planar defects: are interfaces between homogeneous regions of a material, and include:

grain boundaries, stacking faults external surfaces.

Planar Defects in Solids - Twinning

•A shear force that causes atomic displacements such that the atoms on one side of a plane (twin boundary) mirror the atoms on the other side. A reflection of atom positions across the twin plane.

•Displacement magnitude in the twin region is proportional to the atom‟s distance from the twin plane.

•Takes place along defined planes and directions depending upon the system.

•Ex: BCC twinning occurs on the (112)[111] system

Bulk Defects

•Pores (esp. ceramics) - can greatly affect optical, thermal, and mechanical properties

•Cracks - can greatly affect mechanical properties

•Foreign inclusions - can greatly affect electrical, mechanical, and optical properties In summary

• Point, Line, and Area defects arise in solids.

• The number and type of defects can be varied and controlled (e.g., Temperature controls vacancy conc.)

• Defects affect material properties (e.g., grain boundaries control crystal slip).

• Defects may be desirable or undesirable (e.g., dislocations may be good or bad, depending on whether plastic deformation is desirable or not.)

Strengthening Mechanism in Metals

• Plastic deformation - ability of dislocations to move

• Restricting or hindering dislocation motion renders metal harder and stronger

• Strengthening by grain size reduction

• Solid solution hardening

• Strain hardening

• Recovery, Recrystallisation and Grain Growth Mechanical properties of metals and alloys

Strength:- grain refinement, work hardening, texture strengthening, solid solution hardening, solute pinning, precipitation hardening, ordered structures, two – phase strengthening, intermetallic hardening, oxide dispersion strengthening.

Toughness:- grain refinement, dislocation motion, control of particles, stress distribution and concentrators, composites.

Creep:- large grain size, solid solution hardening, coherent precipitates, pinned grain boundaries Fatigue:- small grain size, strengthened surface (ideally in compression – use of shot peening), tough interior

Physical properties of metals and alloys include

Conductivity:- electrical (and resistance increases with temperature), thermal, Magnetic:- soft magnets, hard (permanent) magnets,

Density:- high and three major crystal classes with long range atomic ordering, Melting temperature:- huge range from mercury (-39°C) to tungsten (3400°C).

The properties of transition metals strongly depend on the number of d electrons and the strength of the d-orbital interactions, for cohesive energy, bulk modulus and melting temperature.

Physical properties

• Types of forces (Tensile, compressive, shear, torsion)

• Concepts of Stress and Strain

• Relationship between applied load and deformation

• Tensile, Compressive, Shear stress and strain

• Tension, compressive, shear and torsional tests

• Stress/Strain behaviour

Mechanical properties (Modulus of Elasticity, Hooke‟s Law, Modulus of Rigidity or Shear Modulus, Limit of Proportionality, Elastic and Plastic deformation, Tensile Strength, Proof Stress, Permanent set, Necking, Ductility, Brittleness, Toughness, Poisson Ratio, Yielding and Yield Strength, Resilience, Stiffness, True Stress and True Strain etc.

Types of Materials

Let us classify materials according to the way the atoms are bound together.

Metals: 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.

Semiconductors: the 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: are bound by covalent forces and also by weak van der Waals forces, and usually

Understand the meaning of the following keywords:

Allotropy Amorphous Anisotropy

Atomic packing factor (APF) Body-centered cubic (BCC) Coordination number Crystal structure Crystalline

Face-centered cubic (FCC) Grain

Grain boundary

Hexagonal close-packed (HCP) Isotropic

Lattice parameter

Non-crystalline Polycrystalline Single crystal Unit cell.

Best of Luck in your Test.

Classification of Materials

Like many other things, materials are classified in groups, so that our brain can handle the complexity. One can classify them based on many criteria, for example crystal structure (arrangement of atoms and bonds between them), or properties, or use. Metals, Ceramics, Polymers, Composites, Semiconductors, and Biomaterials constitute the main classes of present engineering materials.

Metals: These materials are characterized by high thermal and electrical conductivity; strong yet deformable under applied mechanical loads; opaque to light (shiny if polished). These characteristics are due to valence electrons that are detached from atoms, and spread in an electron sea that glues the ions together, i.e. atoms are bound together by metallic bonds and weaker van der Waalls forces. Pure metals are not good enough for many applications, especially structural applications. Thus metals are used in alloy form i.e. a metal mixed with another metal to improve the desired qualities. E.g.: aluminum, steel, brass, gold.

