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UNIT – I CONSTITUTION OF ALLOYS

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  1. UNIT – I CONSTITUTION OFALLOYS AND PHASEDIAGRAMS Constitution of alloys – diagrams, isomorphous, eutectoid, eutectic, Solid solutions, substitutional and interstitial – Phase peritectic, and peritectroid reactions – Iron – Iron carbide steel and cast iron, equilibrium diagram – Classification of microstructure, properties andapplications. 2

  2. CRYSTALLIZATION • Crystallization is the transition from the liquid to the solid state or transformation of liquid phase to solid crystallinephase. • It occurs in twostages, • Nucleus formation - Nucleation is a process of formation of stable crystallization centers of a newphase. • Crystal growth - atoms attaching themselves in certain preferred directions, usually along the axes of acrystal. 3

  3. NUCLEATION Nucleation may occur by either homogeneous or heterogeneous mechanism. Presence of foreign particles or other foreign substance in the liquid alloy allows to initiate crystallization at minor value of under cooling (few degrees below the freezing point). This is heterogeneous nucleation. If there is no solid substance present, under cooling of a hundred degrees is required in order to form stable nuclei or “seeds” crystals, providing homogeneousnucleation. 4

  4. CRYSTALGROWTH Number of stable nuclei per unit volume of crystallizing alloy determines the grain size. The difference in potential energy between the liquid and solid states is known as the latent heat offusion. When the temperature of the liquid metal has dropped sufficiently below its freezing point, stable aggregates or nuclei appear spontaneously at various points in theliquid. These nuclei, which have now solidified, act as centers forfurther 5 crystallization.

  5. CRYSTALGROWTH 5. As cooling continues, more atoms tend to freeze, and themselves to already existing nuclei or form newnuclei. attach Each nucleus grows by the attraction of atoms from the liquid into its space lattice. Crystal growth continues in three dimensions, the atoms attaching themselves in certain preferred directions, usually along the axes of a crystal. This gives rise to a characteristic treelike structure which iscalled 6 DENDRITE.

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  7. MECHANISM OFSOLIDIFICATION NUCLEUS FORMATION GROWTH OFCRYSTALLITES GRAIN BOUNDARIES 8

  8. PROCESS OFCRYSTALLIZATION AND DENDRITICGROWTH 9

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  10. GRAIN ANDGRAIN BOUNDARY Since each nucleus is formed by chance, the crystal axes are pointed at random and the dendrites will grow in different directions in each crystal. Finally, as the amount of liquid decreases, the gaps between the arms of the dendrite will be filled and the growth of the dendrite will be mutually obstructed by that of itsneighbors. This leads to a very irregular external shape called asgrains. The area along which crystals meet, known as the grain boundary, is a region ofmismatch. 11

  11. ORMATION OF DENDRITES IN MOLTENMETAL F GRAINBOUNDARY 12

  12. POLYMORPHISM AND ALLOTROPYOF METALS Polymorphism is a physical phenomenon where a material may have more than one crystal structure. A material that shows polymorphism exists in more than one type of space lattice in the solid state. If the change in structure is reversible, then the polymorphic change is known as allotropy. The prevailing crystal structure depends on both the temperature and the external pressure. 13

  13. POLYMORPHISM AND ALLOTROPYOF METALS Polymorphism example is found in carbon: Graphite is the stable polymorph at ambient conditions, whereas Diamond is formed at extremely highpressures. The best known example for allotropy is iron. Whenironcrystallizes at 2800 oF it is B.C.C. (δ -iron), at 2554oFthe structure changes to F.C.C. (γ -iron or austenite), and at 1670 oF it again becomes B.C.C. (α -iron orferrite). 14

  14. ALLOTROPIC BEHAVIOROF PUREIRON 15

  15. CRYSTALIMPERFECTIONS (DEFECTS) • The perfectly regular crystal structure are called as IDEAL CRYSTALS in which atoms are arranged in regularway. • In actual crystals, imperfections or defects are always present, which affect the properties ofcrystals. • The crystallographic defects are classified as, • Point defects or Zero dimensionaldefects. • Line defects or One dimensionaldefects. 3. Surface defects or Plane defects or Two dimensionaldefects. 16

  16. POINTDEFECTS Vacancy – missing atom at a certain crystal latticeposition. Interstitial impurity atom – extra impurity atom in an interstitial position. Self-interstitial atom – extra atom in an interstitial position Substitution impurity atom – impurity atom, substituting an atom in crystal lattice. Frenkel defect – extra self-interstitial atom, responsible for the vacancynearby 17

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  18. LINEDEFECTS Linear crystal defects are edge and screwdislocations. Edge dislocation is an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics: ductility andmalleability. Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part. It is called screw as atomic planes form a spiral surface around the dislocationline. 19

  19. LINEDEFECTS SCREWDISLOCATION EDGEDISLOCATION 20

  20. SURFACEDEFECTS Planar defect is an imperfection in form of a plane between uniform parts of thematerial. Important planar defect is a Grain boundary. Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specificaxis. Tilt boundary – rotation axis is parallel to the boundaryplane; Twist boundary - rotation axis is perpendicular to the boundary plane. 21

