Dr. R. Asmatulu

Department of Mechanical Engineering Wichita State University

1845 Fairmount, Wichita, KS 67260-0133(316) 978-6368

ramazan.asmatulu@wichita.edu

October 19, 2013

(use slide show)

Kansas Science Olympiad TournamentMaterial Science (C)

http://webs.wichita.edu/?u=meweb&p=/FacultyandStaff/Asmatulu/asmatulu/

Materials Science

The materials science tetrahedron illustrates how a material's properties, processing, performance, and structure are interrelated.

Historical Perspective• Beginning of the Material Science - People began to make tools from stone –Start of the Stone Age about two million years ago. Natural materials: stone, wood, clay, skins, etc.

• The Stone Age ended about 5000 years ago with introduction of Bronze in the Far East. Bronze is an alloy (a metal made up of more than one element), copper + < 25% of tin + other elements. Bronze can be hammered or cast into a variety of shapes, can be made harder by alloying, corrode only slowly after a surface oxide film forms.

• The Iron Age began about 3000 years ago and continues today. Use of iron and steel, a stronger and cheaper material changed drastically daily life of a common person.

• The Age of Advanced Materials (?): Throughout the Iron Age, many new types of materials have been introduced (ceramics, semiconductors, polymers, composites…). Understanding of the relationship among structure, properties, processing, and performance of materials. Intelligent design of new materials.

Types of MaterialsMetals: Valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Strong, ductile, conduct electricity and heat well, are shiny if polished. Examples: Iron, silver, copper.

Semiconductors: Bonding is covalent (electrons are shared between atoms). Their electrical properties depend strongly on minute proportions of contaminants. Examples: Si, Ge, GaAs.

Ceramics: Atoms behave like either positive or negative ions, and are bound by Coulomb forces. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Hard, brittle, insulators. Examples: glass, porcelain.

Polymers: They are bound by covalent forces and also by weak van der Waals forces, and usually based on C and H. They decompose at moderate temperatures (100 – 400 C), and are lightweight. Examples: plastics, rubber.

Composites: Engineered materials, combine best characteristics of metals, ceramics, and polymer. Examples: fiber reinforced polymer composites.

Biomaterials

Hip joint is an orthopedic success story, enabling hundreds of thousands of people to live fuller, more active lives. Using metal alloys, high-grade plastics and polymeric materials, orthopedic surgeons can replace a painful, dysfunctional joint with a highly functional, long-lasting hip implant.

Materials PropertiesProperties are the way the material responds to the environment and external forces.

Mechanical properties – response to mechanical forces (strength, ductility, elastic modulus, hardness, etc.).

Electrical properties - response to electrical field (conductivity, resistivity, etc.)

Magnetic properties - response to magnetic fields (ferromagnetic, paramagnetic, etc.).

Thermal properties – response to transmission of heat and heat capacity (thermal conductivity, expansion, shrinkage, etc.).

Optical properties include absorption, transmission and scattering of light.

Chemical stability in contact with the environment (dissolution, degradation, corrosion resistance, etc.).

Future of Materials Science• Miniaturization: “Nanostructured" materials (Nanotechnology), with microstructure that has length scales between 1 and 100 nanometers with unusual properties. Electronic components, materials for quantum computing.

• Smart materials: airplane wings that deice themselves, buildings that stabilize themselves in earthquakes.

• Environment-friendly materials: biodegradable or photodegradable plastics, advances in nuclear waste processing, etc.

• Learning from Nature: shells and biological hard tissue can be as strong as the most advanced laboratory-produced ceramics, mollusces produce biocompatible adhesives that we do not know how to reproduce.

• Materials for lightweight batteries with high storage densities, for turbine blades that can operate at 2500°C, room-temperature superconductors, chemical sensors (artificial nose) of extremely high sensitivity, cotton shirts that never require ironing.

Mechanical Properties

A. Very ductile fracture, soft metals (e.g., Pb, Au, Cu) at room temperature, other metals, polymers, glasses at high temperature.

B. Moderately ductile fracture, typical for ductile metals

C. Brittle fracture, cold metals, ceramics, glass.

Edge, Screw, and Mixed Dislocations

Edge

Screw

Mixed

Most dislocations can exhibit both edge and screw characteristics. These are called mixed dislocations.

Linear Elastic Properties

σ = E ε

F

Fsimple tension test

In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke's law. For an accurate measurement, an extensometer is required.

E is Young's modulus or modulus of elasticity, σ is applied stress, and ε is strain.

∴ Stress has units:N/m2 or lbf/in2

Engineering Stress

Strength of Materials

• Metals: occurs when noticeable necking starts.• Polymers: occurs when polymer backbone chains are aligned and about to break.

• Tensile stress (TS) is the maximum stress on engineering stress-strain curve.• For structural applications, the yield stress (YS) is usually a more important property than the tensile strength, since once the yield stress has passed, the structure has deformed beyond acceptable limits.

Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties.

smaller toughness- unreinforced polymers

Engineering tensile strain, ε

Engineering tensile stress, σ

smaller toughness (ceramics)

larger toughness (metals, PMCs)

Fracture Toughness

Units: MPa√m

Areas under the curves equal to the absorbed energy during fracturing.

roll

AoAd

roll

• Rolling (Hot or Cold Rolling)(I-beams, rails, sheet & plate)

Ao Ad

force

dieblank

force

• Forging (Hammering; Stamping)(wrenches, crankshafts)

often atelev. T

Metal Fabrication Methods

ram billet

container

containerforce die holder

die

Ao

Adextrusion

• Extrusion (rods, tubing)

ductile metals, e.g. Cu, Al (hot)

tensile force

AoAddie

die

• Drawing (rods, wire, tubing)

die must be well lubricated & clean

FORMING CASTING JOINING

• Rare due to low packing density, and only polonium (Po) has this structure• Close-packed directions are cube edges.