Ceramics: These are inorganic compounds, and usually made either of oxides, carbides, nitrides, or silicates of metals. Ceramics are typically partly crystalline and partly amorphous. Atoms (ions often) in ceramic materials behave mostly like either positive or negative ions, and are bound by very strong Coulomb forces between them. These materials are characterized by very high strength under compression, low ductility; usually insulators to heat and electricity.

Examples: glass, porcelain, many minerals.

Polymers: Polymers in the form of thermo-plastics (nylon, polyethylene, polyvinyl chloride, rubber, etc.) consist of molecules that have covalent bonding within each molecule and van der Waals forces between them. Polymers in the form of thermo-sets (e.g., epoxy, phenolics, etc.) consist of a network of covalent bonds. They are based on H, C and other non-metallic elements.

Polymers are amorphous, except for a minority of thermoplastics. Due to the kind of bonding, polymers are typically electrical and thermal insulators. However, conducting polymers can be obtained by doping, and conducting polymer-matrix composites can be obtained by the use of conducting fillers. They decompose at moderate temperatures (100 – 400 C), and are lightweight.

Other properties vary greatly.

Composite materials: Composite materials are multiphase materials obtained by artificial combination of different materials to attain properties that the individual components cannot attain. An example is a lightweight brake disc obtained by embedding SiC particles in Al-alloy matrix. Another example is reinforced cement concrete, a structural composite obtained by combining cement (the matrix, i.e., the binder, obtained by a reaction known as hydration, between cement and water), sand (fine aggregate), gravel (coarse aggregate), and, thick steel fibers. However, there are some natural composites available in nature, for example – wood. In general, composites are classified according to their matrix materials. The main classes of composites are metal-matrix, polymer-matrix, and ceramic-matrix.

Semiconductors: Semiconductors are covalent in nature. Their atomic structure is characterized by the highest occupied energy band (the valence band, where the valence electrons reside energetically) full such that the energy gap between the top of the valence band and the bottom of the empty energy band (the conduction band) is small enough for some fraction of the valence electrons to be excited from the valence band to the conduction band by thermal, optical, or other

forms of energy. Their electrical properties depend extremely strongly on minute proportions of contaminants. They are usually doped in order to enhance electrical conductivity. They are used in the form of single crystals without dislocations because grain boundaries and dislocations would degrade electrical behavior. They are opaque to visible light but transparent to the infrared. Examples: silicon (Si), germanium (Ge), and gallium arsenide (GaAs, a compound semiconductor).

Biomaterials: These are any type material that can be used for replacement of damaged or diseased human body parts. Primary requirement of these materials is that they must be biocompatible with body tissues, and must not produce toxic substances. Other important material factors are: ability to support forces; low friction, wear, density, and cost;

reproducibility. Typical applications involve heart valves, hip joints, dental implants, intraocular lenses. Examples: Stainless steel, Co-28Cr-6Mo, Ti-6Al-4V, ultra high molecular weight poly-ethelene, high purity dense Al-oxide, etc.

Advanced Materials, Future Materials, and Modern Materials needs

Advanced Materials

These are materials used in High-Tech devices those operate based on relatively intricate and sophisticated principles (e.g. computers, air/space-crafts, electronic gadgets, etc.). These materials are either traditional materials with enhanced properties or newly developed materials with high-performance capabilities. Hence these are relatively expensive. Typical applications:

integrated circuits, lasers, LCDs, fiber optics, thermal protection for space shuttle, etc. Examples:

Metallic foams, inter-metallic compounds, multi-component alloys, magnetic alloys, special ceramics and high temperature materials, etc.

Future Materials

Group of new and state-of-the-art materials now being developed, and expected to have significant influence on present-day technologies, especially in the fields of medicine, manufacturing and defense. Smart/Intelligent material system consists some type of sensor (detects an input) and an actuator (performs responsive and adaptive function). Actuators may be called upon to change shape, position, natural frequency, mechanical characteristics in response to changes in temperature, electric/magnetic fields, moisture, pH, etc.

Four types of materials used as actuators: Shape memory alloys, Piezo-electric ceramics, Magnetostrictive materials, Electro-/Magneto-rheological fluids. Materials / Devices used as sensors: Optical fibers, Piezo-electric materials, Micro-electro-mechanical systems (MEMS), etc.

