Metallurgy and Material Science - Module 1

METALLURGY AND MATERIAL SCIENCE

Module 1 (For M G University students prior to 2010 admission)

Prepaired by Sajeev Abraham Department of Mechanical Engineering SAINTGITS College of Engineering Pathamuttom, Kottayam,Kerala,India E mail:[email protected]

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METALLURGY AND MATERIAL SCIENCE (M 304) - SYLLABUS Module 1 Crystallography: Crystal structural determination, crystallographic directions and planes, miller indices, packing of atoms in solids, atomic packing factor, coordination number- Amorphous structure, glass transition temperature -- Effects of crystalline and amorphous structure on mechanical and optical properties -- Mechanism of crystallization: Homogeneous and heterogeneous nuclei formation, dendritic growth and grain boundary irregularity, grain size effects on mechanical & optical properties - Changes within solid materials: Structural imperfections: Point defects - line defect: edge, screw dislocation, burgers vector, forest of dislocations, role of dislocation in the deformation of metals - Surface imperfections: role of surface defect on crack propagation etc – Mode of plastic deformation: mechanism of slip & twinning, dislocation climb & cross slip, dislocation sources, frank-read source – Diffusion in solids, fick’s laws, applications. Module 2 Cold working, strain hardening, recovery, re-crystallization, grain growth, grain size and its effects on mechanical properties-- Hot working, super plasticity – Reasons for alloying, phase transformation phase rules, single phase, multi phase equilibrium diagrams, solid solutions, inter metallic compounds – Equilibrium diagram reactions: monotectic, eutectic, eutectoid, peritectic, peritectoid -- Polymorphism – Detailed discussion of Iron-Carbon diagram with microstructure changes in ferrite, austenite, cementite, graphite, pearlite, martensite, bainite. Module 3 Definition and aims of heat treatment- Annealing, spheroidizing, normalizing, hardening, tempering, austermpering, martempering with microstructure changes -- Surface treatment: Diffusion methods: carburizing, nitriding, cyaniding -- Thermal methods: flame hardening, induction hardening – Deposition methods: hot dipping and coating, impregnation, metal spraying, metal cladding – Various strengthen mechanisms in metals: work hardening, grain boundary hardening, grain size reduction, solid solution hardening, dispersion hardening. Module 4 Alloy steels: Effects of alloying elements on: dislocation movement, polymorphic transformation point, retardation of the transformation rates, improvement in corrosion resistance, mechanical properties -- Nickel steels, chromium steels, etc – Effects on steels, containing molybdenum, vanadium, tungsten, cobalt, silicon, copper and lead – high speed steels - - Cast irons: classifications, gray, white, malleable and spheroidal graphite cast iron, composition, microstructure, properties and applications - Principal non ferrous alloys like aluminum, beryllium, copper, magnesium, nickel, study of composition, microstructure, properties and applications- Reference shall be made to the phase diagrams whenever necessary. Module 5 Fracture: Bonding forces and energies, cohesive strength of metals - Griffith theory –- Crack initiation, growth and crack arrest – Effect of plastic deformation on crack propagation – Factors leading to crack propagation - Cleavage, intercrystalline, brittle, ductile fracture -- Influence of slip on fracture – Effect of impact loading on ductile material and its application in forging etc.-Fatigue: stress cycles – Effects of stress concentration, size effect, surface texture on fatigue – Corrosion and thermal fatigue – Mechanism of fatigue failure -- Creep: Creep curves – Structural change – Mechanism of creep deformation.

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Prepared by Sajeev Abraham, Dept.of Mechanical Engg.,SAINTGITS College of Engineering

METALLURGY AND MATERIAL SCIENCE -Module 1 WHAT ARE MATERIALS?   

Materials may be defined as substance of which something is composed or made. We obtain materials from earth crust and atmosphere. Examples : Silicon and Iron constitute 27.72 and 5.00 percentage of weight of earths crust respectively.  Nitrogen and Oxygen constitute 78.08 and 20.95 percentage of dry air by volume respectively.

WHAT IS MATERIALS SCIENCE AND ENGINEERING?

