WHAT IS MATERIALS ENGINEERING?• It is the field of engineering that encompasses the spectrum of materials types and how to usethem in manufacturing.• Materials engineering is different from Materials science. Materials science involvesinvestigating the relationships that exist between the structures and properties ofmaterials, whereas, Materials engineering, on the basis of these structure–propertycorrelations, design or engineer the structure of a material to produce a predetermined set ofproperties. From a functional perspective, the role of a materials scientist is to develop orsynthesize new materials, whereas a materials engineer is called upon to create new products orsystems using existing materials, and/or to develop techniques for processing materials.What is a Material?• Everything we see and use is made of materials• Engineers make things.• They make them out of materials.Why Study Materials Engineering?In order to be a good designer, an engineer must learn what materials will be appropriate to use indifferent applications.Any engineer can look up materials properties in a book or search databases for a material thatmeets design specifications, but the ability to innovate and to incorporate materials safely in adesign is rooted in an understanding of how to manipulate materials properties and functionalitythrough the control of the material’s structure and processing techniques.
Engineering Materials Metals & Ceramics Advanced Polymers Composites Alloys & Glasses MaterialsMetals and AlloysAtoms in metals and their alloys are arranged in a very orderly manner, and in comparisonto the ceramics and polymers, are relatively dense . Metals have High electricalconductivity, good formability, Castable, machinable. An alloy is a metal that containsadditions of one or more metals or non-metals. Metals and alloys have relatively highstrength, high stiffness, ductility or formability, and shock resistance.CeramicsThermally insulating, Refractories. Ceramics can be defined as inorganic crystallinematerials. Beach sand and rocks are examples of naturally occurring ceramics. Traditionalceramics are used to make bricks, tableware, toilets, bathroom sinks, refractories (heat-resistant material), and abrasives. In general, due to the presence of porosity (smallholes), ceramics do not conduct heat well; they must be heated to very high temperaturesbefore melting. Ceramics are strong and hard, but also very brittle. Advanced ceramicsare materials made by refining naturally occurring ceramics and other special processes.GlassesOptically transparent. Glass is an amorphous material, often, but not always, derived froma molten liquid. The term ―amorphous‖ refers to materials that do not have a regular,periodic arrangement of atoms.
Polymers (Greek; Polys + meros = many + parts)Polyethylene Food packaging Easily formed into thin, flexible, airtight film. Electrically insulatingand moisture-resistant. Polymers are typically organic materials. They are produced using aprocess known as polymerization. Polymeric materials include rubber (elastomers), PE, nylon,PVC, PC, PS, and silicone rubber. and many types of adhesives. Polymers typically are goodelectrical and thermal insulators although there are exceptions such as the semiconductingpolymersCompositesThe main idea in developing composites is to blend the properties of different materials. Thedesign goal of a composite is to achieve a combination of properties that is not displayed by anysingle material, and also to incorporate the best characteristics of each of the componentmaterials.Advanced MaterialsSemiconductorsUsed in Silicon Transistors and integrated circuits. Unique electrical behaviour, converts electricalsignals to light, lasers, laser diodes, etc.BiomaterialsBiomaterials are employed in components implanted into the human body to replace diseased ordamaged body parts. These materials must not produce toxic substances and must be compatiblewith body tissues (i.e., must not cause adverse biological reactions).Smart MaterialsThese materials are able to sense changes in their environment and then respond to thesechanges in predetermined manners. Smart material include some type of sensors, and actuators.Piezoelectric actuators expand and contract in response to an applied electric field .NanomaterialsNanomaterials may be any one of the four basic types; metals, ceramics, polymers, & composites.
