METALLURGICAL CONSIDERATION OF METALS

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METALLURGICAL CONSIDERATION OF METALS:

METALLURGICAL CONSIDERATION OF METALS

INTRODUCTION :

INTRODUCTION

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Metallurgy is a science & technology of metals. Metallurgical field may be divided into.. a) Process metallurgy : deals with obtaining metals from ores. b) Physical metallurgy : study of physical & mechanical properties of metals of alloys. they have three variables : Chemical composition. Mechanical treatment. Heat treatment.

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Physical metallurgy is defined as a science which studies the relationship between composition , structure & porperties of metals & alloys as well as the laws by which these attributes vary owing to external influences, such as thermal, mechanical, chemical, electromegnetc and redioactive effect. As a common man, our application of materials used in everyday life is conditioned by our sense.. i.e. appearance, smell, feel, taste etc…

The concept of crystalline structure of metal, B.C.C.,F.C.C.,H.C.P. ::

The concept of crystalline structure of metal, B.C.C.,F.C.C.,H.C.P. :

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Primary Metallic Crystalline Structures (BCC, FCC, HCP) As pointed out on the previous page, there are 14 different types of crystal unit cell structures or lattices are found in nature. However most metals and many other solids have unit cell structures described as body center cubic (bcc), face centered cubic ( fcc ) or Hexagonal Close Packed ( hcp ). Since these structures are most common, they will be discussed in more detail.

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The body-centered cubic unit cell has atoms at each of the eight corners of a cube (like the cubic unit cell) plus one atom in the center of the cube (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. It is said to have a coordination number of 8. The bcc unit cell consists of a net total of two atoms; one in the center and eight eighths from corners atoms as shown in the middle image below (middle image below). The image below highlights a unit cell in a larger section of the lattice. The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. The bcc structure is often the high temperature form of metals that are close-packed at lower temperatures. The volume of atoms in a cell per the total volume of a cell is called the packing factor . The bcc unit cell has a packing factor of 0.68 . Body-Centered Cubic (BCC) Structure

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Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium, barium, vanadium, alpha-iron and tungsten. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure. It should be noted that there are other important mechanisms for hardening materials, such as introducing impurities or defects which make slipping more difficult. These hardening mechanisms will be discussed latter .

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The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination number of 12. The fcc unit cell consists of a net total of four atoms; eight eighths from corners atoms and six halves of the face atoms as shown in the middle image above. The image below highlights a unit cell in a larger section of the lattice. In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the bcc structure. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls that are aligned in columns and rows. Face Centered Cubic (FCC) Structure

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When a few additional balls are tossed in the box, they will not balance directly on top of the balls in the first layer but instead will come to rest in the pocket created between four balls of the bottom layer. As more balls are added they will pack together to fill up all the pockets. The packing factor (the volume of atoms in a cell per the total volume of a cell) is 0.74 for fcc crystals. Some of the metals that have the fcc structure include aluminum, copper, gold, iridium, lead, nickel, platinum and silver.

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Another common close packed structure is the hexagonal close pack. The hexagonal structure of alternating layers is shifted so its atoms are aligned to the gaps of the preceding layer. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer just like in the fcc structure. However, instead of being a cubic structure, the pattern is hexagonal. (See image below.) The difference between the HPC and FCC structure is discussed later in this section. The hcp structure has three layers of atoms. In each the top and bottom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon. The middle layer has three atoms nestle in the triangular "grooves" of the top and bottom plane. Note that there are six of these "grooves" surrounding each atom in the hexagonal plane, but only three of them can be filled by atoms. Hexagonal Close Packed (HPC) Structure

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As shown in the middle image above, there are six atoms in the hcp unit cell. Each of the 12 atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and each of the three atom in the middle layer contribute 1 atom. The image on the right above attempts to show several hcp unit cells in a larger lattice. The coordination number of the atoms in this structure is 12. There are six nearest neighbors in the same close packed layer, three in the layer above and three in the layer below. The packing factor is 0.74, which is the same as the fcc unit cell. The hcp structure is very common for elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc and zirconium .

