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PDF Drive is your search engine for PDF files. As of today we have 78,, eBooks for Materials Science and Engineering: An Introduction, Eighth Edition. Elsevier Science and Technology Books deliver targeted content that enables the .. Jianjun Guan Associate Professor, Materials Science Engineering, Ohio. This section of instructor's resource materials contains solutions and answers to all problems and questions that appear in the textbook. My penmanship leaves.

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my text, Materials Science and Engineering: An Introduction, Fifth Edition. mations that enhance the learning of key concepts in materials science and engi- . lecturers and researchers who are interested in the fields of materials science, engineering and technology and, specifically, in advanced ceramic materials. Module Name, Download, Description, Download Size. Introduction, Lecture Notes-Introduction, PDF, kb. Atomic Structure,Interatomic Bonding and.

The study of biomaterials is called bio materials science. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into developing new products. Biomaterials science encompasses elements of medicine , biology , chemistry , tissue engineering , and materials science. Biomaterials can be derived either from nature or synthesized in a laboratory using a variety of chemical approaches using metallic components, polymers , bioceramics , or composite materials.

They are often intended or adapted for medical applications, such as biomedical devices which perform, augment, or replace a natural function. Such functions may be benign, like being used for a heart valve , or may be bioactive with a more interactive functionality such as hydroxylapatite coated hip implants.

Biomaterials are also used every day in dental applications, surgery, and drug delivery. For example, a construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time. A biomaterial may also be an autograft , allograft or xenograft used as an organ transplant material.

Electronic, optical, and magnetic[ edit ] Negative index metamaterial [12] [13] Semiconductors, metals, and ceramics are used today to form highly complex systems, such as integrated electronic circuits, optoelectronic devices, and magnetic and optical mass storage media. These materials form the basis of our modern computing world, and hence research into these materials is of vital importance.

Semiconductors are a traditional example of these types of materials. They are materials that have properties that are intermediate between conductors and insulators. Their electrical conductivities are very sensitive to the concentration of impurities, which allows the use of doping to achieve desirable electronic properties. Hence, semiconductors form the basis of the traditional computer.

This field also includes new areas of research such as superconducting materials, spintronics , metamaterials , etc. The study of these materials involves knowledge of materials science and solid-state physics or condensed matter physics.

Computational materials science and engineering[ edit ] With continuing increases in computing power, simulating the behavior of materials has become possible. This enables materials scientists to understand behavior and mechanisms, explain properties formerly poorly understood, and even to design new materials.

Hence materials are strain hardened at low temperatures, thus also called cold working. During plastic deformation, dislocation density increases. And thus their interaction with each other resulting in increase in yield stress.

Strain hardening work hardening is the reason for the elastic recovery. The reason for strain hardening is that the dislocation density increases with plastic deformation cold work due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one Non-Destructive testing NDT NDT is the method of detection and measurement of properties or condition of materials, structures, machines without damaging or destroying their operational capabilities.

All NDTs are used to detect various types of flaws on the surface of material or internal inclusions of impurities and these techniques are also very useful during preventive maintenance and repair. There are few techniques which do not require any special apparatus and are quite simple to handle and only a moderate skill being required.

Some of the applications of NDTs are detecting: Ultrasonic Test High frequency ultrasonic sound waves are applied to the test piece by a Piezoelectric crystal. If the test piece is free from cracks, or flawless, then it reflects ultrasonic waves without distortion. If there are any flaws in the specimen, the time taken by the ultrasonic waves will be less as the reflection of these waves will be from flaw points and not from the bottom of the specimen.

Cathode ray oscilloscope CRO is used to receive the sound signals, whose time base circuit is connected to it. Knowing the time interval between the transmission of the sound pulse and the reception of the echo signal, we can calculate the depth of the crack.

This test is a very fast method of inspection and often used to test aerospace components and automobiles. This test is generally used to detect internal cracks like shrinkage cavities, hot tears, zones of corrosion and non-metallic inclusions. Liquid-Penetration test This test is employed for detection of small defects which are very small to detect with the naked eye.