  21. SURFACEDEFECTS Diffusion along grain boundaries is much faster, than throughout the grains. Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and theirproperties. 22

  22. SOLIDSOLUTIONS A solid solution is simply a solution in the solid state and consists of two kinds of atoms combined in one type of spacelattice. Any solution is composed of two parts a solute and asolvent. The solute is the minor part which is dissolved and the solvent is the major portion of thesolution. The amount of solute that may be dissolved by the solvent is generally a function of temperature, which usually increases with increasing temperature. 23

  23. SOLIDSOLUTIONS There are three possible conditions of asolution, Unsaturated – The solvent is dissolving less of the solute than it could dissolve at a given temperature andpressure. Saturated – The solvent is dissolving the limiting amount ofsolute. Supersaturated – The solvent dissolves more than the solute than it should under equilibrium conditions. 24

  24. SUBSTITUTIONAL SOLID SOLUTION If the atoms of the solvent metal and solute element are of similar sizes (not more, than 15% difference), they form substitution solid solution, where part of the solvent atoms are substituted by atoms of the alloying. Example – Cu-Ni TYPES Ordered Disordered Ni Cu 25 NSK -AAMEC

  25. INTERSTITIAL SOLIDSOLUTION 1. If the atoms of the alloying elements are considerably smaller, than the atoms of the matrix metal, interstitial solid solution forms, where the matrix solute atoms are located in the spaces between large solvent atoms. Smaller Atoms Hydrogen, Carbon, Boron andNitrogen

  26. INTERSTITIAL SOLIDSOLUTION The interstitial solution of carbon in iron constitutes the basis of steelhardening. Very small amount of hydrogen introduced into steels during acid picking (cleaning), plating or welding operations cause a sharp decrease in ductility known as Hydrogenembrittlement. 27

  27. PHASEDIAGRAM Phase Diagram or Equilibrium Diagram or Constitutional Diagrams indicate the structural changes due to variation of temperature and composition. The diagram is essentially a graphical representation of an alloy system. The phase diagram will show the phase relationships under equilibriumconditions. Phasediagramsareplottedwithtemperatureinordinateandalloy 28 composition in weight % as theabscissa.

  28. GIBBS PHASERULE F = C  P +2 F – Degrees ofFreedom C – Number ofComponents P – Number ofPhases For a system inequilibrium 2 = Temperature and Pressure or F  C + P =2 F C +2 P  = What you cancontrol What the systemcontrols Degrees ofFreedom  = Can control the no. of components added andP &T System decided howmany phases to produce given the conditions 29

  29. CLASSIFICATION OFPHASE DIAGRAM UNARY - One component phasediagram. BINARY - Two component phasediagram. TERNARY - Three component phasediagram. 30

  30. UNARY PHASEDIAGRAM The simplest case-Water. Also known as a P-T diagram Sign of [dP/dT]for: Solid-Liquid Liquid-Gas Gas-Solidequilibria 31

  31. BINARY PHASEDIAGRAM Copper-Nickel equilibriumdiagram 32

  32. TERNARY PHASEDIAGRAM 33

  33. PHASEDIAGRAM SYSTEM – A system is a substance so isolated from its surroundings that it is unaffected by these and is subjected to changes in overall composition, temperature,pressure. COMPONENT – A component is a unit of the composition variable of the system. A system that has one component (Unary), two (Binary), three (Ternary) and four (Quaternary). PHASE – A phase is a physically and chemically homogeneous portion of the system, separated from the other portions by asurface, 34 theinterface.

  34. CLASSIFICATION OF PHASEDIAGRAM Phase diagrams are classified according to the relation of the components in the liquid and solidstates. 1. Components completely soluble in the liquidstate, And also completely soluble in solid state. (Isomorphousreaction) 1. But partly soluble in the solid state. (Eutecticreaction). 2. But insoluble in the solid state. (Eutecticreaction) 3. 2. Components completely partially soluble in liquidstate, But completely soluble in the solidstate. 1. And partly soluble in the solidstate. 2. 3. Components completely insoluble in the liquid state and completely insolublein the solidstate. 35

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  36. IRON – IRON CARBIDE EQUILIBRIUMDIAGRAM The following phases are involved in the transformation, occurring with iron-carbon alloys: 1. L - Liquid solution of carbon iniron; 2. δ-ferrite – Solid solution of carbon iniron. Maximum concentration of carbon in δ-ferrite is 0.09% at 2719 ºF (1493ºC) – temperature of the peritectictransformation. The crystal structure of δ-ferrite is BCC (cubic bodycentered). 37