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The face centered cubic and hexagonal close packed structures both have a packing factor of 0.74, consist of closely packed planes of atoms, and have a coordination number of 12 . The difference between the fcc and hcp is the stacking sequence. The hcp layers cycle among the two equivalent shifted positions whereas the fcc layers cycle between three positions. As can be seen in the image, the hcp structure contains only two types of planes with an alternating ABAB arrangement . Notice how the atoms of the third plane are in exactly the same position as the atoms in the first plane . However , the fcc structure contains three types of planes with a ABCABC arrangement . Notice how the atoms in rows A and C are no longer aligned. Remember that cubic lattice structures allow slippage to occur more easily than non-cubic lattices, so hcp metals are not as ductile as the fcc metals. Similarities and Difference Between the FCC and HCP Structure

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Metal Crystal Structure Atomic Radius (nm) Aluminum FCC 0.1431 Cadmium HCP 0.1490 Chromium BCC 0.1249 Cobalt HCP 0.1253 Copper FCC 0.1278 Gold FCC 0.1442 Iron (Alpha) BCC 0.1241 Lead FCC 0.1750 Magnesium HCP 0.1599 Molybdenum BCC 0.1363 Nickel FCC 0.1246 Platinum FCC 0.1387 Silver FCC 0.1445 Tantalum BCC 0.1430 Titanium (Alpha) HCP 0.1445 Tungsten BCC 0.1371 Zinc HCP 0.1332 The table below shows the stable room temperature crystal structures for several elemental metals. A nanometer (nm) equals 10 -9 meter or 10 Angstrom units.

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Space lattice During solidification of metals, their atoms take up fixed position in a regular pattern, referred as space lattice.

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Unit cell The crystal structure of a material or the arrangement of atoms within a given type of crystal structure can be described in terms of its unit cell . The unit cell is a small box containing one or more atoms, a spatial arrangement of atoms . The unit cells stacked in three-dimensional space describe the bulk arrangement of atoms of the crystal . The crystal structure has a three-dimensional shape . The unit cell is given by its lattice parameters , which are the length of the cell edges and the angles between them, while the positions of the atoms inside the unit cell are described by the set of atomic positions ( x i , y i , z i ) measured from a lattice point.

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Unit cell

The concept of solidification of metals, crystal, grain, grain boundaries, dendritic solidification, effect of cooling rates on material properties:

The concept of solidification of metals, crystal, grain, grain boundaries, dendritic solidification, effect of cooling rates on material properties

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The crystallization of a large amount of material from a single point of nucleation results in a single crystal . In engineering materials, single crystals are produced only under carefully controlled conditions. The expense of producing single crystal materials is only justified for special applications, such as turbine engine blades, solar cells, and piezoelectric materials . Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline (more than one crystal) solid forms. The moment a crystal begins to grow is know as nucleation and the point where it occurs is the nucleation point . At the solidification temperature, atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals . The final sizes of the individual crystals depend on the number of nucleation points . The crystals increase in size by the progressive addition of atoms and grow until they impinge upon adjacent growing crystal . Solidification

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a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope.

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In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal without smooth faces because its growth was impeded by contact with another grain or a boundary surface . The interface formed between grains is called a grain boundary. The atoms between the grains (at the grain boundaries) have no crystalline structure and are said to be disordered . Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains . Rapid cooling generally results in more nucleation points and smaller grains (a fine grain structure ). Slow cooling generally results in larger grains which will have lower strength, hardness and ductility.

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2. Dendrites In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages . A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching. The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. The figure to the right shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube . For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines . Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.

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During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other . Eventually , the interdendriticspaces between the dendrite arms crystallize to yield a more regular crystal. The original dendritic pattern may not be apparent when examining the microstructure of a material . However , dendrites can often be seen in solidification voids that sometimes occur in castings or welds, as shown to the right .

3. Grain-boundary:

3. Grain-boundary A grain boundary is the interface between two grains , or crystallites, in a polycrystalline material . Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep . On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve strength, as described by the Hall– Petch relationship.

