MATERIAL SCIENCE AND PROCESS

            This is a branch of science that investigates the relationship between structure of materials and their properties.  Engineering material are classified into the following three types.

  1. Metals and alloys
  2. Ceramics and
  3. Organic Polymers.
Classification of Materials:

Metals: are nothing but elemental substance.  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). Examples: aluminum, steel, brass, gold.

Alloys: are obtained by melting two or more relatively pure metals to form a new metal.  They alloys have quite different properties in comparison to the other two materials used for its manufacture.

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: contains two phases.  A phase is a physically separable and is a homogenous constituent.  The phases may be metallic or non metallic.  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). Examples: glass, porcelain, many minerals.

Organic Materials: are derived from carbon combined with oxygen, Hydrogen etc.  Their structure is fairly complex. Plastics and rubber are the organic engineering materials.  Also called as polymers because of the polymerization process.  Polymerization is the process in which two or more simple molecules are chemically combined to form a massive long chain molecules.

            Other categories are not based on bonding. A particular microstructure identifies 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.

Modern Material's Needs:

Properties of Materials:

Mechanical Properties : Strength, Stiffness, Elasticity, plasticity, ductility, malleability, hardness and brittleness.
Electrical Properties : Conductivity and Resistivity.
Magnetic Properties : Coercive forces and Hysterisys.
Thermal Properties : Conductivity, specific heat, thermal expansion.
Chemical Properties : Corrosion resistance, acidity and alkalinity.
Physical Properties : Dimension and density.
Acoustic Properties : Sound transmission and Reflection.
Optical Properties : Light transmission and light reflection.

Material Structure:

            Depending on the level of magnification the structure of material is classified as follows.

Macrostructure: It is the structure of the material, as seen by the naked eye.  It deals with shape and size. ( Like fracture, flaws on surface etc. )

Microstructure: It is observed with a magnification of X 75 - X 1500.  Optical microscope is used for this purpose.

Substructure: In this the structure is observed with a magnification of X 100000 using a electron microscope.  It provides information on crystal imperfections.

Crystal Structure: This structure tell about the atomic arrangement within the crystal.  X-Ray and electron diffraction techniques are used for this study.

Electronic Structure: This deals with the study of electrons in the outermost shells of individual atoms.  Spectroscopic techniques are used.

Nuclear Structure:  It is studied using Nuclear spectroscopic techniques.

Criteria For Selection of Materials:

           The choice is made based upon taking the following factors.

  1. Service :  There are of paramount importance and should have properties like adequate strength, corrosion resistance, hardness and toughness.
  2. Fabrication : This is also gaining importance.  This includes the possibility to shape a material and join with other materials. These include ductility, Machinability, hardenability, weldability, castability.
  3. Economy.
Atomic Structure:

Atom:

           It is a electrical structure having a diameter of 1 x 10-10 metres or 1 Angstrom.  It has two main parts.  A heavier  nucleus and electrons surrounding it. This nucleus is made of protons and neutrons.

Protons:

           It is positively charged and 1836 times heavier as electron.  It exists with the neutrons in the nucleus of atom.

Neutron:

           It is 1.008 times heavier than proton. It has no electric charge.  For large atoms, the ratio of Mass to Neutron to Mass of Proton is greater than 1.008.

Electron:

           Surrounds the nucleus and at a greater distant from the nucleus.  Its mass is 1/1836 of proton. It is negatively charged.  with a magnitude equal to the charge of protons. Electrons in the outermost orbit are called as valence electrons which determine many of the properties of materials.  Electron and protons are negative and positive charges of the same magnitude, 1.6 × 10-19 Coulombs.

Atomic Number:

           Number of protons or number of electrons.

Mass Number:

           Sum of protons and Neutrons.

Atomic Weight:

           Weight of a atom of element in comparison with weight of a atom of oxygen taken as 16.

Isotopes:

           Same elements having different number of neutrons.

Bonding of solids:

           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 just balances the opposite forces.  There are two types of Chemical bonds.  They are primary bonds and secondary bonds.

Primary Interatomic Bonds:

1. Ionic Bonding:

            This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl- and less around Na, forming Na+. Ionic bonds are the strongest bonds. In real solids, ionic bonding is usually combined with covalent bonding. In this case, the fractional ionic bonding is defined as % ionic = 100 × [1 – exp(-0.25 (XA – XB)2], where XA and XB are the electronegativities of the two atoms, A and B, forming the molecule.

2. Covalent Bonding:

            In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding.

3. Metallic Bonding:

            In metals, the atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place, in a similar way that the electrons in between the H atoms in the H2 molecule bind the protons.