This test is used to detect surface cracks or flaws in non-ferrous metals. This test employs a visible colour contrast dye penetrant technique for the detection of open surface flaws in metallic and non-metallic objects.

The penetrants are applied by spraying over the surface of material to be inspected. The excess penetrant is then washed or cleaned. Absorbent powder is then applied to absorb the penetrants in the cracks, voids which reveals the flaws. This test reveals flaws such as shrinkage cracks, porosity, fatigue cracks, grinding cracks, forging cracks, seams, heat treatment cracks and leaks etc.

If the fluorescent penetrant is used, the developed surface must be examined under ultra violet light to see the presence of defects. This technique is used for non-porous and non- absorbent materials.

Care may be taken to clean the surface so that it is free from dust, scale, etc. Penetrants are highly toxic and flammable and hence proper precautions should be taken both during use and of storage of penetrants. Depending on the type of material, the sectioning operation can be done by using abrasive cutter for metal and metallic composite , diamond wafer cutter ceramics, electronics and minerals or thin sectioning with a microtome plastics. To protects the specimen edge and maintain the integrity of materials surface features.

Fill voids in porous materials. Improves handling of irregular shaped samples. Samples for microstructure evaluation are typically encapsulated in a plastic mount for handling during sample preparation. Large sample or samples for macrostructure evaluation can be prepared without mounting.

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The metallography specimen mounting is done by encapsulating the specimen into: Grinding is accomplished by decreasing the abrasive grit size sequentially to obtain the required fine surface finish prior to polishing. Polishing can be divided into two main steps: Rough polishing The purpose is to remove the damage produced during grinding. Proper polishing will maintain the specimen flatness and retain all inclusions or secondary phases by eliminating the previous damage and maintaining the specimen integrity.

Fine polishing The purpose is to remove only surface damage. Etching alters the microstructural features based on composition; stress or crystal structure and it will develop the surface topology, which can be visible in the microscope. Typically, chemical etching involve immersing the polished surface in the prepared chemical solution for a specified time usually seconds followed by rinsing the etched specimen under running tap water and drying.

The analysis can be done by using a metallurgy microscope. Grain size determination The grain size is often determined when the properties of a polycrystalline material are under consideration. In this regard, there exist a number of techniques by which size is specified in terms of average grain volume, diameter, or area.

Grain size may be estimated by using an intercept method, described as follows.

Straight lines all the same length are drawn through several photomicrographs that show the grain structure. The grains intersected by each line segment are counted; the line length is then divided by an average of the number of grains intersected, taken over all the line segments. The average grain diameter is found by dividing this result by the linear magnification of the photomicrographs. Grain size is measured with a microscope by counting the number of grains within a given area, by determining the number of grains that intersect a given length of random line, or by comparison with standard charts.

The average grain diameter D can be determined from measurements along random lines by the equation where L is the length of the line and N is the number of intercepts which the grain boundary makes with the line. This can be related to the ratio of the grain-boundary surface area S to the volume of the grains, V, by the equation where 1 is the total length of grain boundary n a random plane of polish and A is the total area of the grains on a random plane of polish. A very common method of measuring grain size in the United States is to compare the grains at a fixed magnification with the American Society for Testing and Materials ASTM grain-size charts.

Phase Diagrams Equilibrium Phase Diagrams Give the relationship of composition of a solution as a function of temperatures and the quantities of phases in equilibrium. These diagrams do not indicate the dynamics when one phase transforms into another.

Sometimes diagrams are given with pressure as one of the variables. In the phase diagrams we will discuss, pressure is assumed to be constant at one atmosphere. Binary Isomorphous Systems This very simple case is one complete liquid and solid solubility, an isomorphous system.

The example is the Cu-Ni alloy of Fig. The complete solubility occurs because both Cu and Ni have the same crystal structure FCC , near the same radii, electronegativity and valence. The solidus line is that below which the solution is completely solid does not contain a liquid phase.

Interpretation of phase diagrams Concentrations: Tie-line method a locate composition and temperature in diagram b In two phase region draw tie line or isotherm c note intersection with phase boundaries. Read compositions. The composition of the solid and the liquid change gradually during cooling as can be determined by the tie-line method. Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line.