One common item that presents some interesting material property requirements isthe container for carbonated beverages. The material used for this application mustsatisfy the following constraints: (1) provide a barrier to the passage of carbondioxide, which is under pressure in the container; (2) be nontoxic, non-reactive withthe beverage, and, preferably, recyclable; (3) be relatively strong and capable ofsurviving a drop from a height of several feet when containing the beverage; (4) beinexpensive, including the cost to fabricate the final shape; (5) if optically transparent,retain its optical clarity; and (6) be capable of being produced in different coloursand/or adorned with decorative labels. All three of the basic material types—metal(aluminium), ceramic (glass), and polymer (PE plastic)—are used for carbonatedbeverage containers. All of these materials are non- toxic and un-reactive withbeverages. In addition, each material has its pros and cons. For example, thealuminium alloy is relatively strong (but easily dented), is a very good barrier to thediffusion of carbon dioxide, is easily recycled, cools beverages rapidly, and allowslabels to be painted onto its surface. On the other hand, the cans are optically opaqueand relatively expensive to produce. Glass is impervious to the passage of carbondioxide, is a relatively inexpensive material, and may be recycled, but it cracks andfractures easily, and glass bottles are relatively heavy. Whereas plastic is relativelystrong, may be made optically transparent, is inexpensive and lightweight, and isrecyclable, it is not as impervious to the passage of carbon dioxide as the aluminiumand glass. For example, you may have noticed that beverages in aluminium and glasscontainers retain their carbonization (i.e., ―fizz‖) for several years, whereas those intwo-litre plastic bottles ―go flat‖ within a few months.
The Figure shows three thin disk specimens placed over some printed matter. It isobvious that the optical properties of each of the three materials are different; the oneon the left is transparent, whereas the disks in the center and on the right are,respectively, translucent and opaque. All of these specimens are of the samematerial, aluminum oxide, but the:Leftmost one is what we call a single crystal—that is, it is highly perfect—which givesrise to its transparency.The center one is composed of numerous and very small single crystals that are allconnected; the boundaries between these small crystals scatter a portion of the lightreflected from the printed page, which makes this material optically translucent.The specimen on the right is composed not only of many small, interconnectedcrystals, but also of a large number of very small pores or void spaces. These poresalso effectively scatter the reflected light and render this material opaque.Thus, the structures of these three specimens are different in terms of crystalboundaries and pores, which affect the optical transmittance properties. Furthermore,each material was produced using a different processing technique.
Crystalline Structure Of MetalsThe properties of some materials are directly related to their crystal structures. Thecrystalline structure of a material usually relates to the arrangement of its internalComponents.Subatomic structure involves electrons within the individual atoms and interactions withtheir nuclei.Atomic structure involves arrangement of atoms in materials and defines interactionamong atoms (interatomic bonding).Microscopic structure involves arrangement of small grains of material that can beidentified by microscopy.Macroscopic structure relates to structural elements that may be viewed with the nakedeye. Macroscopic structure Atomic level Subatomic level Microscopic structure
Each atom consists of a very small nucleus composed of protons and neutrons, which isencircled by moving electrons. Both electrons and protons are electrically charged, thecharge magnitude being 1.62*10^-19 C, which is negative in sign for electrons andpositive for protons; neutrons are electrically neutral. Masses for these subatomicparticles are infinitesimally small; protons and neutrons have approximately the samemass, 1.67*10^-27 kg, which is significantly larger than that of an electron, 9.11*10^-31kg.The atomic mass of a specific atom may be expressed as the sum of the masses ofprotons and neutrons within the nucleus.
ATOMIC PACKING FACTOR For BCC APF 0.68 Volume of atoms in unit cell APF For FCC APF 0.74 Volume unit cell Total number of atom s in unit cell Volume of unit atoms For HCP APF ? Volume unit cell Show that the atomic packing factor for the BCC crystal structure is 0.68. Show that the atomic packing factor for the FCC crystal structure is 0.74. What is the atomic packing factor for the HCP crystal structure?. Show that for HCP the c/a ratio is 1.633 3 3 a2 Area of a Hexagon 2 Numerical Problems1. Calculate the volume of an BCC unit cell in terms of the atomic radius R.2. Calculate the volume of an FCC unit cell in terms of the atomic radius R.3. Calculate the volume of an HCP unit cell in terms of the atomic radius R.