Material science is the investigation of the relationship among processing, structure, properties, and performance of materials Structure:  

At the atomic level: arrangement of atoms in different ways. (Gives different properties for graphite than diamond both forms of carbon.) At the microscopic level: arrangement of small grains of material that can be identified by microscopy. (Gives different optical properties to transparent vs. frosted glass.)

Properties are the way the material responds to the environment. For instance, the mechanical, electrical and magnetic properties are the responses to mechanical, electrical and magnetic forces, respectively. Other important properties are thermal (transmission of heat, heat capacity), optical (absorption, transmission and scattering of light), and the chemical stability in contact with the environment (like corrosion resistance). Processing of materials is the application of heat (heat treatment), mechanical forces, etc. to affect their microstructure and, therefore, their properties.

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WHY STUDY MATERIALS SCIENCE AND ENGINEERING?   

To be able to select a material for a given use based on considerations of cost and performance. To understand the limits of materials and the change of their properties with use. To be able to create a new material that will have some desirable properties.

All engineering disciplines need to know about materials. Even the most "immaterial", like software or system engineering depend on the development of new materials, which in turn alter the economics, like software-hardware trade-offs. Increasing applications of system engineering are in materials manufacturing (industrial engineering) and complex environmental systems. TYPES OF MATERIALS Materials are classified as follows. Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Metals have high strength, high stiffness, high melting point and have good ductility Examples: aluminum, steel, brass, gold. Semiconductors: the bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs. Ceramics: atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). They are brittle, high melting temperature, low density, high strength, stiffness, hardness, wear resistance, and corrosion resistance. Many ceramics are good electrical and thermal insulators. Examples: glass, porcelain, many minerals. Polymers: are bound by covalent forces and by weak van der Waals forces, and usually based on H, C and other non-metallic elements. Polymers are useful because they are lightweight, are corrosion resistant, are easy to process at low temperatures, and are generally inexpensive. One of the distinct properties of polymers is that they are poor conductors of electricity and heat, which makes them good insulators. Examples: plastics (nylon, Teflon, polyester) and rubber. Other categories are not based on bonding. Composites: made of different materials in intimate contact (example: fiberglass, concrete, wood) to achieve specific properties. Biomaterials:Can be any type of material that is biocompatible and used, for instance, to replace human body parts. TYPES OF SOLIDS Crystallitne solid  Atoms self-organize in a regular and periodic arrangement of atoms or molecules in three dimensions

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   

It possesses a long range order of atoms ions or molecules. May be made up of metallic crystals or non-metallic crystals They shows sharp melting point They are anisotropic (ie., physical properties are different in different directions)

Amorphous Solids :  Lacks a systematic atomic arrangement.  there is no long-range order. when a melt or a solution is cooled rapidly we get an amorphous solid Eg. glass  No sharp melting point (because all bonds are not strong.)  isotropic

Amorphous: lacks a systematic atomic arrangement

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CRYSTAL STRUCTURE A regular and repetitious pattern in which atoms of a crystalline material arrange them selves is known as the crystal structure A crystal is a repeating array. In describing this structure, we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell). The most fundamental property of a crystal lattice is its symmetry. In three-dimensions, unit cells stack like boxes, filling the space, making the crystal.

Cubic Lattice Structure

Hexagonal Lattice Structure

UNIT CELL The unit cell is the smallest structural unit or building block that can describe the the crystal structure. Repetition of the unit cell generates the entire crystal.Different choices of unit cells possible, generally choose parallelepiped unit cell with highest level of symmetry Example: 2D honeycomb net can be represented by translation of two adjacent atoms that form a unit cell for this 2D crystalline structure SPACE LATTICE or CRYSTAL LATTICE Space lattice is defined as an infinite array of points in three dimensions in which every point has surroundings identical to those of every other point in the array. If the centers of the points are considered to be connected together by straight lines, then the system will be obtained comprising a great number of equal parallelepipeds. The three dimensional array formed by the unit cells of a crystal is called space lattice. When a crystalline solid starts to form from the molten or gaseous state, these unit cells will tend to stack in a three-dimensional array, with each cell perfectly aligned, and they will form a crystal. If crystals are growing in a melt at the same time, the crystals will eventually meet and form grains. The junction of the grains is called grain boundaries.