The composition of the liquid phase evolves by diffusion, following the equilibrium values that can be derived from the tie-line method. However, diffusion in the solid state is very slow. Hence, the new layers that solidify on top of the grains have the equilibrium composition at that temperature but once they are solid their composition does not change. This lead to the formation of layered cored grains Fig. Binary Eutectic Systems Interpretation: Solvus line: Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature.

The eutectic structure then adds when the remaining liquid is solidified when cooling further. The eutectic microstructure is lamellar layered due to the reduced diffusion distances in the solid state. To obtain the concentration of the eutectic microstructure in the final solid solution, one draws a vertical line at the eutectic concentration and applies the lever rule treating the eutectic as a separate phase.

Eutectoid and Peritectic Reactions The eutectoid eutectic-like reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. There are associated eutectoid temperature or temperature , eutectoid phase, eutectoid and proeutectoid microstructures.

The inverse reaction occurs when heating. Precipitation hardening and the treating of steel to form tempered matrensite are totally different phenomena, even though the heat treatment procedures are similar. Precipitation reactions A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid.

That insoluble compound is called a precipitate Kinetics of nucleation and growth From a micro structural standpoint, the first process to accompany a phase transformation is nucleation- the formation of very small particles or nuclei, of the new phase which are capable of growing. The second stage is growth, in which the nuclei increase in size; during this process, some volume of the parent phase disappears.

The transformation reaches completion if growth of these new phase particles is allowed to proceed until the equilibrium fraction is attained. As would be expected, the time dependence of the transformations rate which is often termed the kinetics of a transformation is an important consideration in the heat treatment of materials. With many investigations, the fraction of reaction that has occurred is measured as a function of time, while the temperature is maintained constant. Transformation progress is usually ascertained by either microscopic examination or measurement of some physical property.

Data are plotted as the fraction of transformed material versus the logarithm of time; an S-shaped curve, represents the typical kinetic behavior for most solid state reactions.

Solid Solutions A solid solution may be formed when impurity atoms are added to a solid, in which case the original crystal structure is retained and no new phases are formed. Concentrations are usually given in weight percent. The possible phases are: An intermetallic compound. It is not important in practice. Austenite has a maximum C concentration of 2. It is not stable below the eutectic temperature C unless cooled rapidly Chapter Austenite has FCC cubic face centered crystal structure, permitting high solubility of carbon i.

Cementite is a hard and brittle substance, influencing the properties of steels and cast irons. Below this temperature austenite does not exist.

This transformation is known as eutectic reactionand is written symbolically as: Another set of invariant reactions that occur often in binary systems are - peritectic reaction where a solid phase reacts with a liquid phase to produce a new solid phase. For their role in mechanical properties of the alloy, it is important to note that: Ferrite is soft and ductile Cementite is hard and brittle. Thus, combining these two phases in solution an alloy can be obtained with intermediate properties.

Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed. When it cools slowly it forms perlite, a lamellar or layered structure of two phases: Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid pearlite. Hypereutectoid alloys contain proeutectoid cementite plus pearlite. Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite. Thus, the strength of Fe—C alloys increase with C content and also with the addition of other elements.

Time-temperature transformation TTT diagrams measure the rate of transformation at a constant temperature. In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is measured, for example by dilatometry. Obviously a large number of experiments is required to build up a complete TTT diagram. When cooling proceeds below the eutectoid temperature oC nucleation of pearlite starts.

The S-shaped curves fraction of pearlite vs. The family of S-shaped curves at different temperatures can be used to construct the TTT Time- Temperature-Transformation diagrams For these diagrams to apply, one needs to cool the material quickly to a given temperature To before the transformation occurs, and keep it at that temperature over time.

The horizontal line that indicates constant temperature To intercepts the TTT curves on the left beginning of the transformation and the right end of the transformation ; thus one can read from the diagrams when the transformation occurs. The formation of pearlite indicates that the transformation occurs sooner at low temperatures, which is an indication that it is controlled by the rate of nucleation.

This reduced grain growth leads to fine-grained microstructure fine pearlite. At higher temperatures, diffusion allows for larger grain growth, thus leading to coarse pearlite.