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Effect of grain boundary & grain size on the properties of metal : It is difficult to polish the coarse grain metals . The greater depth of hardening is found in coarse grained steel . The toughness of coarse grain metals is less . The surface finish of coarse grain metals is poor . The coarse grains given better creep resistance at higher temperatures . During heat treatment the chances of crack formation in fine grain steel is less .

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7. Better fatigue resistance is offered by the fine grain metals. Grater stiffness, resilience & toughness are offered by the fine grain matels . Ductility & malleability of fine grain metals are more . Fine & coarse - grain structure

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The temperature-composition diagram shows the changes of structures taking0 place during heating & cooling . this diagram is known as equilibrium diagram . In this diagram on x-axis metal composition and on y-axis temperature scales are plotted . The full name of this diagram is “Thermal equilibrium diagram ”. It is also known as phase diagram constitutional diagram . The terminologies related to equilibrium diagram are as under . Definition of equilibrium diagram

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System : part of the substance which is completely separated from the surrounding . Component : basic chemical substance whose mass is constant and which can be used to form a chemical mixture . Phase : the part of material system uniformly mixed physically . Constituent : the part of multiphase mixture which can be identified separately . Liquidus : the locus of temperatures of a transformation at which solidification start . Solidus : the locus of temperature at which solidification is complete . Below solidus only solids are stable .

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Solvent : the major element of a solid solution. Solute : the minor element of a solid solution.

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A uniform mixture of substances in solid form. Solid solutions often consist of two or more types of atoms or molecules that share a crystal lattice, as in certain metal alloys. Much of the steel used in construction , for example, is actually a solid solution of iron and carbon. The carbon atoms, which fit neatly within the iron's crystal lattice, add strength to its structure. D efinition of solid solution

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The solid solution are of two types as stated below :

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Example of solid solution Brass is a solid solution of copper and zinc. Steel is a solid solution of iron and carbon . Monel metal is a solid solution of nickel & copper . Au- ag alloy is a solid solution of gold and silver . Sterling silver is a solid solution of silver and copper. Certain stainless steel are solid solution of iron chromium and nickel. Au-cu alloy is a solid solution of gold and copper .

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Size Factor : If the difference of atomic diameter of solvent and solute metal is more than 15% , the possibility of forming solution is 100 %. If this difference is more than 15% the possibility of forming solid solution is limited . The monel is an alloy of copper and nickel formed by substitutional solid solution has Atomic diameter of copper= dA 2.551 A Atomic diameter of nickel= dA 2.487A

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Chemical affinity factor : The greater affinity of two metals tends to form an intermediate phase instead of solid solution . Due to their greater affinitythe metals are further apart elements in periodic table . Crystal structure factor : The crystal lattice structure of the metals should be same, means both must possess BCC or FCC or HCP structure. The similar structure gives complete solubility . Dissimilar structure of metal does not allow them to merge into each other. For complete solubility the size factor should also be complied.

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Relative valency factor : The metal of high valency electron can dissolve in small amount in a lower valency metal . While the lower valency metals have good solubility in the higher valency metal. Al-Ni alloy, Ni has lower valency than Al , hence Ni can dissolve only 0.04% of Al and Al can dissolve 5% of Ni in their solid states.

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Substitutional solid solution : The substitutional solid solution is formed when the atoms of solute metal occupy the position of the atoms of solvent metal . Due to the size difference in atoms of the two metals, there is possibility of lattice distortion . Eg . Cu-Ni and Bi- Sb alloys . As per the arrangements of the solvent and solute atoms in the substitutional solid solution there are two types stated below : 1) Random or disordered substitutional solid solution. 2) Ordered substitutional solid solution.

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(b) Disordered substitutional solid solution : While forming the solid solution, solute atoms instead of occupying specific position get distributed random in the lattice structure of the solvent . This alloy is in the disordered condition . In this type of condition the concentration of solute atoms can vary considerably in the lattice structure.