AllotropyAllotropy is the ability of an element to exist in different structural forms while in thesame state of matter. The allotropes depend on both the allotropy temperature andthe external pressure. For example, the allotropes of carbon include diamond (wherethe carbon atoms are bonded together in a tetrahedral lattice arrangement),graphite (where the carbon atoms are bonded together in sheets of a hexagonallattice). Graphite is the stable polymorph at ambient conditions, whereas diamond isformed at extremely high pressures. Also, pure iron has a BCC crystal structure atroom temperature, which changes to FCC iron at 912◦ C
Crystallographic Directions, and PlanesDeformation under loading (slip) occurs on certain crystalline planes and in certaincrystallographic directions. Before we can predict how materials fail, we need to knowwhat modes of failure are more likely to occur.• It is often necessary to be able to specify certain directions and planes in crystals.• Many material properties and processes vary with direction in the crystal.• Directions and planes are described using three integers; Miller Indices Method of describing Miller indices for Directions • Draw vector, and find the coordinates of the head, h1,k1,l1 and the tail h2,k2,l2. • Subtract coordinates of tail from coordinates of head • Remove fractions by multiplying by smallest possible factor • Enclose in square brackets
Draw the following direction vectors in cubic unit cells:(a)  and  (b) (c)  (d) (a) The position coordinates for the direction indices  and  direction are (1, 0,0) and (1, 1, 0), respectively (Fig. a).(b) The position coordinates for the  direction are obtained by dividing thedirection indices by 2 so that they will lie within the unit cube. Thus the positioncoordinates are (1/2,1/2,1) (Fig. b).(c) The position coordinates for the [Ī10] direction are (-1, 1, 0) (Fig. c). Note that theorigin for the direction vector must be moved to the lower-left front corner of the cube.(d) The position coordinates for the direction are obtained by first dividingall the indices by 3, the largest index. This gives -1,-2/3,-1/3 which are shown in Fig. d.
Method of describing Miller indices for PlanesThe procedure for determining the Miller indices for a cubic crystal plane is as follows:1. Choose a plane that does not pass through the origin at (0, 0, 0).2. Determine the intercepts of the plane in terms of the crystallographic x, y, and zaxes for a unit cube. These intercepts may be fractions.3. Form the reciprocals of these intercepts.4. Clear fractions and determine the smallest set of whole numbers that are in thesame ratio as the intercepts. These whole numbers are the Miller indices of thecrystallographic plane and are enclosed in parentheses without the use of commas.The notation (hkl) is used to indicate Miller indices in a general sense, where h, k, andl are the Miller indices of a cubic crystal plane for the x, y, and z axes, respectively.Draw the following crystallographic planes in cubic unit cells:(a) (101) (b) (110) (c) (221)
Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom,(d) large substitutional atom, (e) Frenkel defect, and (f) Schottky defect.
Vacancies: A vacancy is produced when an atom or an ion is missing from itsnormal site in the crystal structure. When atoms or ions are missing (i.e., whenvacancies are present), the overall randomness or entropy of the material increases,which increases the thermodynamic stability of a crystalline material. All crystallinematerials have vacancy defects. Vacancies are introduced into metals and alloysduring solidification, at high temperatures, or as a consequence of radiation damage.Interstitial Defects: An interstitial defect is formed when an extra atom or ion isinserted into the crystal structure at a normally unoccupied position. Interstitial atomssuch as hydrogen are often present as impurities, whereas carbon atoms areintentionally added to iron to produce steel.A substitutional defect is introduced when one atom or ion is replaced by a differenttype of atom or ion.A Frenkel defect is a vacancy-interstitial pair formed when an ion jumps from anormal lattice point to an interstitial site.A Schottky defect, is unique to ionic materials and is commonly found in manyceramic materials. When vacancies occur in an ionically bonded material, astoichiometric number of anions and cations must be missing from regular atomicpositions if electrical neutrality is to be preserved. For example, one Mg+2 vacancyand one O-2 vacancy in MgO constitute a Schottky pair.
Iron ores are rocks and minerals from which metallic iron can be economicallyextracted. The ores are usually rich in iron oxides and vary in colour from dark grey,bright yellow, deep purple, to rusty red. The iron itself is usually found in the form ofmagnetite (Fe3O4), hematite (Fe2O3), goethite (FeO(OH)), limonite (FeO(OH)n(H2O))or siderite (FeCO3).SmeltingTo convert it to metallic iron it must be smelted or sent through a direct reductionprocess to remove the oxygen. Oxygen-iron bonds are strong, and to remove the ironfrom the oxygen, a stronger elemental bond must be presented to attach to theoxygen. Carbon is used because the strength of a carbon-oxygen bond is greaterthan that of the iron-oxygen bond, at high temperatures. Thus, the iron ore must bepowdered and mixed with coke, to be burnt in the smelting process.