At lower temperatures nucleation starts to become slower, and a new phase is formed, bainite. Since diffusion is low at low temperatures, this phase has a very fine microscopic microstructure.

Spheroidite is a coarse phase that forms at temperatures close to the eutectoid temperature. The relatively high temperatures caused a slow nucleation but enhances the growth of the nuclei leading to large grains. A very important structure is martensite, which forms when cooling austenite very fast quenching to below a maximum temperature that is required for the transformation.

It forms nearly instantaneously when the required low temperature is reached; since no thermal activation is needed, this is called an athermal transformation. Martensite is a different phase, a body- centered tetragonal BCT structure with interstitial C atoms.

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Martensite is metastable and decomposes into ferrite and pearlite but this is extremely slow and not noticeable at room temperature. In the examples, we used an eutectoid composition.

For hypo- and hypereutectoid alloys, the analysis is the same, but the proeutectoid phase that forms before cooling through the eutectoid temperature is also part of the final microstructure. Carbon steel is broken down into four classes based on carbon content: Mild and low-carbon steel Mild steel also known as plain-carbon steel, is the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications, more so than iron.

Low-carbon steel contains approximately 0. Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing. It is often used when large quantities of steel are needed, for example as structural steel.

The density of mild steel is approximately 7. Low-carbon steels suffer from yield-point run out where the material has two yield points. The first yield point or upper yield point is higher than the second and the yield drops dramatically after the upper yield point. If low-carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop louder bands.

Low-carbon steels contain less carbon than other steels and are easier to cold-form, making them easier to handle.

Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red- short, that is, brittle and crumbly at working temperatures. Low-alloy carbon steel, such as A36 grade, contains about 0. Manganese is often added to improve the harden ability of low-carbon steels. These additions turn the material into low-alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.

Low carbon steel Less than 0. Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components. High-carbon steel Approximately 0. Very strong, used for springs and high-strength wires. Ultra-high-carbon steel Approximately 1. Steels that can be tempered to great hardness. Used for special purposes like non-industrial-purpose knives, axles or punches.

Most steels with more than 1. Note that steel with carbon content above 2. Alloy steel Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1. Alloy steels are broken down into two groups: Smith and Has hemi define the difference at 4.

Most commonly, the phrase "alloy steel" refers to low-alloy steels. According to the World Steel Association, there are over 3, different grades of steel, encompassing unique physical, chemical and environmental properties. In essence, steel is composed of iron and carbon, although it is the amount of carbon, as well as the level of impurities and additional alloying elements that determines the properties of each steel grade. The carbon content in steel can range from 0.

Elements such as manganese, phosphorus and sulphur are found in all grades of steel, but, whereas manganese provides beneficial effects, phosphorus and sulphur are deleterious to steel's strength and durability. Different types of steel are produced according to the properties required for their application, and various grading systems are used to distinguish steels based on these properties. Carbon Steels 2.

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Alloy Steels 3. Stainless Steels 4. Tool Steels 1 Carbon Steels: Carbon steels can be further categorized into three groups depending on their carbon content: Alloy steels contain alloying elements e. Applications for alloys steel include pipelines, auto parts, transformers, power generators and electric motors. These steels can be divided into three groups based on their crystalline structure: Austenitic steels form the largest portion of the global stainless steel market and are often used in food processing equipment, kitchen utensils and piping.

These magnetic steels cannot be hardened with heat treatment, but can be strengthened by cold works. These magnetic and heat-treatable steels are used in knives, cutting tools, as well as dental and surgical equipment. Tool steels contain tungsten, molybdenum, cobalt and vanadium in varying quantities to increase heat resistance and durability, making them ideal for cutting and drilling equipment.

These products are commonly used in the automotive and construction sectors. These materials are mainly used in automotive parts, appliances, packaging, shipbuilding, and construction. Cast iron Cast iron is iron or a ferrous alloy which has been heated until it liquefies, and is then poured into a mould to solidify.

It is usually made from pig iron. The alloy constituents affect its colour when fractured: Grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks.