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Different choices of unit cells possible, generally choose parallelepiped unit cell with highest level of symmetry. CRYSTAL SYSTEMS The crystal systems are a grouping of crystal structures according to the axial system used to describe their lattice. Each crystal system consists of a set of three axes in a particular geometrical arrangement. There are seven unique crystal systems.(Refer page 7)

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METALLIC CRYSTAL STRUCTURES The most common types of unit cells are the face centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP).There are 14 different types of crystal unit cell structures or lattices are found in nature. However most metals and many other solids have unit cell structures described as body center cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp). FACE-CENTERED CUBIC (FCC) CRYSTAL STRUCTURE  Atoms are located at each of the corners and on the centers of all the faces of cubic unit cell  Cu, Al, Ag, Au have this crystal structure

 The hard spheres or ion cores touch one another across a face diagonal the cube edge length, a= 4R/√2  The coordination number, CN = the number of closest neighbors to which an atom is bonded = number of touching atoms, CN = 12  Number of atoms per unit cell, n = 4. (For an atom that is shared with m adjacent unit cells, we only count a fraction of the atom, 1/m). In FCC unit cell we have:  6 face atoms shared by two cells: 6 x 1/2 = 3  8 corner atoms shared by eight cells: 8 x 1/8 = 1

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 Atomic packing factor, APF = fraction of volume occupied by hard spheres = (Sum of atomic volumes)/(Volume of cell) = 0.74 (maximum possible)  Corner and face atoms in the unit cell are equivalent  FCC crystal has APF of 0.74, the maximum packing for a system equal-sized spheres FCC is a close-packed structure  FCC can be represented by a stack of close-packed planes (planes with highest density of atoms)

BODY-CENTERED CUBIC (BCC) CRYSTAL STRUCTURE  

Atom at each corner and at center of cubic unit cell Cr, α-Fe, Mo have this crystal structure

  

The hard spheres touch one another along cube diagonalthe cube edge length, a= 4R/√3 The coordination number, CN = 8 Number of atoms per unit cell, n = 2 Center atom (1) shared by no other cells: 1 x 1 = 1 8 corner atoms shared by eight cells: 8 x 1/8 = 1

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 

Atomic packing factor, APF = 0.68 Corner and center atoms are equivalent

HEXAGONAL CLOSE-PACKED CRYSTAL STRUCTURE  HCP is one more common structure of metallic crystals  Six atoms form regular hexagon, surrounding one atom in center. Another plane is situated halfway up unit cell (c-axis), with 3 additional atoms situated at interstices of hexagonal (close-packed) planes.  Cd, Mg, Zn, Ti have this crystal structure

 Unit cell has two lattice parameters a and c. Ideal ratio c/a = 1.633  The coordination number, CN = 12 (same as in FCC)  Number of atoms per unit cell, n = 6. 3 mid-plane atoms shared by no other cells: 3 x 1 = 3 12 hexagonal corner atoms shared by 6 cells: 12 x 1/6 = 2 2 top/bottom plane center atoms shared by 2 cells: 2 x 1/2 = 1  Atomic packing factor, APF = 0.74 (same as in FCC)  All atoms are equivalent Simple Cubic Structures  the cube edge length, a= 2R  The coordination number, CN = 4+2=6 Number of atoms per unit cell, n = 1 8 corner atoms shared by eight cells: 8 x 1/8 = 1  Atomic packing factor, APF = 0.52

)

Close-packed Structures (FCC and HCP  Both FCC and HCP crystal structures have atomic packing factors of 0.74 (maximum possible value)  Both FCC and HCP crystal structures may be generated by the stacking of close-packed planes

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 The difference between the two structures is in the stacking sequence

HCP: ABABAB... FCC: Stacking Sequence ABCABCABC

FCC: ABCABCABC…

...

Third plane is placed above the “holes” of the first plane not covered by the second plane

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HCP: Stacking Sequence ABABAB...

Third plane is placed directly above the first plane of atoms POLYMORPHISM AND ALLOTROPY

Some materials may exist in more than one crystal structure, this is called polymorphism. If the material is an elemental solid, it is called allotropy.An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.

Pure, solid carbon occurs in three crystalline forms – diamond, graphite; and large, hollow fullerenes. Two kinds of fullerenes are shown here: buckminsterfullerene (buckyball) and carbon nanotube SINGLE CRYSTALS AND POLYCRYSTALLINE MATERIALS

Single crystal: atoms are in a repeating or periodic array over the entire extent of the material Polycrystalline material: comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called grain boundaries.