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(c) Ordered solid solution : If an alloy od disordered condition , cooled slowly because of diffusion , its atoms get arranged . The diffusion produces uniform distribution of solvent and solute atoms move into definite orderly positions in the lattice. The structure is then known as super lattice or ordered substitutional solid solution . The prolonged annealing of the alloy can produce still more uniform and ordered solid solution.

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(d) Interstitial solid solution : When the solute atoms are smaller in size than solvent atoms , this type of solid solution is formed . The solute atoms in this case occupies the vacant space between the solvent atoms. Due to mixing of small atomic radii elements like carbon , nitrogen and hydrogen in the metal interstitial solid solution is formed. In this type of solid solution the constituent are in flexible proportions . They have very strong bonding force between, The alloys Fe3C and Fe4N are solid solution of this type. This solid solution is shown in the figure.

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Thermal & electrical properties are reduced due to the formation of solid solution . The strength and hardness of all the alloy metals increase due to solid solution . Solid solution reduces the ductility & malleability of the alloys . The density , specific heat & heat distribution get changed according to the proportions of alloying elements . By changing the proportion of the alloying element there is no possible to changed the stiffnes any more . Solid solution alloys exibit differential freezing . Solid solutions are conductors but not as good as the pure metals on which the are based . Properties of solid solution

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An alloy is an intimate mixture with metallic properties and is composed of tow or more element of which at least one is a metal . Alloy can be formed by combining the different element in different ways. The constituents of the solid solution cannot be detected by the microscope. It is simply a solution in the solid state consisting of tow kinds of atoms combined in one type of space-lattice. Definition of an alloy Binary and ternary alloy pure metals are considered as single phase like wise the solid solution is also a single phase . If pure metal and chemical compound are in existence together in an alloy then in that alloy there exists two phases which are different from eachother .

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In the way solid solution & pure metal together will give tow phase . This phase formed by physical & chemical mixture occupies different positions with respect to one-another . The relationship between the no. of phase in equilibrium (p) no. of component (c) and the no. of degree of freedom (f) for an alloy system is determined by Willard gibbs phase rule stated below . P= c+e-f Where e = no. of environmental factors such as temperature pressure electric fields magnetic fields etc . If temperature is the only consideration we have p+f =c+1 Now if no of degree of freedom f=0 c=2 e=1 \

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P=2+1-0=3 binary alloy. If f=0 c=3 and e=1 then P=3+1-0=4 this is ternary alloy .

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Choice of alloy : Alloys are preferred to pure metal because : They have higher strength . They have lower melting point and hence easier to shape by casting . They are harder and hence more durable . They offer higher resistance to air acid action . They have more luster and can also retain it better . They offer more resistance to electricity.

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Alloys are of various types and hence generally classified as : Single phase, Two phase etc … based on metallurgical structure . Alluminium alloys , copper alloys etc … based on principal alloys . Cast alloys, Wrought alloys … based on method of fabrication . Solder alloys, bearing alloys… based on application . Binary ternary etc… based on number of elements mixed .

Cooling curves for pure metals & alloys ::

Cooling curves for pure metals & alloys : When metal solidifies from liquid phase, structural changes occur . During this process for a period of time the temperature does not fall . Heat is still being lost in a form of latent heat which is compensated for, by heat from the kinetic energy released by the atoms when they solidify . When pure metal is allowed to cool down from higher temperature and there is no change in its state, we have the cooling curve as shown in fig. This curve is uniform & regular from the start to the end .

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The material which is non-crystalline, like glass when cool-down, this type of cooling curve is obtained. When salt is mixed is pure metal, the curve as shown in fig. is obtained . Upto the poind ‘q’, curve pq shows regular cooling . Qr represents the transformation into solid & latent heat liberation . While rs represents the cooling of solid state . It means that the liquid metal is converted completely into solid at the pt , r & ‘ qr ’ is the solidification process .

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In the fig. , the point-’x’ is available due to super-cooling . Due to super cooling the formation of nuclius & crystal takes place at maximum rate . During solidification due to liberation of latent heat this process slows down and solidification takes place . There is much variation in solubility in each other in case of binary alloy . The elements of some alloys, the alloying elements are completely soluble in complete range in both liquid & solid state e.g. copper-Nickel, Potassium-Rhodium, silver- plantinum .