Carbon C and silicon Si are the main alloying elements, with the amount ranging from 2. Iron alloys with less carbon content are known as steel. While this technically makes these base alloys ternary Fe—C—Si alloys, the principle of cast iron solidification is understood from the binary iron—carbon phase diagram. Cast iron's properties are changed by adding various alloying elements, or alloyants.

Next to carbon, silicon is the most important alloying because it forces carbon out of solution. Instead the carbon forms graphite which results in a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfur, when present, forms iron sulfide, which prevents the formation of graphite and increases hardness.

The problem with sulfur is that it makes molten cast iron sluggish, which causes short run defects. To counter the effects of sulfur, manganese is added because the two form into manganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt so it tends to float out of the melt and into the slag.

The amount of manganese required to neutralize sulfur is 1. Nickel is one of the most common alloying elements because it refines the pearlite and graphite structure, improves toughness, and evens out hardness differences between section thicknesses.

Chromium is added in small amounts to the ladle to reduce free graphite, produce chill, and because it is a powerful carbide stabilizer; nickel is often added in conjunction. A small amount of tin can is added as a substitute for 0. Copper is added in the ladle or in the furnace, on the order of 0. Molybdenum is added on the order of 0. Titanium is added as a degasser and deoxidizer, but it also increases fluidity.

In malleable iron melts, bismuth is added, on the scale of 0. In white iron, boron is added to aid in the production of malleable iron; it also reduces the coarsening effect of bismuth. Grey cast iron Grey cast iron is characterised by its graphitic microstructure, which causes fractures of the material to have a grey appearance.

It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2. Grey cast iron has less tensile strength and shock resistance than steel, but its compressive strength is comparable to low and medium carbon steel. White cast iron It is the cast iron that displays white fractured surface due to the presence of cementite. With a lower silicon content graphitizing agent and faster cooling rate, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite.

These eutectic carbides are much too large to provide precipitation hardening as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix. Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures.

In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet.

White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces impeller and volute of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket although cast medium-carbon martensitic steel is more common for this application.

It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron.

The resulting casting, called a chilled casting, has the benefits of a hard surface and a somewhat tougher interior. High-chromium white iron alloys allow massive castings for example, a tonne impeller to be sand cast, i.

These high-chromium alloys attribute their superior hardness to the presence of chromium carbides. The main form of these carbides are the eutectic or primary M7C3 carbides, where "M" represents iron or chromium and can vary depending on the alloy's composition.

The eutectic carbides form as bundles of hollow hexagonal rods and grow perpendicular to the to the hexagonal basal plane.

Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes.

They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron. Ductile cast iron A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes.

Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections. Most of the typical uses include: Any material in question should be evaluated as a part of a larger system and treatment plans should be based upon consideration of all relevant factors.

Heat Treatment Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming. Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation.

Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.

Steels are heat treated for one of the following reasons: Softening 2. Hardening 3. Material modification Softening: Softening is done to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size or change the electromagnetic properties of the steel.

Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold-rolling operation or wiredrawing. Annealing — full Process, spheroidizing, normalizing and tempering austempering, martempering are the principal ways by which steel is softened.

Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.

Material Modification: Heat treatment is used to modify properties of materials in addition to hardening and softening. These processes modify the behavior of the steels in a beneficial manner to maximize service life, e.

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It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. At this point, it can be cooled in room temperature air with natural convection.

The steel becomes soft and ductile. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite.

The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling. This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite- Austenite region, line A1on the diagram.

This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace. Stress Relief Anneal is used to reduce residual stresses in large castings, welded parts and cold- formed parts.

Such parts tend to have stresses due to thermal cycling or work hardening. This is done by one of the following ways: Hold the temperature for a prolonged time and follow by fairly slow cooling. All these methods result in a structure in which all the Cementite is in the form of small globules spheroids dispersed throughout the ferrite matrix.

This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.

Tempering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of Martensite. This brittleness is removed by tempering.

Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability. Tempering is not to be confused with tempers on rolled stock-these tempers are an indication of the degree of cold work performed.

