Materials Technology

What is Materials Technology?

• Metals, Plastics and Ceramics have completely different properties which means technology involved for production is also different.

• Evolving discipline with new materials leading to new applications.

Raw Materials

Processing of materials

Required shapes &

forms

Specific Applications

Metallic Non-Metallic

Ferrous Non-Ferrous

Steels Cast Iron

Plain Carbon

Alloy

GreyWhiteMalleableDuctile

AlluminiumCopperMagnesiumTin Zinc LeadNickel and their alloys

Organic Inorganic

PlasticsWood PaperRubberLeatherPetroleum Products

Minerals CementGlassCeramicsGraphite

Materials used in automobiles

Material Science

Why Mechanical properties

• Materials used in cryogenics where the working temperature is below -157 deg C

• Materials that are used at high temperatures such as blast furnace where the temperature is more than

1600 deg C

• In the manufacture of railway wheels, dies and punches, where the load delivered may be as high as

4500 tones.

• In the manufacture of valves and pipes used in chemical industries where the material can undergo

severe corrosion.

• In the design of concrete structures for civil structures

• In the design of space craft where the temperature in the nose region may be as high as 1400 deg C

when it re-enters the earth’s atmosphere.

Few Mechanical Properties

• Stress • Strain • Elasticity • Creep• Strength• Maelability• Toughness • Tensile

• Elongation• Ductile• Fracture• Tension• Flexural• Plasticity• Resiliance• Yield

ELASTIC AND PLASTIC BEHAVIOUR 

ELASTICITY

“Elasticity is the physical property of materials which return to their original shape after the stress that caused their

deformation is no longer applied.”(within elastic limit)

PLASTICTY

Is the property of a material where it undergoes permanent deformation

under the load.

• Elastic deformation: Small – Time dependant- Fully recoverable- obeys hookes law- occurs in metal within elastic limit

• Elastometric deformation: Very Large- Time dependant –Fully recoverable- does not obeys hookes law- occurs in elastomers.

• Plastic deformation or inelastic deformation: Large – Permanent – Time independent-obeys hookes law- occurs beyond plastic limits.

• Anelastic deformation: Small-Fully recoverable-time dependant-may or may not obey hooke’s law-occurs in rubber and plastics and metals due to thermoelastic phenomenon

• Viscoelastic deformation: Time dependant-partially elastic and partially permanent-obeys hookes law along with newtons law- occurs in polymers

Types of Deformation

ELASTICITY IN METALS

• It is convenient to express the elasticity of the material with the ratio of stress is to strain, a parameter also termed as young’s modulus.

• The stress and strain relationship is unique for each metal.

BRITTLE MATERIALS

• Brittle materials, which includes cast iron, glass, and stone, are characterized by the fact that rupture occurs without any noticeable prior change in the rate of elongation.

DUCTILE MATERIALS

• They include steel, copper, tungsten etc 

• The yield strength or yield point of a material is defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.

• Ultimate tensile strength is the maximum stress that a material can withstand while being stretched or pulled before necking.

• Necking is when large amount of strain is applied and there is a prominent decrease in the cross-sectional area, which provides the name “necking”.  

POLYMERS• A polymer is a large molecule (macromolecule) composed of repeating structural units

. These sub-units are typically connected by covalent chemical bonds.

 • Examples of polymers are plastic, rubber, proteins etc

 • Elastic properties of polymers differ from metals.

 • Their elastic moduli are very small when compared to those of metals

 • They endure large deformation without rupture and can still return to their original

shape.

• Their elastic moduli is increased with temperature.

MECHANISMS OF PLASTIC DEFORMATION

• This type of deformation is irreversible.• However, an object in the plastic deformation range will first have undergone elastic

deformation, which is reversible.• Thermoplastics have large plastic deformation when compared to brittle and ductile materials.  • Plastic deformation is characterized by a strain hardening region and a necking region and

finally, fracture (also called rupture). • During strain hardening the material becomes stronger through the movement of 

atomic dislocations.(dislocations are imperfections in crystal structure which increases as strain increases)

• There are two types of dislocations: edge and screw.• The modes of deformation are twinning and slip.

• Necking, in engineering or materials science, is a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material. The resulting prominent decrease in local cross-sectional area provides the basis for the name "neck".  

• This type of deformation is also irreversible. A break occurs after the material has reached the end of the elastic, and then plastic, deformation ranges. At this point forces accumulate until they are sufficient to cause a fracture. All materials will eventually fracture, if sufficient forces are applied.

Mechanism of elastic and plastic deformation

• TWINNING• Common in hcp and bcc

structures• Limited deformation

but help in plastic deformation in hcp and bcc crystals.

• Occurs on specific twinning planes and twinning directions

• SLIP• Dislocations move on a

certain crystallographic plane: slip plane

• Dislocations move in a certain crystallographic direction: slip direction

• The combination of slip direction and slip plane is called a slip system.

Yield stress for real crystals

• The stress level at which a metal or other material ceases to behave elastically. The stress divided by the strain is no longer constant. The point at which this occurs is known as the yield point.

• The initial elastic strain is caused by the simple stretching of bonds. Hooke's Law applies to this region.

• At the yield point, stage I begins. The crystal will extend considerably at almost constant stress. This is called easy glide, and is caused by slip on one slip system.

• The geometry of the crystal changes as slip proceeds.• In this stage of deformation, known as stage II, dislocations are gliding on two slip

systems, and they can interact.• Consequently, the crystal becomes more difficult to extend. This phenomenon is

called work hardening.• Stage III corresponds to extension at high stresses, where the applied force becomes

sufficient to overcome the obstacles, so the slope of the graph becomes progressively less steep. The work hardening saturates.