Secondary Bonding (Van der Waals):

1. Fluctuating Induced Dipole Bonds:

            Since the electrons may be on one side of the atom or the other, a dipole is formed: the + nucleus at the center, and the electron outside. Since the electron moves, the dipole fluctuates. This fluctuation in atom A produces a fluctuating electric field that is felt by the electrons of an adjacent atom, B. Atom B then polarizes so that its outer electrons are on the side of the atom closest to the + side (or opposite to the – side) of the dipole in A. This bond is called van der Waals bonding.

2. Polar Molecule-Induced Dipole Bonds:

            A polar molecule like H2O (Hs are partially +, O is partially – ), will induce a dipole in a nearby atom, leading to bonding.

3. Permanent Dipole Bonds:

            This is the case of the hydrogen bond in ice. The H end of the molecule is positively charged and can bond to the negative side of another dipolar molecule, like the O side of the H2O dipole.

Crystals:

           These are solids in which the atoms are arranged in some regular repetitive pattern in three dimension.  This arrangement is called as crystal structure.

Crystal Imperfections:

           No crystal structure is perfect.  It is associated with imperfections, which is often helpful for understanding the properties of crystals.  The following are the crystal defects.

  1. Thermal Vibration.
  2. Point defects ( Vacancies, Interstitial cies and electron defects )
  3. Line defects ( Edge and screw dislocations )
  4. Surface defects and
  5. Volume defects.
Solid Study:

           There are two approaches.  They are structural approach ( electrons, valence bonds ) and Compositional approach ( Deals with the phase of materials ).  This compositional approach is used to know the state and condition of solid and to change the condition from thermodynamic point of view.

System:

           it is a substance or group of substance unaffected by the surroundings.  It is subjected to change in composition, temperature, pressure or total volume only to the extend allowed by the person investigating it.  The system may be composed of solid, liquid or gas or a composition of all three.

State:

           Of a system is a physical condition defined by quantities.  E.g. Length and angles define the state of triangles.

Phase:

           It is physically and chemically homogenous.  Homogenous in the sense that the smallest adjacent part is indistinguishable form the other.  Each phase has its own physical and chemical properties.

Gibbs Rule:

           F = C - P + 2

           F - Degrees of freedom.
           C - Number of components at equilibrium.
           P - Number of phases that can co exist at equilibrium.

Mechanical Properties and testing:

           Since a number of properties are best evaluated by testing under various conditions, mechanical testing are carried out to provide useful data to a designer.  However certain assumptions are made about the materials.  The materials are continuous, homogenous and isotropic. Stress:

           When a material is subjected to a load, it does not deform instantaneously, but increases steadily till it stops. During the process of deformation, the material exerts continuously increases it resistance to the load.  The moment the deformation stops, the body is in state of equilibrium.

               Applied load = Internal resistance of the body.

           Both are equal and opposite indirection.  The sum total of interatomic forces that prevails in the body to counteract the externally applied load is called stress and the resultant deformation is expressed a fraction change in dimension called as strain.

True Stress:

           In this instead of taking the original area into account for the calculation of stress, that area at any instant on applying the load it taken into account.

Different Mechanical properties of materials:

Strength:

           It is the capacity of the materials to withstand load without destruction, under the action of external load.  It is the ability of the material to with stand stress without failure.  This strength varies according to the type of loading ( Whether, tensile load or compressive load or shear load ).  Materials with covalent bond are the strongest.  Then comes Ionic bond, Metallic and Molecular bond.

Stiffness:

           It is the resistance of the material to elastic deformation.  A material having only slight deformation has a high amount of stiffness.

Flexibility:

           This is opposite to stiffness.  It is related to bending.

Resilience:

           It is the capacity of body to absorb energy elastically, and return it when unloaded.  The maximum energy that can be stored upto elastic limits is called as proof resilience.  This property is associated with high elastic limits.  Materials with high resilience is used tin springs.

               Modulus or resilience = Proof resilience / Volume.

Plasticity:

           It is the property of the material to undergo permanent deformation without rupture.  Plastic deformation occurs beyond the elastic limits.  Plasticity increases with increase in temperature.

Ductility:

           It is a measure of tensile property.  It enables a material to be easily drawn to wires.  Percentage increase in elongation and percentage reduction in are the two measures used. Rivets are made of ductile material.

Machinability:

            It is the ease with which the metals could be removed from operation like turning, drilling etc.

Malleability:

           It is a measure of compressive property. It is the ability of material to be flattened into sheets without cracking by rolling and hammering.