The mechanism of tempering depends on the steel and the tempering temperature. The prevalent Martensite is a somewhat unstable structure. When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form.

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Tool steels for example, lose about 2 to 4 points of hardness on the Rockwell C scale. Even though a little strength is sacrificed, toughness as measured by impact strength is increased substantially. Springs and such parts need to be much tougher — these are tempered to a much lower hardness. Tempering is done immediately after quench hardening.

In this region a softer and tougher structure Troostite is formed. This has less strength than Troostite but more ductility and toughness. Heating in a bath also ensures that the entire part has the same temperature and will undergo the same tempering. Regardless of the bath, gradual heating is important to avoid cracking the steel. After reaching the desired temperature, the parts are held at that temperature for about 2 hours, then removed from the bath and cooled in still air.

Hardening Hardness is a function of the Carbon content of the steel. Hardening of a steel requires a change in structure from the body-centered cubic structure found at room temperature to the face- centered cubic structure found in the Austenitic region.

The steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is extremely soft. Usually when hot steel is quenched, most of the cooling happens at the surface, as does the hardening.

This propagates into the depth of the material. Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application.

Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vaporizes, and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots. Water is a good rapid quenching medium, provided good agitation is done.

However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking. Salt Water: Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately.

Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard.

Precipitation hardening is achieved by: This leads to a supersaturated solid solution that remains stable metastable due to the low temperatures, which prevent diffusion.

If the process is continued for a very long time, eventually the hardness decreases. This is called over aging. Materials with structure at the nanoscale often have unique optical, electronic, or mechanical properties. The field of nanomaterials is loosely organized, like the traditional field of chemistry, into organic carbon-based nanomaterials such as fullerenes, and inorganic nanomaterials based on other elements, such as silicon.

Examples of nanomaterials include fullerenes , carbon nanotubes , nanocrystals , etc. Main article: Biomaterial The iridescent nacre inside a nautilus shell A biomaterial is any matter, surface, or construct that interacts with biological systems.

The study of biomaterials is called bio materials science. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into developing new products. Biomaterials science encompasses elements of medicine , biology , chemistry , tissue engineering , and materials science.

Biomaterials can be derived either from nature or synthesized in a laboratory using a variety of chemical approaches using metallic components, polymers , bioceramics , or composite materials.

They are often intended or adapted for medical applications, such as biomedical devices which perform, augment, or replace a natural function. Such functions may be benign, like being used for a heart valve , or may be bioactive with a more interactive functionality such as hydroxylapatite coated hip implants.

Biomaterials are also used every day in dental applications, surgery, and drug delivery. For example, a construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time. A biomaterial may also be an autograft , allograft or xenograft used as an organ transplant material.

Electronic, optical, and magnetic[ edit ] Negative index metamaterial [12] [13] Semiconductors, metals, and ceramics are used today to form highly complex systems, such as integrated electronic circuits, optoelectronic devices, and magnetic and optical mass storage media. These materials form the basis of our modern computing world, and hence research into these materials is of vital importance.

Semiconductors are a traditional example of these types of materials. They are materials that have properties that are intermediate between conductors and insulators. Their electrical conductivities are very sensitive to the concentration of impurities, which allows the use of doping to achieve desirable electronic properties.

Hence, semiconductors form the basis of the traditional computer.Chemical vapor deposition can place a film of a ceramic on another material.

It maybe the same type of atom as the others self interstitial or an impurity atom. Ordinary dislocation is of mixed character of edge and screw type. Two methods are used for carburizing steel.

Crystals of ZnS are used in lasers that operate in the mid-infrared part of the spectrum. In general, when dislocations are close and their strain fields add to a larger value, they repel, because being close increases the potential energy it takes energy to strain a region of the material. The results of creep rupture tests are most commonly presented as the logarithm of stress versus the logarithm of rupture lifetime. Now, at least part of the atom was to be composed of Thomson's particulate negative "corpuscles", although the rest of the positively charged part of the atom remained somewhat nebulous and ill-defined.

Before deformation the grains are equiaxed, or have approximately the same dimension in all directions. Besides material characterization, the material scientist or engineer also deals with extracting materials and converting them into useful forms.