NUCL320 Exam 3 Review

Concentration gradient

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

Activation energy

The minimum energy required for atoms to move and initiate 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 alloy

An alloy with a composition greater than the eutectic composition.

Hypoeutectic alloy

An alloy with a composition less than the eutectic composition.

Intermetallic compound

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

Isomorphous system

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

A two-component system where both components are completely soluble in each other in both liquid and solid states.

Binary eutectic systems

A two-component system where components have limited solid solubility and a eutectic composition.

Binary systems with intermediate phases

Systems where the components form additional solid phases or compounds at specific compositions.

The iron-carbon system

A binary system involving iron and carbon, determining properties of steel and cast iron.

Key properties of phases

Ferrite offers ductility, austenite provides strength at high temperatures, cementite contributes to hardness.

Homogeneous system

A single-phase system.

Heterogeneous systems

Systems with two or more phases.

Component

A chemical constituent, element, or compound of an alloy, specifying its composition.

Composition/Concentration

The relative content of a particular element expressed in wt% or at%.

Solution

Single phase solid, liquid, or gas solutions.

Solvent

Majority species in a solution.

Solute

Minority species in a solution.

Mixture

More than one phase present.

Determination of phases

Knowing temperature and composition allows determination of phase presence.

Determination of phase compositions

Knowing temperature and composition allows determination of phase composition.

Determination of phase weight fractions

Knowing temperature and composition allows determination of weight fraction of each phase.

Case Hardening

Process of diffusing carbon atoms into the surface of iron.

Solidification

Transformation of liquid metal into a solid phase as it cools.

Inhomogeneous materials

Materials that can become homogeneous through diffusion.

Active diffusion

Occurs when temperature overcomes energy barriers to atomic motion.

Concentration gradient

Driving force for interdiffusion.

Factors influencing diffusion

Temperature, diffusion mechanism, species properties, and microstructures affect diffusion rate.

Rate of transformation

Depends on time and temperature.

Superheating

Raising a material's temperature significantly above its melting point.

Supercooling

Cooling a liquid material below its melting point.

Nucleation process

Nuclei act as templates for crystal growth, requiring faster atom addition than loss.

Volume Free Energy

Energy saved by atoms bonding in a crystalline structure.

Surface free energy

Energy from solid-liquid phase boundary during solidification.

Homogeneous nucleation

Requires considerable supercooling to form nuclei in the bulk of liquid.

Heterogeneous nucleation

Easier nucleation at structural inhomogeneities with slight supercooling.

Difference between nucleations

Homogeneous occurs uniformly, while heterogeneous occurs at inhomogeneities.

Hardness comparison

Martensite is harder than fine pearlite.

Tempering martensite

Heat treatment process to reduce brittleness.

Difference between tempered and non-tempered martensite

Tempered martensite is less brittle with reduced internal stresses.