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The cooling curve for them is shown in fig.The alloying elements of certain alloys are insoluble in liquid and solid state e.g. cooper-molybdenum, silicon-titanium, etc., while some of them are soluble in liquid state but partly soluble in solid state e.g., Iron-zinc, silver-nickel, cadmium-zinc etc . The cooling curve shown in fig. is for the alloy whose alloying elements are soluble in liquid state but partly soluble or insoluble in solid state . During this cooling the crystal of element is obtained during “ xy ” remaining liquid solidifies during yz during which temperature remains constant . After “z” the cooling of solid state takes place.

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Sequence of steps for plotting the equilibrium diagram with the help of cooling curves : There are many alloys used for industrial production . Therefore it is difficult to remember the properties of each of them and also to classify the metals . The equilibrium is one of the basis for classifying the metal . The reclassification of metal is done on the ground of solubility of alloying element in liquid and solid states . Therefore the classification is based on the formation of intermediate phase or solid solution .

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Generally the following types of equilibrium diagrams are plotted : Type- I -> Solid Solution Type- II -> System Eutectic type Type-III- > Eutectoid type Type-V -> Peritectic & Peritectoid system. The equilibrium diagram of solid solution of Gold (Au) and silver (Ag) is already shown in fig. The gold & silver are the element soluble in both liquid and solid states . The possible contents of shown in the fig. The melting points & freezing points of each alloy can be shown by the temperatures on vertical lines.

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The vertical lines in the above fig. shown the melting point & freezing point of the alloys formed by various proportion of the alloying elements . The points 1, 2, 3 and 4 are the melting points and corresponding freezing points are 5, 6, 7 and 8. By the points 1, 2,3 & 4 we get liquidus line and by joining 5, 6, 7 and 8 we get solidus line . Following steps are followed to draw the equilibrium diagram : Show composition on x-axis & temperature on y-axis . Show the different compositions of the alloys on x-axis . Draw cooling curves of each alloy .

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Show melting point & freezing point of the alloy on each curve. The melting point & freezing point of the alloy on each curve . The melting point and freezing points are such points where a change of state occurs. E.g., solid -> solid liquid <- liquid etc . Draw a curve joining each melting point. The line obtained is the liquids line above this line & at lower temperatures the metal is in solid state . Draw a curve joining each freezing point. The line obtained is the solidus line below this line & at lower temperatures the metal is in liquid as well as in solid states.

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On the basis of relation between alloying elements in liquid and solid states, the equilibrium diagram is classified as Components completely soluble in solid state . Completely soluble in solid state. Party soluble in solid state Insoluble in solid state. Paratactic reaction . Components Partly soluble in liquid state but Completely soluble in solid state. Party soluble in solid state . Components completely insoluble in liquid and solid state. Classification of Equilibrium Diagram:

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1) Components completely soluble in liquid & solid state

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Antimony-Bismuth , Nickel-Cooper. Gold-Silver, chromium-molybdenum and tungsten- molybdenum alloy represents the solid solution of included in this equilibrium diagram . The conditions for unlimited solubility in solid state are (1) the lattice & crystal system of the alloy should be similar, and (2) the size of their atoms should be equal . If the difference of size of atom is more there 15 percent , the crystal lattice of the solvent gets distorted, due to which the solid solution can’t be formed . Therefore the substitution solid solution of the two metals which are completely soluble in solid state can be formed . Two metals completely soluble in liquid & solid state :

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By studying the equibrium diagram we know that only two lines are generated, which the solidus and liquidus lines . Above the solidus line, there exists solid solution of any alloy. In between the solidus & liquidus lines, the liquid & solid solution are mixed in each other . Considering the alloy of 70% Antimony and 30% Bismuth as shown in fig. with the cooling commences immediate freezing at 582 temperatures . At point’s components of the alloy becomes solid . At point N the complete alloy slowly reaches at room temperature and becomes completely solid.