Toughness:

           It is the ability of material to withstand both elastic and plastic deformation ( is the ability to withstand high deformations and high stress without fracture. )  It is the amount of energy that it could absorb before rupture.  It is not possible to measure toughness but it is the area under the Stress-Strain curve.  There is a difference between ductility and toughness.  Ductility deals with only deformation.

Hardenability:

           Indicates the degree of hardness that could be imparted to particular steel, by the process of hardening is connected with the transformation of characteristic of steel.

Brittleness:

           It is the property of breaking of a material without much permanent deformation ( Glass ),  Tensile stress of a brittle material is only a fraction of their compressive stress.

Fatigue:

           80% - 90% of total machine failure is because of fatigue.  The term fatigue is used to describe the failure of the material under repeated stress.  The stress necessary to cause failure when it is applied a large number of times is much below the actual breaking strength.  Thus fatigue deals with cyclic loading in which the maximum stress applied / cycle is within the elastic limits.  If failure occurs, the material has poor fatigue strength.

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

Fatigue stress:

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

           To resist fatigue failure there should be good surface finish, No stress raisers and control of corrosion and erosion.

Creep:

           A material is subject to constant tensile load at an elevated temperature will creep and undergo a time dependent deformation.  This slow and progressive deformation of the material under constant stress is called as creep.  This creep continues until sufficient strain has occurred in necking down and reducing the cross sectional area, and finally the material ruptures.  Creep occurs at stress below the elastic limits. Yield Point:

           This is the horizontal portion of stress-strain curve.  It is the point, where the material yields without any increase in load.  Yield strength is defined as that stress at which there is a great increase in strain, without the corresponding increase in stress.  For materials which does not have a clear cut yield point, it is determined by offset test.

Tensile strength:

           Beyond the yield point the load can again be increased to a minimum value, when a necking down occurs and there is a reduction in cross sectional area.  This load is called as tensile load.

Anelastic:

           Refers to stress and time dependent of elastic strain.  Fully recoverable by time dependent deformation is called anelatic deformation.  Upon removal of load, the material does not regain it shape instantaneously.  This asymptotic approach to reach the equilibrium value is called as elastic after effect.  This can be understood, by understanding the concept of relaxation time.

Elastic after effect:

            At t=0 a stress is applied which is followed by a instantaneous strain, which again is followed by delayed strain in time 't' which asymptotically attains a final value.  When loading is removed the strain decreases by the same amount by which it increase while loading.  The mechanism which produces elastic after effect is internal fricition

Plastic Deformation:

           It is a function of stress, temperature and rate of straining.

Fracture:

           It is the failure caused by stress, separating the material into two or more pieces.  Following are the different modes of failure

  1. Yielding
  2. Fracture
  3. Deflection
  4. Wear
  5. Corrosion and
  6. Caustic embrittlement

Types of fracture:

            There are two types of fracture.  They are ductile and brittle fracture.  Following are the differences between brittle and ductile fracture

  Ductile Brittle
deformation extensive little
track propagation slow, needs stress fast
type of materials most metals (not too cold) ceramics, ice, cold metals
warning permanent elongation none
strain energy higher lower
fractured surface rough smoother
necking yes no

1. Ductile Fracture

Stages of ductile fracture

The interior surface is fibrous, irregular, which signify plastic deformation.

2. Brittle Fracture

            There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (or faceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular.

3. Brittle Fracture of Ceramics

            The brittle fracture of ceramics limits applications.  It occurs due to the unavoidable presence of microscopic flaws (micro-cracks, internal pores, and atmospheric contaminants) that result during cooling from the melt.  The flaws need to crack formation, and crack propagation (perpendicular to the applied stress) is usually transgranular, along cleavage planes. The flaws cannot be closely controlled in manufacturing; this leads to a large variability (scatter) in the fracture strength of ceramic materials. 

            The compressive strength is typically ten times the tensile strength.  This makes ceramics good structural materials under compression (e.g., bricks in houses, stone blocks in the pyramids), but not in conditions of tensile stress, such as under flexure. 

            Plastic deformation in crystalline ceramics is by slip, which is difficult due to the structure and the strong local (electrostatic) potentials.  There is very little plastic deformation before fracture. Non-crystalline ceramics, like common glass deform by viscous flow (like very high-density liquids).  Viscosity decreases strongly with increases temperature. 

            Following are the different theories of failure.

  1. Maximum principle stress theory - Rankine theory
  2. Maximum shear shtress theory - Coloums theory
  3. Maximum strain energy theory - venants theory
  4. Maximum strain theory - Haigh theory
  5. Distortion energy theory - von misses theory.