Typical applications: By incorporating sensors, actuators and chip processors into system, researchers are able to stimulate biological human-like behavior; Fibers for bridges, buildings, and wood utility poles; They also help in fast moving and accurate robot parts, high speed helicopter rotor blades; Actuators that control chatter in precision machine tools; Small microelectronic circuits in machines ranging from computers to photolithography prints; Health monitoring detecting the success or failure of a product.

Mechanical Properties of Metals

Most of the materials used in engineering are metallic in nature. The main reason is because of the versatile nature of their properties those spread over a very broad range compared with other kinds of materials. Many engineering materials are subjected to forces both during processing/fabrication and in service. When a force is applied on a solid material, it may result in translation, rotation, or deformation of that material. Aspects of material translation and rotation are dealt by engineering dynamics. We restrict ourselves here to the subject of material deformation under forces. Deformation constitutes both change in shape, distortion, and change in size/volume, dilatation. Solid material are defined such that change in their volume under applied forces is very small, thus deformation is used as synonymous to distortion. The ability of material to with stand the applied force without any deformation is expressed in two ways, i.e.

strength and hardness. Strength is defined in many ways as per the design requirements, while the hardness may be defined as resistance to indentation of scratch.

Material deformation can be permanent or temporary. Permanent deformation is irreversible i.e.

stays even after removal of the applied forces, while the temporary deformation disappears after removal of the applied forces i.e. the deformation is recoverable. Both kinds of deformation can be function of time, or independent of time. Temporary deformation is called elastic deformation, while the permanent deformation is called plastic deformation. Time dependent recoverable deformation under load is called anelastic deformation, while the characteristic recovery of temporary deformation after removal of load as a function of time is called elastic aftereffect. Time dependent i.e. progressive permanent deformation under constant load/stress is called creep. For visco-elastic materials, both recoverable and permanent deformations occur together which are time dependent. When a material is subjected to applied forces, first the material experiences elastic deformation followed by plastic deformation. Extent of elastic- and plastic- deformations will primarily depend on the kind of material, rate of load application, ambient temperature, among other factors. Change over from elastic state to plastic state is characterized by the yield strength (σ

0) of the material.

Forces applied act on a surface of the material, and thus the force intensity, force per unit area, is used in analysis. Analogous to this, deformation is characterized by percentage change in length per unit length in three distinct directions. Force intensity is also called engineering stress (or simply stress, s), is given by force divided by area on which the force is acting. Engineering strain (or simply strain, e) is given by change in length divided by original length. Engineering strain actually indicates an average change in length in a particular direction. According to definition, s and e are given as

where P is the load applied over area A, and as a consequence of it material attains the final length L from its original length of L

0.

Because material dimensions changes under application of the load continuously, engineering stress and strain values are not the true indication of material deformation characteristics. Thus the need for measures of stress and strain based on instantaneous dimensions arises. Ludwik first proposed the concept of, and defined the true strain or natural strain (ε) as follows:

There are certain advantages of using true strain over conventional strain or engineering strain.

These include (i) equivalent absolute numerical value for true strains in cases of tensile and compressive for same intuitive deformation and (ii) total true strain is equal to the sum of the incremental strains. As shown in the figure below, if L

1=2 L

0 and L

2=1/2 L

1=L

0, absolute numerical value of engineering strain during tensile deformation (1.0) is different from that during compressive deformation (0.5). However, in both cases true strain values are equal (ln [2]).

True stress (σ) is given as load divided by cross-sectional area over which it acts at an instant.

It is to be noted that engineering stress is equal to true stress up to the elastic limit of the material. The same applies to the strains. After the elastic limit i.e. once material starts deforming plastically, engineering values and true values of stresses and strains differ. The above equation relating engineering and true stress-strains are valid only up to the limit of uniform deformation i.e. up to the onset of necking in tension test. This is because the relations are developed by assuming both constancy of volume and homogeneous distribution of strain along the length of the tension specimen.