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An isothermal line drawn paralled to x-axis, intersects the solidus & liquidus lines in points ‘O’ and ‘P’ respectively . These are the points at 482 c temperature, where solid and liquid phases are in equilibrium condition . At point ‘O’, 14% bismuth is present in the solid phases while at point p about 62% bismuth is present in the liquid phase . At temperature 649 c & point1, 30% bismuth and 70% antimony alloy is in single liquid phase . At temperature 482 c & point-2 this alloy exist in solid & liquid i.e. in solid state the chemical composition is not uniform. Therefore non uniform structure is formed. Due to the chemical composition difference the grains of alloy are heterogeneous or cored.

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Eutectic System:

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The eutectic system consists a specific alloying element which forms eutectic mixture at minimum temperature in comparison to other chemical compostions . The elements of an alloy solidify at constant temperature and components of the system solidify at lowest temperature is known as eutectic alloy . The eutectic system is two types as stated below : Type – II ->In this system two pure metals are insoluble in the solid state . Type – III ->In the system two metals are completely soluble in liquid state, But solubility decreases with the decrease in temperature. Therefore this is known as the partly soluble system.

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The component of the liquid solution solidify at constant temperature in eutectic reaction and forms two or more phases is known as eutectic system . In this system the components which become solid at lower temperature is known as eutectic alloy . The point obtain due to solidification is known as eutectic system. Eutectic system:

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The alloy I soluble in liquid state but pure metal is solid state has laser solubility after solidification both pure metals get crystalised separately . The decrease of solid solubility with the decrease in temperature happens. Many alloys of this type shown the eutectic reaction. Two metals completely soluable in the liquid state but insoluble in the solid state:

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In this type of solid solution it is impossible for the two pure metals remaining insoluble, as per technical view point. In many cases the solubility it is said as insoluble e.g. Tin-zinc, Bismuth-cadmium etc. Now consider the compositions, 1, 2 and 3 of the equilibrium diagram given in fig. to understand this type of eutective system .

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Point -1 - > 20% cd + 80% Bismuth With the mixture of cadmium, first pure Bismuth crystals are started forming. Due to reduction of Temperature to 140 c, the liquid composition comes to point E Where 40% cadmium & 60% bismuth becomes solid. In this system There is possibility of differed proportions & coring to takes place. Point- 2 -> 40% cadmium + 60% bismuth Till it reaches to a temperature of 140 c, the eutectic composition Does not becoming solid. At eutectic temperature two pure metal Crystalline together and good characterized mass known as eutectic Is formed. This alloy formed is this way is known as eutectic alloy. Point-3- > 80% cadmium + Bismuth As shown in fig. at temperature T1 the nucleus of pure cadmium is Formed. When temperature reduces T2 from nesleus of the crystal Dendritic growth starts. When temperature reduces to 140 c from T2, during this period the dendrites grows & rich mass of the Bismuth forms. At eutectic temperature 140 c remaining 40% Cadmium + 60% bismuth forms the eutectic composition. Thus We can say that in this type of system two metals are completely Insoluble in solid state.

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c) Two metals completely soluble in liquid state but party soluble in solid state : In this case allotropic transformation of alloy formed by solid solution takes place. Due to that solid solution is divided into two phases. The two phases are obtained here is due to the liquid-solid transformation. The reaction known as eutectoid reaction is as under: These types of reactions are seen in Al-cu, Cu-Zn and Iron-Carbon diagram. Fig. shows the equilibrium diagram for the lead-tin alloy. The melting points of lead and tin are 327 c and 232 c respectively are shown at point A and C respectively. If tin is added in lead or lead is added in molten tin, the freezing point goes down. Which is shown in the diagram by the lines AE and CE. As a result of which at temperature 183 c or at eutectic point 61.9% tin and 38.1 % lead forms eutectic alloy.