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Simulation of annealing of a polycrystalline grain structure

ANISOTROPY

Different directions in a crystal have a different packing.For instance, atoms along the edge of FCC unit cell are more separated than along the face diagonal. This causes anisotropy in the properties of crystals, for instance, the deformation depends on the direction in which a stress is applied. In some polycrystalline materials, grain orientations are random, so bulk material properties are isotropic Some polycrystalline materials have grains with preferredorientations (texture), so properties are dominated by those relevant to the texture orientation and the material exhibits anisotropic properties

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MILLER INDICES (hkl)

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DEFECTS IN SOLIDS

Defects have a profound impact on the macroscopic properties of materials

The processing determines the defects

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TYPES OF DEFECTS

Point defects: Point defects are lattice errors at isolated lattice points. As the name implies, they are imperfect point like regions in the crystal and ,therefore they are referred to as zero dimensional imperfections.

Vacancy - A lattice position that is vacant because the atom is missing. Vacancies an important part in diffusion of atoms through the lattice. 18


Interstitialcies – In a close packed arrangement of atoms if the atomic packing factor is low, an extra atom may be lodged within the crystal structure. This is known as interstitials.Self-interstitials in metals introduce large distortions in the surrounding lattice

Di-vacancies and interstitials

Vacancies are not only present as a result of solidification but can be produced by raising the temperature or by irradiation with fast moving nuclear particles.

Schottky Defect: is obtained when an atom or ion is removed from a normal lattice site and replaced by an ion on the surface of the crystal. In non-metallic crystals, the formation of a vacancy involves a local readjustment in the surrounding crystal such that charge neutrality is maintained in the crystal as a whole. Thus if in an ionic crystal there is a vacancy in a positive ion site, charge neutrality may be achieved by creating a vacancy in a neighboring negative ion site. Such a pair of vacant sites is called Schottky defect.

Frenkel defect :- an ion displaced from the lattice site into an interstitial site. If the charge neutrality is maintained by having an ion in an interstitial position, the pair constitutes a Frenkel defect. 19


Impurities –Foreign atoms either occupy lattice sites from which the regular atoms are missing or they occupy positions between the atoms of the ideal crystal.These impurity atoms are responsible for the functioning of most semiconductor devices. Impurities two types-interstitial & substitutional impurities Interstitial impurity is a small–sized atom occupying an interstice or space between the regularly positioned atoms. Substitutional impurity is created when a foreign atom substitutes for or places a parent atom in the lattice. In brass, zinc is a substitutional atom in the copper lattice.

Dislocations—Linear Defects Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. A dislocation may be defined as a disturbed region between two perfect parts of a

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crystal. Dislocation is responsible for the phenomenon of slip, by which most metals deform plastically. Dislocations can be observed in crystalline materials using electron-microscopic techniques. Virtually all crystalline materials contain some dislocations that were introduced during solidification, during plastic deformation, and as consequence of thermal stresses that result from rapid cooling. The importance of dislocations to the metal user is that dislocation interactions within a metal are a primary means by which metals are deformed and strengthened. When metals deform by dislocation motion, the more barriers the dislocations meet, the stronger the metal. Two simple types of dislocation are:  

Edge dislocation Screw dislocation

Edge dislocation Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line.

The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. This line runs along the top of the extra half-plane. The interatomic bonds are significantly distorted only in the immediate vicinity of the dislocation line.

Edge dislocation

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Understanding the movement of a dislocation is key to understanding why dislocations allow deformation to occur at much lower stress than in a perfect crystal. Dislocation motion is analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move its entire body at once. Instead it moves the rear portion of its body forward a small amount and creates a hump. The hump then moves forward and eventual moves all of the body forward by a small amount.

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They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction. Screw dislocation Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

Burgers Vector To describe the magnitude and the direction of the main lattice distortion(strain component of dislocation) caused by a dislocation, we use Burgers vector b. To find the Burgers vector, we should make a circuit from from atom to atom counting the same number of atomic distances in all directions. If the circuit encloses a dislocation it will not close. The vector that closes the loop is the Burgers vector b.

