in Mg-Zn).

The iron-carbon system (steel and cast iron): A binary system involving iron and carbon, with key phases like ferrite, austenite, cementite, and graphite, forming different microstructures (e.g., pearlite, martensite) depending on composition and thermal processing. It governs the behavior of steel and cast iron.

Key properties and applications of each phase: ferrite offers ductility, austenite provides strength at high temperatures, cementite contributes to hardness, and graphite enhances machinability in cast iron.

Phase – A homogeneous portion of a system that has uniform physical and chemical characteristics.

homogeneous system - A single-phase system

mixtures or heterogeneous systems - systems with two or more phases

Component – A chemical constituent, an element, or a compound, of an alloy,

which may be used to specify its composition

Composition/Concentration – The relative content of a particular element or

constituent within an alloy expressed in wt% or at%

Solution – solid, liquid, or gas solutions, single phase

• Solvent – Majority species; • Solute – Minority species

• Mixture – more than one phase

Solubility Limit: Maximum concentration for which only a single phase solution exists.

phase diagram - a map of equilibrium phases associated with various

combinations of temperature, composition, and pressure.

isomorphous system - This system possess complete liquid and solid solubility from 0 ~ 100%.

Determination of phase(s) present - If we know T and Co, then we know: which phase(s) is (are) present.

Determination of phase compositions - f we know T and C0, then we can determine: the composition of each phase.

Determination of phase weight fractions - f we know T and C0, then can determine: the weight fraction of each phase.

Case Hardening - Diffuse carbon atoms into the host iron atoms at the surface.

solidification - the process by which a liquid metal transforms into a solid phase as it cools, resulting in the formation of a microstructure that influences the material's properties.

Diffusion - material transport by atomic motion. Inhomogeneous materials can become homogeneous by diffusion. For an active diffusion to occur, the temperature should be high enough to overcome energy barriers to atomic motion.

Interdiffusion (or impurity diffusion) occurs in response to - a concentration gradient

Self-diffusion - diffusion in one-component material, when all atoms that

exchange positions are of the same type.

What are the two mechanisms by which diffusion occurs? - Vacancy diffusion and Interstitial diffusion

Vacancy diffusion - depends on vacancies being present so

that neighboring atoms can jump into them.

Interstitial diffusion - the diffusion of a species along

interstitial sites, not atom sites

How is the rate of diffusion expressed? - expressed as a diffusion flux, J, which is defined as the mass of atoms diffusing through a cross-section of the specimen per unit time.

Steady state diffusion - when diffusion flux does not change with time, constant J value

Fick’s first law - the diffusion flux along direction x is proportional to the concentration gradient

What is the driving force of diffusion? - the concentration gradient the steeper the concentration gradient - the greater the flux

What does fick’s first law assume? - the concentration gradient is constant with time, as this is not the reality, the law applies only to snapshot instances in time

Diffusion is a _____ activated process - thermally

What kind of dependance does D have with T? - exponential

Factors that influence diffusion: Temperature - diffusion rate increases very rapidly with increasing temperature

Factors that influence diffusion: Diffusion mechanism - diffusion by interstitial mechanism is usually faster than by vacancy mechanism

Factors that influence diffusion: Diffusing and host species - Do, QD are different for every solute, solvent pair

Factors that influence diffusion: Microstructures - Thus far we have talked about only diffusion in the lattice, but there are other types.

Three main types of diffusion -Lattice, diffusion through the lattice or volume; Surface Diffusion, diffuse across the open sites of a surface; Grain Boundary, diffusion along the grain boundaries

How does the rate of transformation depend on time and temperature? -

Is it possible to slow down transformations so that non-equilibrium structures

are formed? -

Are the mechanical properties of non-equilibrium structures more desirable than equilibrium ones? -

The rate of approaching equilibrium depends on - the amount of heating or cooling, like Heating to melt a metal, Cooling to solidify a metal, Heating a iron to transform it to g iron for hardening, Heat treating a quenched g iron to convert it to a + Fe3C

Superheating - raising a materials temperature significantly above its melting point

Supercooling - cooling a liquid material below its melting point to help it to solidify faster

Nucleation process - nuclei (seeds) act as templates on which crystals grow, for nucleus to form, rate of addition of atoms to nucleus must be faster than rate of loss, once nucleated, growth proceeds until equilibrium is attained

The desire to nucleate increases with - increase in change of temperature, superheating (peritectic) or supercooling (eutectic, eutectoid)

What is the effect of small supercooling on nucleation rate - slow nucleation rate, few nuclei, large crystals

What is the effect of large supercooling on nucleation rate - rapid nucleation rate, many nuclei, small crystals

Homogeneous nucleation - nuclei form in the bulk of liquid metal, requires considerable supercooling (typically 80-300oC)

Heterogeneous nucleation – much easier since stable “nucleating surface” is already present, e.g., mold wall, impurities in liquid phase, grain boundaries. only very slight supercooling (0.1-10ºC).

How does solid precipitate from the liquid phase? - Precipitation of solid from a liquid phase results from the competition of two types of energy (volume free energy and surface free energy)

Volume Free Energy - results from the amount of energy saved by the atoms bonding together and forming a crystalline structure.

Surface free energy - results from the formation of the solid–liquid phase boundary during the solidification

transformation

What is the effect of small cluster sizes (small r) on free energy? - surface energy addition dominates, Clusters that are too small have a high surface energy and are actually at higher energy than the liquid itself and therefore melt back to liquid

What is the effect of large cluster sizes (larger) on free energy? - volume energy reduction dominates, Clusters that are large enough have low surface energy and can save energy by bonding and forming crystals, stable equilibrium structures.

Differences between homogeneous and heterogeneous nucleation - The distinction between them is made according to the site at

which nucleating events occur. For the homogeneous type, nuclei of the new phase form uniformly throughout the parent phase. For heterogeneous type, nuclei form preferentially at structural inhomogeneities, such as, container surfaces, insoluble impurities, grain boundaries, dislocations, and so on. The solidification can occur much more rapidly in heterogeneous

nucleation because the surface free energy is not so high, and

volume free energy takes over earlier.

Compare the hardness of martensite and fine pearlite - pearlite « martensite

Compare the hardness and ductility of fine pearlite, coarse pearlite, and martensite - hardness fine>coarse>spheroidite, ductility fine<coarse<spheroidite

How to temper martensite - heat treat it

What are the differences between tempered and non-tempered martensite - tempered is less brittle and has reduced internal stresses