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This type of system has the freezing characteristic as described in type-III. Now by studying the equilibrium diagram given in fig. following explanation can be given : From solidus lines AB & CD it is clear that in solid state maximum 19.5% tin gets dissolved in lead and 2.6% lead dissolves in tin, which has maximum solubility upto eutectic temp 183 c, which is similar to type-I . Now consider 90% lead and 10% tin alloy. Its solidification starts from point X and completes at point y and in between x and y in any part, the alloy is partly solid & partly liquid. Therefore at any temp. Solidification in the xy range is shown by line Y1Y, where crystals get separated and the remaining is shown by liquids line xx1. Therefore the cooling of the alloy is show, which creates diffusion and uniform solid solution of 90% lead and 10% tin is obtained.

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Due to the solubility of tin in lead, BF line shows the fall of temperature. Therefore within yz temp. Range, without any change the solid solution of 90% lead and 10 % tin remains unaltered. The solubility of tin in lead decreases at temperature which is seen from the ZF line . Alloy containing 19.5% Tin when reaches to eutectic temperature of 183 & when solidification starts then 19.5% tin & 80.5% lead as saturated solid solution separated and at eutectic temp it becomes soli permanently, which is the saturated solution of composition, which is seen from the points B and D. in solid solution there are 19.5% Tin & 80.5% lead and solid solution there are 97.4% Tin and 2.6% lead are present . Due to fast cooling in atmosphere saturated solid solution is converted into super saturated solid solution. But after some time due to precipitation the hardness and strength of alloy increases. If alloy is heated then precipitation rate in short duration increases. Therefore this change is utilized as a principle in age hardening .

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The lead and antimony is used in bearing metal. Silver & copper alloy is used in coins.

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The alloying elements completely soluble in liquid state and limited soluble in solid state when alloyed and plotted on equilibrium diagram, they represent peristaltic type of alloy. The solid solution crystal after solidifying reacts with definite composition of liquid alloy forming solid solution of which new crystal forms. The paratactic reaction is not common. This reaction takes place at constant temperature. The reaction taking place during solidification of alloy is known as peristaltic reaction, where the solid phase of different composition reacts with liquid phase. (D) Peritectic Reaction:

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The following information can be obtained from the equilibrium diagram . The melting & freezing temperatures of an alloy for the proportion of constituents present in it . At a given temperature, number of constituents, their types and proportions is an alloy, can be found . The proportions of each phase can be known in two phase alloy . At given a temperature what type of microstructures present in an alloy can be assumed . Information obtained from equilibrium diagram:

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It is guide for heat treatment as it provides information related to the type of changes taking place transformation. The information of phase of alloy at a point can be known from the temperature & composition . At a given temperature, from the diagram proportions of the constituent present at any point in an alloy, can be found.

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This rule gives us the information of number of phase and the chemical composition of the phase. This rule is useful to find the percentage of phases in two phase system . In fig. the point E lies in the portion between the solidus and liquidus lines . By drawing horizontal line from this point E, the lever rule can be applied . At temp. ‘t’ the horizontal line GH shows the weight of both the constituents . Lever rule

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The GE line shows the phase that has achieved is solid state. The line EH shows the phase, Which is still in liquid state . % of phase in solid state = length GE / length GH * 100 % of phase in liquid state = length EH / length GH * 100 As per rule point J represents 100 % solid state & point K represents 100 % liquid state. This rule can be applied to any binary equilibrium diagram where two phase are present.

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The phases finally formed in a heat affected zone during cooling or subsequent heating depend upon time and temperature. TTT diagram shows the time required for transformation to various phases at constant temperature, and, therefore, gives a useful initial guide to likely transformations . The TTT diagram for an eutectoid (about 0.8%) carbon steel. Austenite is stable above A 1 temperature line, and below this line, austenite is unstable, i.e., it can transform into pearlite, bainite or martensite . In addition to the variations in the rate of transformation with temperature, there are variations in the structure of the transformation products also. Time Temperature Transformation TTT Diagram -

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Transformations at temperatures between approximately 1300°F (705°C) and 1020°F (550°C) result in the characteristic lamellar microstructure of pearlite. At a temperature just below A 1 line, nucleation of cementite from austenite will be very slow, but diffusion and growth of nuclei will proceed at maximum speed, so that there will be few large lamellae and the pearlite will be coarse. However, as the transformation temperature is lowered, i.e., it is just above the nose of the C-curve, the pearlite becomes fine . At temperature between 1020°F and 465°F (the approximate, Ms temperature line), transformation becomes more sluggish as the temperature falls, for, although austenite becomes increasingly unstable, the slower rate of diffusion of carbon atoms in austenite at lower temperatures outstrips the increased urge of the austenite to transform .