Iron Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pigiron through the use of a blast furnace. From pig iron many other types of iron and steel areproduced by the addition or deletion of carbon and alloys. The following paragraphs discuss thedifferent types of iron and steel that can be made from iron ore.PIG IRON.— Pig iron is composed of about 93% iron, from 3% to 5% carbon, and variousamounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limiteduse and approximately ninety percent produced is refined to produce steel. Cast-iron pipe andsome fittings and valves are manufactured from pig iron.Carbon 3.0–4.5%Manganese 0.15–2.5%Phosphorus 0.1–2.0%Silicon 1.0–3.0%Sulphur 0.05–0.1%WROUGHT IRON.— Wrought iron is made from pig iron with some slag mixed in duringmanufacture. Almost pure iron, the presence of slag enables wrought iron to resist corrosion andoxidation. The chemical analyses of wrought iron and mild steel are just about the same. Thedifference comes from the properties controlled during the manufacturing process. Wrought ironcan be gas and arc welded, machined, plated, and easily formed; however, it has a low hardnessand a low-fatigue strength.CAST IRON.— Cast iron is any iron containing greater than 2% carbon alloy. Cast iron has ahigh compressive strength and good wear resistance; however, it lacks ductility, malleability, andimpact strength.
All of the phosphorus and most of the manganese will enter the molten iron. Oxides ofsilicon and sulphur compounds are partially reduced, and these elements also becomepart of the resulting metal. Other contaminant elements, such as calcium, magnesium,and aluminium, are collected in the limestone-based slag and are largely removedfrom the system. The resulting pig iron tends to have roughly the followingcomposition:Carbon 3.0–4.5%Manganese 0.15–2.5%Phosphorus 0.1–2.0%Silicon 1.0–3.0%Sulphur 0.05–0.1%
Modern Blast Furnace1: Iron ore + Calcareous sinter2: coke3: conveyor belt4: feeding opening, with a valvethat prevents direct contact with theinternal parts of the furnace5: Layer of coke6: Layers of sinter, iron oxidepellets, ore,7: Hot air (around 1200°C)8: Slag9: Liquid pig iron10: Mixers11: Tap for pig iron12: Dust cyclon for removing dustfrom exhaust gasses beforeburning them in 1313: Air heater14: Smoke outlet (can beredirected to carbon capture &storage (CCS) tank)15: feed air for Cowper air heaters16: Powdered coal17: cokes oven18: cokes bin19: pipes for blast furnace gas
PRODUCTION OF IRONIron is the fourth most plentiful element in the earth’s crust, it is rarely found in themetallic state. Instead, it occurs in a variety of mineral compounds, known as ores, themost attractive of which are iron oxides coupled with companion impurities. To producemetallic iron, the ores are processed in a manner that breaks the iron–oxygen bonds.Ore, limestone, coke (carbon), and air are continuously introduced into specificallydesigned furnaces and molten metal is periodically withdrawn.The production of iron in a blast furnace is a continuous process. The furnace is heatedconstantly and is re-charged with raw materials from the top while it is being tapped fromthe bottom. Iron making in the furnace usually continues for about ten years before thefurnace linings have to be renewed. Blast furnace is a furnace for smelting of iron from iron oxide ores (hematite, Fe2O3 or magnetite, Fe3O4). Coke, limestone and iron ore are poured in the top, which would normally burn only on the surface. The hot air blast to the furnace burns the coke and maintains the very high temperatures that are needed to reduce the ore to iron. The reaction between air and the fuel generates carbon monoxide. This gas reduces the iron oxide in the ore to iron. Fe2O3(s) + CO(g) Fe(s) + CO2(g)
BLAST FURNACE CHEMISTRY FOR THE PRODUCTION OF IRONThe significant reactions occurring within the Blast Furnace can be described as follows:1. Iron is extracted from its ores by the chemical reduction of iron oxides with carbon in afurnace at a temperature of about 800 C - 1900 C.2. Coke, the source of chemical energy in the blast furnace, is burnt both to release heat energyand to provide the main reducing agent:3. Calcium oxide, formed by thermal decomposition of limestone, reacts with the silicon oxidepresent in sand, a major impurity in iron ores, to form slag (which is less dense than molten iron).Overall, the chemical processes can be summarized by these equations: At 500o C 3Fe2O3 +CO -> 2Fe3O4 + CO2 Fe2O3 +CO -> 2FeO + CO2 At 850o C Fe3O4 +CO -> 3FeO + CO2 At 1000o C FeO +CO -> Fe + CO2 At 1300 oC CO2 + C -> 2CO At 1900o C C+ O2 -> CO2 FeO +C -> Fe + CO