  37. 4. Austenite – interstitial solid solution of carbon inγ-iron. Austenite has FCC crystal structure, permitting high solubility of carbon – up to 2.06% at 2097 ºF (1147ºC). Austenite does not exist below 1333 ºF (733ºC) and maximum carbon concentration at this temperature is0.83%. α-ferrite – solid solution of carbon inα-iron. α-ferrite has BCC crystal structure and low solubility of carbon – up to 0.25% at 1333 ºF (733ºC). α-ferrite exists at roomtemperature. Cementite – iron carbide, intermetallic compound, having fixed composition Fe3C. It is hard andbrittle. 38

  38. The following phase transformations occur with iron-carbon alloys: Alloys, containing up to 0.51% of carbon, start solidification with formationofcrystalsofδ-ferrite.Carboncontentin δ-ferrite increases up to 0.09% in course solidification, and at 2719 ºF (1493ºC) remaining liquid phase and δ-ferrite perform peritectic transformation, resulting in formation ofaustenite. Alloys, containing carbon more than 0.51%, but less than 2.06%, form primary austenite crystals in the beginning of solidification and when the temperature reaches the curve ACM primary cementite stars to form. 39

  39. 3. Iron-carbon alloys, containing up to 2.06% of carbon, are called3. Iron-carbon alloys, containing up to 2.06% of carbon, are called steels. Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic transformation at 2097 ºF (1147 ºC). The eutectic concentration of carbon is4.3%. In practice only hypoeutectic alloys are used. These alloys (carbon content from 2.06% to 4.3%) are called cast irons. When temperature of an alloy from this range reaches 2097 ºF (1147 ºC), it contains primary austenite crystals and some amount of the liquid phase. The latter decomposes by eutectic mechanism to a fine mixture of austenite and cementite, calledledeburite. 40

  40. 6. All iron-carbon alloys (steels and cast irons) experience eutectoid transformation at 1333 ºF (733ºC). The eutectoid concentration of carbon is 0.83%. 7. When the temperature of an alloy reaches 1333 ºF (733ºC), austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions). 41

  41. CRITICALTEMPERATURES Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoidalloys. Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoidalloys. Lower critical temperature (point) A1 is the temperature ofthe austenite-to-pearlite eutectoid transformation. Below this temperature austenite does notexist. Magnetic transformation temperature A2 is the temperaturebel4o2w

  42. Phase compositions of the iron-carbonalloys at roomtemperature Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of primary (proeutectoid) ferrite andpearlite. Eutectoid steel (carbon content 0.83%) entirely consists ofpearlite. Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid)cementite andpearlite. Cast irons (carbon content from 2.06% to 4.3%) consist of proeutectoid cementite C2 ejected from austenite according to the curve ACM, pearlite and transformed ledeburite (ledeburite in which austenite transformed to pearlite). 43

  43. CLASSIFICATIONOF STEEL Classification of steels bycomposition Carbonsteels Low carbon steels (C <0.25%); Medium carbon steels (C =0.25% to0.55%); High carbon steels (C >0.55%). 44

  44. DESIGNATION OFSTEEL American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE) have established four-digit (with additional letter prefixes) designation system: SAE1XXX First digit 1 indicates carbon steel (2-9 are used for alloysteels); Second digit indicates modification of the steel. 0 - Plain carbon, non-modified 1 - Resulfurized 2 - Resulfurized andrephosphorized 5 - Non-resulfurized, Mn over1.0% Last two digits indicate carbon concentration in0.01%. 45

  45. A letter prefix before the four-digit number indicates thesteel makingtechnology: 1. A - Alloy, basic openhearth B - Carbon, acidBessemer C - Carbon, basic openhearth D - Carbon, acid openhearth E - Electricfurnace Example: AISI B1020 means non modified carbon steel, produced in acid Bessemer and containing 0.20% ofcarbon. 46

  46. 1. Low alloy steels(alloyingelements <8%); High alloy steels (alloying elements >8%). According to the four-digit classification SAE-AISIsystem: First digit indicates the class of the alloysteel: 2- Nickel steels; 3- Nickel-chromiumsteels; 4- Molybdenumsteels; 5- Chromiumsteels; 6- Chromium-vanadiumsteels; 7- Tungsten-chromiumsteels; 9- Silicon-manganese steels. Second digit indicates concentration of the major element in percents (1 means 1%). Last two digits indicate carbon concentration in0,01%. Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% ofcarbon. 47

  47. CLASSIFICATION OFSTEELS BYAPPLICATION Stainless steels AISI has established three-digit system for the stainlesssteels: 2XX series – chromium-nickel-manganese austenitic stainlesssteels; 3XX series – chromium-nickel austenitic stainless steels; 4XX series – chromium martensitic stainless steels or ferritic stainless steels; 5XX series – low chromium martensitic stainlesssteels. 48

  48. TOOL AND DIESTEELS Designation system of one-letter in combination with a number is accepted for tool steels. The letter means: W - Water hardened plain carbon toolsteels; O - Oil hardening cold work alloy steels; A - Air hardening cold work alloysteels; D -Diffused hardening cold work alloysteels; S – Shock resistant low carbon toolsteels; T – High speed tungsten toolsteels; M - High speed molybdenum tool steels; H – Hot work toolsteels; 49

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