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In an ideal crystal

Dislocation Line: A dislocation line is the boundary between slip and no slip regions of a crystal Burgers vector: The magnitude and the direction of the slip is represented by a vector b called the Burgers vector, Line vector A unit vector t tangent to the dislocation line is called a tangent vector or the line vector.

Edge dislocation

 

An edge dislocation lies ┴ to its Burgers vector An edge dislocation moves (in its slip plane) in the direction of the Burgers vector (slip direction).

Screw Dislocation 

A screw dislocation lies  to its burgers vector.

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

A screw dislocation moves (in the slip plane) in a direction ┴ to the Burgers vector (slip direction)

ine

w re

Di

nL tio ca o l s t

Sc

b || t b







Find the Burgers vector of a screw dislocation.

Surface Defects Or Imperfections

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Grain Boundaries

Polycrystalline material comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called crystal or grain boundaries. Surfaces and interfaces are reactive and impurities tend to segregate there. Since energy is associated with interfaces, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures. High and Low Angle Grain Boundaries

Depending on misalignments of atomic planes between adjacent grains we can distinguish between the low and high angle grain boundaries. When the orientation difference between neighbouring grains is more than100- 140, boundaries are called high-Angle Grain Boundaries

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TILT AND TWIST GRAIN BOUNDARIES Low angle grain boundary is an array of aligned edge dislocations. This type of grain boundary is called tilt boundary (consider joint of two wedges) Twist boundary - the boundary region consisting of arrays of screw dislocations (consider joint of two halves of a cube and twist an angle around the cross section normal)

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TWIN BOUNDARIES

This gives rise to shape memory metals, which can recover their original shape if heated to a high temperature. Shape-memory alloys are twinned and when deformed they untwin. At high temperature the alloy returns back to the original twin configuration and restore the original shape.Twins may come into existence during the growth of the crystal or they may arise during deformation of materials.Twins formed during the process of recrystallisation are annealing twins; and those

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Sequence in an ideal fcc crystal may be described as ABCABC ABC….. But the stacking fault might change the sequence to ABCACABC…

BULK OR VOLUME DEFECTS

   

Pores - can greatly affect optical, thermal, mechanical properties Cracks - can greatly affect mechanical properties Foreign inclusions - can greatly affect electrical, mechanical, optical properties

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DIFFUSION IN SOLIDS

Diffusion is 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.

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INTERDIFFUSION AND SELF-DIFFUSION Interdiffusion (or impurity diffusion) occurs in response to a concentration gradient.

Self-diffusion is diffusion in one-component material,when all atoms that exchange positions are of the same type. DIFFUSION MECHANISMS Vacancy diffusion

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To jump from lattice site to lattice site, atoms need energy to break bonds with neighbors, and to cause the necessary lattice distortions during jump. This energy comes from the thermal energy of atomic vibrations. Materials flow (the atom) is opposite the vacancy flow direction. Interstitial diffusion

Interstitial diffusion is generally faster than vacancy diffusion because bonding of interstitials to the surrounding atoms is normally weaker and there are many more interstitial sites than vacancy sites to jump to. Requires small impurity atoms (e.g. C, H, O) to fit into interstices in host

Diffusion Flux The flux of diffusing atoms, J, is used to quantify how fast diffusion occurs. The flux is defined as either in number of atoms diffusing through unit area and per unit time (e.g., atoms/m2-second) or in terms of the mass flux - mass of atoms diffusing through unit area per unit time, (e.g., kg/m2-second). J = M / At (1/A) (dM/dt) (Kg m-2 s-1) where M is the mass of atoms diffusing through the area A during time t.

STEADY-STATE DIFFUSION Steady state diffusion: the diffusion flux does not change with time.

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Concentration profile: concentration of atoms/molecules of interest as function of position in the sample. Concentration gradient: dC/dx (Kg.m-3): the slope at a particular point on concentration profile.

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

The concentration gradient is often called the driving force in diffusion (but it is not a force in the mechanistic sense).The minus sign in the equation means that diffusion is down the concentration gradient

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Non steady-State Diffusion: Fick’s second law In most real situations the concentration profile and the concentration gradient are changing with time. The changes of the concentration profile is given in this case by a differential equation, Fick’s second law.

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Applications of diffusion

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Metallurgy and Material Science - Module 1

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