PRODUCTION OF STEEL When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. In order to convert the pig iron into steel, it must be melted and reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. This liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as cast steel ingots. The ingots are then heated in a soaking pit and hot rolled into slabs, blooms, or billets. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails. In modern foundries these processes often occur in one assembly line, with ore coming in and finished steel coming out. Sometimes after steel’s final rolling it is heat treated for strength, however this is relatively rare. Iron as obtained from blast furnace contains from 3-4% of Carbon, and variable amount of silicon, manganese sulphur and phosphorus.1. Dead mild steel — up to 0.15% carbon2. Low carbon or mild steel — 0.15% to 0.45% carbon3. Medium carbon steel — 0.45% to 0.8% carbon4. High carbon steel — 0.8% to 1.5% carbon
STEEL MAKING PROCESSES Bessemer Process Crucible Process Open Hearth Process Electric Process (Arc, Induction) Duplex Process Linz Donnawitz Process Kaldo Process Modern Process
Bessemer ProcessBessemer process was invented in1875 by Thomas Gilchrest. In Bessemer processthe molten pig iron from blast furnace is poured into converter. The converter ismade of steel plates lined inside with refractory material. In the bottom of convertervessels, a no of holes are introduced through which air is blown at a pressure of200-250KN/m2. Based on full capacity, the converter is charged with 100-150 ton,and this charge is carried out from 10 to 15 ton at different time intervals. Their firstoxidizes silicon, and manganese which together with iron oxide rise to the top fromslag. During this air blowing process the carbon begins to burn and blowingcontinued, until 0.25% of carbon is eliminated. In Bessemer process acids are usedto burn and eliminated silicon and phosphorus. The finished steel is then poured intoladles and from ladle it is poured into ingot moulds for subsequently rolling andforging process.
Crucible ProcessIn Crucible process wrought iron together with a small amount of pig iron, necessaryalloying metals and slagging materials are placed in a clay or clay-graphitecrucible, covered with an old crucible bottom and melted in a gas or coke-firedfurnace. After the charge is entirely molten, with sufficient time allowed for the gasesand impurities to rise to the surface, the Crucible is withdrawn, the slag removedwith a cold iron bar, and the resulting steel poured into a small ingot which issubsequently forged to the desire shape.There are three types of Crucible Furnaces:(a) lift-out crucible,(b) stationary pot, from which molten metal must be ladled, and(c) tilting-pot furnace
Open Hearth ProcessThe open-hearth furnace is rectangular and rather low, holding from 15 to 200 ton ofmetal in a saucer-like shallow pool. It is heated either by producer gas, oil, tar, mixedblast furnace and coke oven gas, pitch, mixture of creosote and pitch and heavy fueloil. Flames come from first one end and then the other. Waste gases pass throughregenerators. The furnace is charged with ore and limestone. The lime stone beginsto decompose in carbon di-oxide and calcium oxide.A. gas and air enterB. pre-heated chamberC. molten pig ironD. hearthE. heating chamberF. gas and air exit.
Electric FurnaceAn Electric Arc Furnace (EAF) is a furnace that heats charged material by meansof an electric arc. An electric arc furnace used for steelmaking consists ofa refractory-lined vessel, usually water-cooled in larger sizes, covered with aretractable roof, and through which one or more graphite electrodes enter thefurnace. Arc furnaces differ from induction furnaces in that the charge material isdirectly exposed to an electric arc, and the current in the furnace terminals passesthrough the charged material.An induction furnace is an electrical furnace in which the heat is appliedby induction heating of metal. The advantage of the induction furnace is a clean,energy-efficient and well-controllable melting process compared to most othermeans of metal melting.The one major drawback to Electric furnace usage in a foundry is the lack of refiningcapacity; charge materials must be clean of oxidation products and of a knowncomposition and some alloying elements may be lost due to oxidation. Induction furnace is based on the principle of heating by induced currents. If a conductor isplaced within a coil through which an alternating current is flowing, a current is induced in theconductor. By the normal la of electricity this conductor is heated. The magnitude of the currentgenerated depends on:_the physical dimensions of the coil;_the resistivity of the conductor and_the frequency of the current.