Elastic deformation and Plastic deformation Elastic deformation

Elastic deformation is reversible i.e. recoverable. Up to a certain limit of the applied stress, strain experienced by the material will be the kind of recoverable i.e. elastic in nature. This elastic

strain is proportional to the stress applied. The proportional relation between the stress and the elastic strain is given by Hooke’s law, which can be written as follows:

where the constant E is the modulus of elasticity or Young‟s modulus,

Though Hooke‟s law is applicable to most of the engineering materials up to their elastic limit, defined by the critical value of stress beyond which plastic deformation occurs, some materials won‟t obey the law. E.g.: Rubber, it has nonlinear stress-strain relationship and still satisfies the definition of an elastic material. For materials without linear elastic portion, either tangent modulus or secant modulus is used in design calculations. The tangent modulus is taken as the slope of stress-strain curve at some specified level, while secant module represents the slope of secant drawn from the origin to some given point of the σ-ε curve, as shown in the figure below.

Tangent and Secant moduli for non-linear stress-strain relation.

If one dimension of the material changed, other dimensions of the material need to be changed to keep the volume constant. This lateral/transverse strain is related to the applied longitudinal strain by empirical means, and the ratio of transverse strain to longitudinal strain is known as Poisson’s ratio (ν). Transverse strain can be expected to be opposite in nature to longitudinal strain, and both longitudinal and transverse strains are linear strains. For most metals the values of ν are close to 0.33, for polymers it is between 0.4 – 0.5, and for ionic solids it is around 0.2.

Stresses applied on a material can be of two kinds – normal stresses, and shear stresses. Normal stresses cause linear strains, while the shear stresses cause shear strains. If the material is subjected to torsion, it results in torsional strain. Different stresses and corresponding strains are shown in the figure below.

Schematic description of different kinds of deformations/strains.

Analogous to the relation between normal stress and linear strain defined earlier, shear stress (τ) and shear strain (γ) in elastic range are related as follows:

where G is known as Shear modulus of the material. It is also known as modulus of elasticity in shear. It is related with Young‟s modulus, E, through Poisson‟s ratio, ν, as

Similarly, the Bulk modulus or volumetric modulus of elasticity K, of a material is defined as the ratio of hydrostatic or mean stress (σ

m) to the volumetric strain (Δ). The relation between E and K is given by

Let σ

x, σ

y and σ

z be the linear stresses and ε

x, ε

y and ε

z are the corresponding strains in X-, Y- and Z- directions, then

Using the above equations, it is possible to find strains from stresses and vice versa in elastic range.

The basis for elastic deformation is formed by reversible displacements of atoms from their equilibrium positions. On an atomic scale, elastic deformation can be viewed as small changes in the inter-atomic distances by stretching of inter-atomic bonds i.e. it involve small changes in inter-atomic distances. Elastic moduli measure the stiffness of the material. They are related to the second derivative of the inter-atomic potential, or the first derivative of the inter-atomic force vs. inter-atomic distance (dF/dr) (see the Figure below). By examining these curves we can tell which material has a higher modulus.

Elastic modulus can also be said as a measure of the resistance to separation of adjacent atoms, and is proportional to the slope of the inter-atomic force Vs inter-atomic distance curve. Hence values of modulus of elasticity are higher for ceramic materials which consist of strong covalent and ionic bonds. Elastic modulus values are lower for metal when compared with ceramics, and are even lower for polymers where only weak covalent bonds present.

Moreover, since the inter-atomic forces will strongly depend on the inter-atomic distance as shown in the figure below, the elastic constants will vary with direction in the crystal lattice i.e.

they are anisotropic in nature for a single crystal. However, as a material consists of number of randomly oriented crystals, elastic constants of a material can be considered as isotropic.

Graph showing variation of inter-atomic forces against inter-atomic distance for both weak and strong inter-atomic bonds.

The elastic moduli are usually measured by direct static measurements in tension or torsion tests.

For more precise measurements, dynamic techniques are employed. These tests involve measurement of frequency of vibration or elapsed time for an ultrasonic pulse to travel down and back in a specimen. Because strain cycles occur at very high rates, no time for heat transfer and thus elastic constants are obtained under adiabatic conditions. Elastic modulus obtained under isothermal and adiabatic conditions are related as follows:

where α is volume coefficient of thermal expansion, and c is the specific heat. It can be observed that with increasing temperature, the modulus of elasticity diminishes. This is because, intensity of thermal vibrations of atoms increases with temperature which weakens the inter-atomic bonds.

Plastic deformation

When the stress applied on a material exceeds its elastic limit, it imparts permanent

When the stress applied on a material exceeds its elastic limit, it imparts permanent