• Coordination # = 6(# nearest neighbors)

(Courtesy P.M. Anderson)

Simple Cubic (SC) Structure

Coordination # = 8

Atoms are at center of cubic unit cell, and they touch each other along cube diagonals.

Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.

Body Centered Cubic Structure (BCC)

Ex: Cr, W, Fe (α), Ta, Mo

2 atoms/unit cell: 1 center + 8 corners x 1/8

Coordination # = 12

Atoms are located at each of the corners and on the centers of all the faces of cubic unit cell.

Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.

Face Centered Cubic Structure (FCC)

Ex: Al, Cu, Ni, Pb, Au, Pt, Ag

4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8

• APF for a simple cubic (SC) structure = 0.52 (52% atoms and 48% void).• APF for BCC is 0.68, while FCC is 0.74.

Atomic Packing Factor (APF)

Ionic bonding (by transferring electrons from one atom to another )

Covalent bonding (by sharing of pairs of electrons and atoms)

Metallic bonding is the bonding between atoms within metals (e.g., Cu-Cu, Fe-Fe, Ni-Ni or alloys).

Hydrogen bonding (weak) occurs when an atom of hydrogen is attracted (e.g., H2O).

van der Waals bonding (weak, everywhere).

Types of Bonding

Poisson's Ratio

–ν > 0.50 density increases

–ν < 0.50 density decreases (voids form)

εν = − L

ε

metals:  ν ~ 0.33ceramics: ν ~ 0.25polymers: ν ~ 0.40

Materials subject to tension shrink laterally. The ratio of lateral and axial strains is called the Poisson's ratio ν.

ν is dimensionless, and this sign shows that lateral strain is in opposite sense to longitudinal strain.

Theoretical value for isotropic material: 0.25Max value: 0.50, and typical value: 0.24 - 0.30

εL

ε

‐ν

Fatigue Limit

• Fatigue limit, Sfat: --no fatigue if S < Sfat

Sfat

case for steel (typ.)

N = Cycles to failure103 105 107 109

unsafe

safe

S = stress amplitude

• Sometimes, the fatigue limit is zero!

case forAl (typ.)

N = Cycles to failure103 105 107 109

unsafe

safe

S = stress amplitude

Fatigue limit (endurance limit) occurs for some materials (e.g., Fe and Ti allows). In this case, the S—N curve becomes horizontal at large N. The fatigue limit is a maximum stress amplitude below which the material never fails, no matter how large the number of cycles is.

In most alloys, S decreases continuously with N. In this case, the fatigue properties are described by:Fatigue strength: stress at which fracture occurs after specified number of cycles (e.g. 10^7).Fatigue life: Number of cycles to fail at specified stress level.

Creep Rate 1. Instantaneous deformation, mainly elastic.2. Primary/transient creep: Slope of strain vs. time decreases with time: work-hardening3. Secondary/steady-state creep: Rate of straining is constant - balance of work-hardening and recovery.4. Tertiary: Rapidly accelerating strain rate up to failure - formation of internal cracks, voids, grain boundary separation, necking, etc.

Constant load at elevated temperature.

Single Crystal

TEM image of a single-crystal Bi film, showing individual atoms. All the atoms in the crystal lattice system are well ordered/oriented.

A single crystal, also called monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries (one grain only). In other words, solids in which there is long-range atomic order are called crystalline solids.

Single crystal gold, Magnification: 600000x, 200 KV

Polycrystal (Multicrystal)

Polycrystalline material (also referred to as polymaterial) is formed of many small single crystals with various lattice orientations.

The opposite of a single crystal sample is a polycrystalline sample, which is made up of a number of smaller crystals known as crystallites, separated by grain boundaries.

Amorphous

Sometimes, atoms are randomly ordered in an amorphous structure. There are some mix atoms. For example, silicon nanocrystals embedded in an amorphous silicon film.

An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous ceramic, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids.

ViscosityViscosity is a measure of a non-crystalline (glass or liquid) material’s resistance to deformation. High-viscosity fluids resist flow; low-viscosity fluids flow easily. How readily a moving layer of fluid molecules drags adjacent layers of molecules along with it determines its viscosity.

•Units are Pa-s, or Poises (P)•1 P = 0.1 Pa-s

• Viscosity of water at room temp is ~ 10^-3 P • Viscosity of typical glass at room temp >> 10^16 P

• Viscosity, η:

- relates shear stress and velocity gradient:y

vdd

η=τ

velocity gradient

dvdy

t

tglass dv

dy

Surface Tension and Contact Angle

Contact angle (θ) can be measured by producing a drop of liquid on a solid.Surface tension (γ) is measured as the energy required to increase the surface area of a liquid by a unit of area

Microscopes

Examples of other powerful microscopes at nanoscale:

Atomic force microscope (AFM)Scanning electron microscope (SEM)Transmission electron microscope (TEM)Scanning tunneling microscope (STM) ….

A microscope is an optical instrument used to visualize objects that are not visible by a naked eye. Organic, inorganic and biological materials can be seen under the microscopes.

• Useful up to 2000X magnification.• Polishing removes surface features (e.g., scratches)• Etching changes reflectance, depending on crystal orientation.

Thank You

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Prepared by Dr. Asmatulu for educational purpose only