Linz-Donawitz (LD) or Basic oxygen steel making (BOS) ProcessThe basic oxygen steel-making process is as follows:1. Molten pig iron from a blast furnace is poured into a large refractory-lined container called a ladle. Besides the BOS vessel is one-fifth filled with steel scrap.2. The metal in the ladle is sent directly for basic oxygen steelmaking or to a pre- treatment stage. Pre-treatment of the blast furnace metal is used to reduce the refining load of sulphur, silicon, and phosphorus. In desulfurising pre-treatment, several hundred kilograms of powdered magnesium are added. Sulphur impurities are reduced to magnesium sulphide in a violent exothermic reaction. The sulphide is then raked off. Similar pre-treatment is possible for desiliconisation and dephosphorisation using lime as reagents. The decision to pretreat depends on the quality of the blast furnace metal and the required final quality of the BOS steel.3. Fluxes (lime or dolomite) are fed into the vessel to form slag, which absorbs impurities of the steelmaking process. During blowing the metal in the vessel forms an emulsion with the slag, facilitating the refining process.
Kaldo Process:The Kaldo process, is a modification of LD process. It was originally developed in Sweden by Prof.Kalling. This process is based on the advantage of evolution of heat by high phosphorus(2%) pigiron to as low as 0.02% P.The converter in Kaldo Process is inclined at 150 to 200 with the horizontal, and rotated at a speedof 25-30 r.p.m. The oxygen lance is introduced through the open end of the vessel, which also actsas the outlet for the exhaust gases. The use of oxygen allows simultaneous removal of carbon andphosphorus from the (p 1.85%) pig iron. The rotation of the converter ensures better slag-metalreaction.
MODERN STEEL MAKING PROCESSVacuum Induction Melting process:This process is similar to the induction melting process with suitable arrangement for creating avacuum. This process is used for making super alloys containing nickel and cobalt as base metals.It is very suitable process for further remelting for investment casting. Due to vacuum prevailing inthe chamber , non-metallic inclusions can be minimized and composition of chemically reactiveelements like titanium , boron and aluminium can be controlled accurately. New alloys of steelpossessing greater uniformly and reproducibility of properties accompanied by greater strength,creep resistance, etc can be produced.Consumable Electrical Vacuum Arc Melting Process: It is direct arc steel melting process in which the electrode is consumed during melting. Thisprocess was originally used for titanium. Since this process eliminates hydrogen, oxygen, andvolatile materials, it is extensively used for special-purpose steels, as in moving parts of aircraftengines, due to need of high strength, uniformity of properties, greater toughness and freedom fromtramp and volatile elements.Electric slag refining (ESR) Process:This process is commonly known as ESR. It is a larger form of the original welding process . It is theelectrical resistance heating process that remelts the preformed electrode into a water-cooledcrucible. Due to resistance to flow of current, the metal melts and drops onto the crucible through alayer of slag around the ingot. The process is used for making high alloy, high quality steels forobtaining superior properties normally not achieved in conventional processing. For example, ultrahigh strength weldable steel.
EQUILIBRIUM PHASE DIAGRAMA phase may be defined as a homogeneous portion of asystem that has uniform physical and chemicalcharacteristics.One-component or Unary Phase Diagram(P-T Diagram)An equilibrium phase diagram is a graphic mapping ofthe natural tendencies of a material or a materialsystem, assuming that equilibrium has been attainedfor all possible conditions. There are three primaryvariables to be considered:temperature, pressure, and composition. The simplestphase diagram is a pressure–temperature (P–T)diagram for a fixed-composition material. Areas of thediagram are assigned to the various phases, with theboundaries indicating the equilibrium conditions oftransition.P–T phase diagrams are rarely used for engineering applications. Most engineeringprocesses are conducted at atmospheric pressure, and variations are more likely tooccur in temperature and composition.