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From the same bar cut large number of small specimens . Place the specimen quickly in molten salt bath held at the proper austinitizing temperature. For 1080- eutectoid steel the approximate temperature of bath is 1425 c. keep them in the bath for long enough to complete the formation of austenite . Transfer the specimen quickly in other molten bath held at about 1310 F . Allow the specimen to react isothermally for certain time and then quench in cold water or iced brine . The quenching of specimen in cold water stops isothermal reaction & cause the remaining austenite to change almost instantly to martensitic. Steps to construct a T.T.T. diagram:

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When large number specimens isothermally reacted for varying time periods are matallographically examined, the result is the reaction curve. When data is obtained from a series of isothermal reaction curves over the whole temperature range of austenite instability for a given composition of steel is summarized, the result is T.T.T. diagram for steel.

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To understand the ferrous metallurgy with ease, iron-carbon diagram is important. With the help of this diagram heat-treatment of steel, effect of alloying element on alloy steel and special purpose steel, the properties of steel specially used for construction can be understood very easily. The equilibrium diagram can be produced by plotting the changing of structure happening due to heating & cooling of metals. The Iron-carbon diagram shows the changes occurred in iron by varying the percentage of carbon . By studying the iron-carbon equilibrium diagram shown in fig., the clarification of following points are obtained. Iron-carbon Equilibrium diagram and its characteristics:

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Iron has body centered atomic structure and is generally known as ‘ferrite .’It is magnetic . Gamma iron has face centered structure & is not possessing magnetic effects . Iron-carbide, Fe3cC is generally known as cementite . Pure iron metals at 1539 c, which is represented by ‘A’ in fig.,is containing carbon element in liquid mixture . Iron-carbon mixture containing 1.7% of carbon is known steel. While the mixture containing more than 1.7% of carbon is known as cast-iron.

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When carbon mixes in liquid iron then the solidification of iron-carbon mixture is decreased, which is shown in the diagram by the line AE . At point ‘A’ 100% iron is present and has solidification temp. of 1539 c. when more & more carbon is added in liquid iron its solidification temperature slowly drops up to point E, where the temperature is 1130 c and carbon 4.3% in iron carbon mixture by adding carbon more than is 4.3% the solidification temperature start increasing which is represented by line CD in the diagram. When carbon in steel is 0.8%, the solidification of steel proceeds along the line XYSZ. The steel containing up to 0.8% carbon is known as hypo eutectoid steel and steel containing 0.8% to 2.0% of carbon is known as hyper eutectoid steel . When the temperature of molten steel falls, the solidification starts, which is shown as point X at 1430 c the process stops at 1370 c temp at point Y. It means the solidification carbon steel does not takes place at any specific temp but it has the range of solidification. Steel is partly liquid and partly solid in between the line XY. The solidification starts and stops and points X and Y respectively .

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At point Y iron is in from & in solid solution there is 0.8% of carbon. It is known as austenite. Here the carbon chemically units with iron, in other words its can be side as uniform solid solution of iron & iron carbide . When temp falls from Y to S, the iron and cemented from solid solution temp falls below points, the iron is transformed in to iron. By studying the diagram it can be learnt that the iron can dissolved 0.025% carbon iron 0.7% carbon and iron 0.1% of carbon this the maximum capacity of them to dissolve the carbon the. The alpha, gamma, delta irons are generally known as ferrite, austenite and delta ferrite .

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Uses iron-carbon equibrium diagram : Predict number types and compositions of phases . Predict amount of phases . Guide heat treater . Predict micro structure of an alloy.

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It does not give information about the of ferrite or cementite from ejected from austenite on cooling . It does not give information about the size of micro constituents present . It does not give information about physical & mechanical properties of the material. Limitation of Iron-carbon equilibrium Diagram:

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