DIFFERENT HARDENING MECHANISMS

Solid solution hardening:

            This is the common way to increase the hardness and yield strength and particularly its straining rate.

Dispersion Hardening:

            This means a strengthening a metal by creating a fine dispersion of insoluble particles o a second phase within the metal.  The insoluble particles may be slag inclusions, inter metallic compound formed between the alloying elements and any other impurity atoms.  The presence of finely distributed hard particles obstruct the flow pattern of the stress deformation and causes rapid hardening.  The effect depends upon the size, shape, concentration and physical characteristics.

Age hardening:

            This is the phenomenon observed in many non-ferrous alloys like Al, Si, Mg alloys whereby the hardness of the material increase with time.  The essential requirement for precipitation to occur in solution is the decreasing solubility of a solute with decreasing temperatures.  This results in super saturated solid solution that being unstable tends to decompose according to the relation.

Super saturated a solid solution = saturated solution a +  b  precipitation.

            Age hardening involves the following mentioned stages

Heating: The alloy is first solutionized by heating into a single phase reaction, held there long enough to dissolve all existing soluble precipitate particles.

Quenching: After solutionizing, the alloy is rapidly quenched into the two phase reaction region.  The rapidity of the quench prevents the formation of equilibrium precipitates and thus produces the supersaturated solid solution.  The quenching medium is usually water.

Aging: On aging at or above room temperature, fine scale transition structures as small as  100 Angstrom is formed.

Strain hardening:

            In most of the metals and alloys it is observed that the yield strength of the material increases after the material undergoes plastic deformation from the stress-strain curve shown.  Strain hardening or work hardening is the phenomenon which results in an increase in hardness and strength of a metal subjected to plastic deformation at temperatures lower than the re-crystallisation range.  Strain hardening however reduces ductility and plasticity.

            An important characteristic of plastic deformation of metals is that the shear stress required to produce slip continuously increases with shear strain.  When the metal is loaded, the strain increases with stress and the curve reaches a point A in the plastic region.  If at this stage, the specimen is unloaded, the strain does not  recover along the original part AO, but moves along AB.  If the specimen is reloaded immediately the curve again rises from B to A, and reaches the point C, after which it still follows the curvature, if loading is continued.  IF the specimen would not have been unloaded, after point A, the stress-strain curve would have followed the dotted path AD'.

            The figure shows the stress strain curve of FCC crystal.  There are three regions of hardening and are experimentally distinguishable.  The forest dislocation theory stages that when a material is stressed the dislocation starts moving which results in plastic deformation.  Even as the stress increase the number of dislocations present in the body increase exponentially by frank reed source mechanism.  The movement of a large number of dislocation along different slip lanes creates a traffic jam like situation and there by making it difficult for any movement of dislocation.  Therefore further plastic deformation requires more stress or more load.  

            Stage I or the easy glide region, immediately follows the yield point and is characterized by little strain hardening undergone by the crystal.  During easy glide the dislocation are able to move over relatively large distances without encountering barriers.

            Stage II region marks a rapid increase in work hardening, the slope of which is approximately independent of applied stress, temperature, orientation alloy content.  In this region slip occurs on both primary and secondary slip systems.  As a result, several new lattice irregularities may be formed which will include.

            There are three theories that explain the hardening mechanism at this stage.  They are pile-up theory, forest theory and jog theory.  The pile up theory states that some of the dislocations give out by the frank reed sources are eventually stopped at barriers, according to this theory, the hardening is principally due to long range internal stresses from piled up groups interacting with guide dislocations.

            Stage III is the region of decreasing rate of strain hardening.  At the sufficiently high stress value or temperature in region 3, the dislocations help up in stage 2 are able to move by a process that had been suppressed at lower stresses and temperatures.  In this mechanism, dislocations can by-pass the obstacles in their guide plane and do not have to interact strongly with them.  For this reason, this stage exhibits a lower rate of work hardening.

Grain boundary hardening:

            It is a relevant fact that the dislocations are obstructed by the grain boundaries during plastic deformation of the material.  This is basically due to the disordered at grain boundaries, that is in the grain boundary the atoms are not arranged in any particular fashion by arranged randomly.  It requires large amount of force for the dislocations to travel through the disordered structure, than along the slip planes.  Transmission electron microscope picture have revealed that dislocations get piled up like grain boundary as the deformation process at this stage the stress concentration near the grain  boundary must be sufficient to nucleate slip in the next grain.  In a material with fine grains the area of grain boundary within gives a volume that is going to be very high compared to materials with large grains.  So the materials with fine grain will have higher strain.  This effect is called grain boundary strengthening or hardening.

Last updated on December 14, 2003 , 05:55 PM