COMPLETE SOLUBILITY IN BOTH LIQUID AND SOLID STATESThe upper line is the liquidus line, the lowest temperature for which the material is 100%liquid. Above the liquidus, the two materials form a uniform-chemistry liquid solution. Thelower line, denoting the highest temperature at which the material is completely solid, isknown as a solidus line. Below the solidus, the materials form a solid-state solution inwhich the two types of atoms are uniformly distributed throughout a single crystallinelattice. Between the liquidus and solidus is a freezing range, a two-phase region whereliquid and solid solutions coexist. Binary Phase DiagramCONDITIONS FOR UNLIMITED SOLID SOLUBILITY1. Size factor: The atoms or ions must be of similar size, with no more than a 15%difference in atomic radius.2. Crystal structure: The materials must have the same crystal structure; otherwise,there is some point at which a transition occurs from one phase to a second phase witha different structure.3. Valence: The ions must have the same valence; otherwise, the valence electrondifference encourages the formation of compounds rather than solutions.
INTERPRETATION OF PHASE DIAGRAMSIn a phase diagram, for each point of temperature and composition, following threepieces of information can be obtained:1. The phases present: The stable phases can be determined by simply locating thepoint of consideration on the temperature–composition mapping and identifying theregion of the diagram in which the point appears.2. The composition of each phase: If the point lies in a two-phase region, a tie-linemust be constructed. A tie-line is simply an isothermal (constant-temperature) line drawnthrough the point of consideration, terminating at the boundaries of the single phaseregions on either side. The compositions where the tie-line intersects the neighbouringsingle-phase regions will be the compositions of those respective phases in the two-phase mixture.2. Amount of each phase:1. The tie line is constructed across the two-phase region at the temperature of the alloy.2. The overall alloy composition is located on the tie line.3. The fraction of one phase is computed by taking the length of tie line from the overallalloy composition to the phase boundary for the other phase, and dividing by the total tieline length.4. The fraction of the other phase is determined in the same manner.
PARTIAL SOLID SOLUBILITYMany materials do not exhibit complete solubility in the solid state. Each is often solublein the other up to a certain limit or saturation point, which varies with temperature.INSOLUBILITYIf one or both of the components are totally insoluble in the other, the diagrams will alsoreflect this phenomenon. The following Figure illustrates the case where component A iscompletely insoluble in component B in both the liquid and solid states.
The three-phase reaction that occurs upon cooling through 183°C can be written as: The lead–tin phase diagram
Figure given below summarizes the various forms of three-phase reactions that mayoccur in engineering systems, along with the generic description of the reaction shownbelow the figures. These include the eutectic, peritectic, monotectic, and syntecticreactions, where the suffix -ic denotes that at least one of the three phases in thereaction is a liquid. If the same prefix appears with an -oid suffix, the reaction is of asimilar form but all phases involved are solids. Two such reactions are the eutectoid andthe peritectoid. The separation eutectoid produces an extremely fine two-phase mixture,and the combination peritectoid reaction is very sluggish since all of the chemistrychanges must occur within (usually crystalline) solids.If components A and B form a compound, and the compound cannot tolerate anydeviation from its fixed atomic ratio, the product is known as a stoichiometricintermetallic compound and it appears as a single vertical line in the diagram
IRON–CARBON PHASE DIAGRAMSteel, composed primarily of iron and carbon, is the most important of the engineeringmetals. For this reason, the iron–carbon equilibrium diagram assumes specialimportance. We normally are not interested in the carbon-rich end of the Fe-C phasediagram and this is why the full iron–carbon (Fe-C) diagram is not normally encountered,but we examine the Fe-Fe3C diagram as part of the Fe-C binary phase Diagram.In the Figure, stoichiometric intermetalliccompound, Fe-Fe3C, is used to terminatethe carbon range at 6.67 wt% carbon.Immediately after solidification, iron formsa BCC structure called δ-ferrite. Onfurther cooling, the iron transforms to aFCC structure called γ, or austenite.Finally, iron transforms back to the BCCstructure at lower temperatures; thisstructure is called α, or ferrite. Both of theferrites (α and δ) and the austenite aresolid solutions of interstitial carbon atomsin iron.
The fourth single phase is the stoichiometric intermetallic compound which goes by thename cementite, or iron–carbide. Like most intermetallics, it is quite hard and brittle, andcare should be exercised in controlling the structures in which it occurs. Alloys withexcessive amounts of cementite, or cementite in undesirable form, tend to have brittlecharacteristics. Because cementite dissociates prior to melting, its exact melting point isunknown, and the liquidus line remains undetermined in the high-carbon region of thediagram.