Material Science and Process

8/13/2019 Material Science and Process

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shape and si&e. ,i*e fracture$ flaws on surface etc. /

Microstructure : -t is observed with a magnification of 0 0 1 44. Opticalmicroscope is used for this purpose.

Substructure : -n this the structure is observed with a magnification of 0 144444 using aelectron microscope. -t provides information on crystal imperfections.

Crystal Structure : This structure tell about the atomic arrangement within the crystal. 0)ay and electron diffraction techni!ues are used for this study.

Electronic Structure : This deals with the study of electrons in the outermost shells ofindividual atoms. (pectroscopic techni!ues are used.

Nuclear Structure : -t is studied using 5uclear spectroscopic techni!ues.

Criteria For Selection of Materials :

The choice is made based upon ta*ing the following factors.1. (ervice ' There are of paramount importance and should have properties li*e

ade!uate strength$ corrosion resistance$ hardness and toughness.2. 6abrication ' This is also gaining importance. This includes the possibility to

shape a material and 7oin with other materials. These include ductility$Machinability$ hardenability$ weldability$ castability.

3. Economy.

Atomic Structure :

Atom :

-t is a electrical structure having a diameter of 1 # 14 14 metres or 1 "ngstrom. -thas two main parts. " heavier nucleus and electrons surrounding it. This nucleus ismade of protons and neutrons.

Protons :

-t is positively charged and 1839 times heavier as electron. -t e#ists with theneutrons in the nucleus of atom.

Neutron :

-t is 1.448 times heavier than proton. -t has no electric charge. 6or large atoms$the ratio of Mass to 5eutron to Mass of roton is greater than 1.448.

Electron :


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(urrounds the nucleus and at a greater distant from the nucleus. -ts mass is 1:1839of proton. -t is negatively charged. with a magnitude e!ual to the charge of

protons. Electrons in the outermost orbit are called as valence electrons which determinemany of the properties of materials.

Atomic Number :

5umber of protons or number of electrons.

Mass Number :

(um of protons and 5eutrons.

Atomic Weight :

;eight of a atom of element in comparison with weight of a atom of o#ygen ta*en

as 19. Isotopes :

(ame elements having different number of neutrons.

Bon in! of soli s :

The arrangement of atoms in a solid element is determined by the character$strength of chemical bonds or cohesive forces. The bonds may be attractive or repulsive$which hold the atoms at a particular spacing and which 7ust balances the opposite forces.There are two types of Chemical bonds. They are primary bonds and secondary bonds.

rimary bonds are interatomic bonds$ which include -onic bonds$ Covalent bonds$and metallic bonds.

(econdary bonds are intermolecular bonds and include


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3. ,ine defects Edge and screw dislocations /=. (urface defects and. C ? 2

6 +egrees of freedom. C 5umber of components at e!uilibrium. 5umber of phases that can co e#ist at e!uilibrium.

Mec#anical Properties an testin! :

(ince a number of properties are best evaluated by testing under variousconditions$ mechanical testing are carried out to provide useful data to a designer.%owever certain assumptions are made about the materials. The materials arecontinuous$ homogenous and isotropic.

Continuous 5o voids or space. %omogenous -dentical properties at all points.


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-sotropic ;ith respect to some property. That property does not vary withdirection.

"nisotropy " body where the property varies in different directions.

Stress :

;hen a material is sub7ected to a load$ it does not deform instantaneously$ butincreases steadily till it stops. +uring the process of deformation$ the material e#ertscontinuously increases it resistance to the load. The moment the deformation stops$ the

body is in state of e!uilibrium.

"pplied load > -nternal resistance of the body.

@oth are e!ual and opposite indirection. The sum total of interatomic forces that prevails in the body to counteract the e#ternally applied load is called stress and theresultant deformation is e#pressed a fraction change in dimension called as strain.

True Stress :

-n this instead of ta*ing the original area into account for the calculation of stress$that area at any instant on applying the load it ta*en into account.

Different Mec#anical properties of materials :

Strength :

-t is the capacity of the materials to withstand load without destruction$ under the

action of e#ternal load. -t is the ability of the material to with stand stress withoutfailure. This strength varies according to the type of loading ;hether$ tensile load orcompressive load or shear load /. Materials with covalent bond are the strongest. Thencomes -onic bond$ Metallic and Molecular bond.

Stiffness :

-t is the resistance of the material to elastic deformation. " material having onlyslight deformation has a high amount of stiffness.

le!ibility :

This is opposite to stiffness. -t is related to bending.

"esilience :

-t is the capacity of body to absorb energy elastically$ and return it whenunloaded. The ma#imum energy that can be stored upto elastic limits is called as proofresilience. This property is associated with high elastic limits. Materials with high


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resilience is used tin springs.

Modulus or resilience > roof resilience :


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84B 4B of total machine failure is because of fatigue. The term fatigue is usedto describe the failure of the material under repeated stress. The stress necessary to causefailure when it is applied a large number of times is much below the actual brea*ingstrength. Thus fatigue deals with cyclic loading in which the ma#imum stress applied :cycle is within the elastic limits. -f failure occurs$ the material has poor fatigue strength.

Mechanism ' This fatigue begins at irregularities at the surface or at points of high stressor stress concentration. 6racture so formed is brittle even in a ductile material.

atigue stress :

The stress at which the material fails because of fatigue is called as fatigue stress.6or most materials$ there is a limiting stress within which it can be applied for aindefinitely large number of times without causing failure. This is called as endurancelimit or fatigue limit. The presence of stress concentrators reduce the endurance limit.

"s tensile strength increase$ the endurance limit increases. "s temperature decreases below the ambient temperature$ the endurance limitincreases.

To resist fatigue failure there should be good surface finish$ 5o stress raisers andControl of corrosion and erosion.

Creep :

" material is sub7ect to constant tensile load at an elevated temperature will creepand undergo a time dependent deformation. This slow and progressive deformation of

the material under constant stress is called as creep. This creep continues until sufficientstrain has occurred in nec*ing down and reducing the cross sectional area$ and finally thematerial ruptures. Creep occurs at stress below the elastic limits.

"t low temperatures$ the creep rate usually decreases with time and logarithmiccreep curve is obtained.

"t high temperatures$ T > 4. 4. Tm / the creep rate does not decreasegradually. This is due to mechanical recovery.

"t very high temperatures T D 4 . Tm / the creep is primarily due to diffusionand stress applied has little effect.

&iel Point :

This is the hori&ontal portion of stress strain curve. -t is the point$ where thematerial yields without any increase in load. ield strength is defined as that stress atwhich there is a great increase in strain$ without the corresponding increase in stress. 6ormaterials which does not have a clear cut yield point$ it is determined by offset test.

Tensile stren!t# :


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@eyond the yield point the load can again be increased to a minimum value$ whena nec*ing down occurs and there is a reduction in cross sectional area. This load is calledas tensile load.

'ar ness Test :

1. @rinell %ardness ' -t is used for iron and steel. The different combination of thetest are 14 mm ball with a applied load of 44 Fg or 3444 Fg. "nd a mm ballwith a applied load of 4 Fg.

2. )oc*well hardness.3.

Material Science and Process

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