• Stage III ends with the failure of the crystal.

SHEAR STRENGTH FOR PERFECT CRYSTALS

• It is a term used to describe the strength of a material against structural failure(fracture), where the material or component fails due to deforming force( shear force).

• STRENGTH TERMS:-• Tensile strength or ultimate tensile strength is a limit state of tensile stress that leads to

tensile failure in the manner of ductile failure. or brittle failure. • Compressive strength is a limit state of compressive stress that leads to failure in the

manner of ductile failure or brittle failure. • Fatigue strength is a measure of the strength of a material or a component under cyclic

loading, and is usually more difficult to assess than the static strength measures. • Impact strength, is the capability of the material to withstand a suddenly applied load

and is expressed in terms of energy.

STRENGHTENING MECHANISMS

• Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials.

• • Work hardening (such as beating a red-hot piece of metal on anvil) has also been

used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

• • Q) What is strengthening?• • A) Plastic deformation occurs when large numbers of dislocations move and

multiply so as to result in macroscopic deformation. In other words, it is the movement of dislocations in the material which allows for deformation. If we want to enhance a material's mechanical properties (i.e. increase the yield and tensile strength), we simply need to introduce a mechanism which prohibits the mobility of these dislocations.

WORK HARDENING

• Work hardening is the result of many contributing factors.• The primary species responsible for work hardening are

dislocations. • Dislocations interact with each other by generating stress

fields in the material.• As a material is plastically deformed, dislocations move

extensively throughout the crystal, and in addition the dislocation density increases.

• The effect of this is to increase the number of entanglements - these are points where dislocations interact in such a way that their further motion is hindered.

SOLID SOLUTIONING

• Metals usually form homogenous liquid solutions in their liquid state.

• Even after to solid crystalline state, the metals retain their homogeneity and consequently their solubility, this is called solid solution.

• There are two types of solid solution:-1) Substitutional( (a)disordered and (b)ordered)2) Interstitial

SUBSTITUTIONAL SOLID SOLUTION

• In this there is a direct substitution of one type of atom for the another so that the solute atoms (Cu) enter the crystals to take the position normally occupied by solvent atoms.(Ni)

• In disordered substitutional solution the atoms do not occupy any paticular position and are disordered.

• In ordered solution ,the alloy is in disordered condition and if it is cooled slowly, it undergoes rearrangment of atoms due to diffusion that takes place due to cooling.

INTERSTITIAL SOLID SOLUTION• It is formed when solute

atoms are very small as compared to solvent atoms, they are unable to substitute solvent atoms(because of large difference in diameters) and can only fit into the interstices or spaces in the crystal lattice of the solvent atom.

Grain boundary strengthening• Is a method of strengthening materials by changing

their average crystallite (grain) size.• It is based on the observation that grain boundaries

impede dislocation movement and that the number of dislocations within a grain have an effect on how easily dislocations can traverse grain boundaries and travel from grain to grain.

• So, by changing grain size one can influence dislocation movement and yield strength.

• For example ,heat treatment after plastic For example ,heat treatment after plastic deformation and changing the rate of solidification are ways to alter grain size.[

deformation and changing the rate of solidification are ways to alter grain size.

GRAIN BOUNDARY STRENGTHENING

DISPERSION HARDENING• Dispersion hardening is a mean of strengthening a metal by

creating a fine dispersion of insoluble particles within the metal.• So metals containing finely dispersed particles are much stronger

than the pure metal matrix.• This effect depends on the size, shape, concentration and physical

characteristics of particles.• Dispersion hardened materials can be produced with the help of

powder metallurgy- a process in which powder(of materials) of required shape, size and distribution are mixed in desired proportions and then compacted and sintered at the appropriate temperature.

PARTICULATE STRENGTHENED SYSTEMS

• The difference between particulate and dispersion strengthened systems are in the size of dispersed particles and their volumetric concentration.

• In dispersion strengthening the particle size are small as compared to particulate strengthened systems

• Because of their size the particle can not interfere with dislocations and exhibits a strengthening effect by hydrostatically restraining the movement of the matrix close to it.

• Particulate composites sre made mainly by powder metallurgy techniques that may involve solid or liquid state sintering(atomic diffusions preferably at high temperatures) or even impregnation by molten metals

• Examples are Tungsten-nickel-iron system obtained as a liquid –sintered composite.

SUPERPLASTICITY• In materials science, superplasticity is a state in

which solid crystalline material is deformed well beyond its usual breaking point, usually over about 200% during tensile deformation.

• Such a process happens at a very high temperature.• Examples of superplastic materials are some fine-grained

metals and ceramics.• Other non-crystalline materials (amorphous) such as silica

glass ("molten glass") and polymers also deform similarly, but are not called superplastic, because they are not crystalline

• Superplastically deformed material gets thinner in a very uniform manner, rather than forming a "neck" (a local narrowing) which leads to fracture.

SUPERPLASTICITY OF NANOCRYSTALLINE COPPER AT ROOM TEMPERATURE

DEFORMATION OF NON-CRYSTALLINE MATERIALS

• In non-crystalline materials, permanent deformation is often related to localized slip and/or viscous flow (low stress or high temperature)

• Viscous flow is due to permanent displacement of atoms in different locations within the material.

• Glass transition temperature is an important factor to the deformation in non-crystalline material.

• The glass-liquid transition (or glass transition for short) is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state.