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Unit 3 PPT.pptx

  1. 1. Unit – 3, Solidification, Phase Diagram, Iron Carbon system Prof. A. V. Dube
  2. 2. UNIT-III Solidification, Phase Diagram, Iron Carbon system Solidification: Mechanism of solidification, Homogenous and Heterogeneous Nucleation, crystal growth. Cast metal structures. Phase diagram: Diffusion in solids, Fick’s laws of diffusion, Solid solutions, Hume-Rothery rules, substitution, and interstitial solid solutions, Gibbs phase rule, construction of equilibrium diagrams, equilibrium diagrams involving complete and partial solubility, lever rule. Numericals on phase diagrams. Iron Carbon system: Iron carbon equilibrium diagram description of phases, Solidification of steels and cast irons and invariant reactions
  3. 3. Basics of unit 3
  4. 4. System A system is a set of interacting or inter dependant components parts forming a complex / intricate whole is called system. Or the substances that are isolated and unaffected by their surrounding are known as system Or A part of the universe under study is called system. Phase A phase is defined as homogeneous, physically distinct and mechanically separable part of the system under study. Variable A particular phase exists under various conditions of temperature, pressure and concentration. These parameters are called as the variables of the phase
  5. 5. Component • The component of the system may be elements, ions or compounds. They refer to the independent chemical species that comprise the system is called as component. • Cu-Au
  6. 6. Solid solution. • It is an alloy in which, the atoms of solute are distributed in the solvent and has the same structure as that of the solvent. • Solid Solution have different compositions with similar structure and are like liquid solutions as sugar in water. • The atoms of one element become a part of the space lattice of the other element, thus forming a solid solution. • The element present in the alloy in the largest proportion is referred as base metal or parent metal or solvent and the elements are referred as alloying elements or solute.
  7. 7. Substitutional Solid Solution
  8. 8. • The solute may incorporate into the solvent crystal lattice substitutionally by replacing a solvent particle in the lattice. • Solute atoms sizes are roughly similar to solvent atoms. • Due to similar size solute atoms occupy vacant site in solvent atoms
  9. 9. In interstitial solid solution, the atoms of alloying elements occupy the interstitial sites of the base metal.
  10. 10. • Many substances exist in more than one stable crystalline form. The various forms have the same composition but different crystal structures. • Change in crystal structure is observed due to either change in pressure or temperature or both. Such a change of structure is called polymorphism. (Observed in element and chemical composition)
  11. 11. • Some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon known as polymorphism. • When found in elemental solids, the condition is often termed allotropy.
  12. 12. • Enantiotropy Enantiotropy forms are mutually transformable reversibly at some temperature. • Examples: Fe, Zr, Ti, etc. • Monotropy Monotropic forms are irreversible in the solid state and cannot be transformed one into the other. Monotropic transition occurs at a temperature above the melting point of the material . • Examples: Phosphorus, Alumina etc.
  13. 13. Mechanism of solidification
  14. 14. • Casting Process. • Solidification involves two steps. 1) Nuclei of a solid phase (crystallite) form from the liquid. 2) Solid crystallites begins to grow as an atoms until complete liquid solidifies. • When the metal is cooled below its melting point, nuclei begin to form in different parts of the melt at the same time. • The rate of formation of nuclei depends upon the degree of undercooling or super cooling and on the presence of impurities which mainly facilitate nucleation
  15. 15. • At any temperature below the melting point, a nucleus has to be of a certain minimum size so that it will grow. This size is called as critical size of nucleus. •The critical size is maximum near the melting point, but there are less chances of forming such a large nucleus. •If the size of nuclei is smaller than it gets dissolved by the vigorous bombardment of neighbouring atoms and cannot grow. These are known as embryos. Some level of undercooling is done to promote the growth of nuclei. Thus, with undercooling i.e. lowering of temperature, the vibration of atoms decreases and the chances of survival of nuclei increases
  16. 16. It means, some degree of undercooling is necessary to start solidification i.e. nucleation and its growth. •Growth of nuclei occur by diffusion process which is a function of temperature. Therefore, the rate of nucleation (R.N.) and grain growth (G.G.) are functions of temperature.
  17. 17. Homogenous and Heterogeneous Nucleation Nucleation is the beginning of a phase transformation. It is marked by the appearance in the molten metal of tiny regions called nuclei of the new phase which grow to solid crystals until the transformation is complete.
  18. 18. • Prenucleants are assumed to exist in the liquid metal. In many processes, homogeneous nucleation is assumed to occurs Homogeneous or Self Nucleation Homogeneous nucleation is a type of the nucleation process where formation of nuclei starts in the interior of a uniform substance such as pure liquid metals.
  19. 19. • Homogeneous nucleation occurs when where there are no special objects inside a phase which can cause nucleation. • It is the slower process and occurs with much more difficulty. Heterogeneous Nucleation • Heterogeneous nucleation consisting of dissimilar elements or ingredients; not having uniform quality. • Heterogeneous nucleation is a type of nucleation which starts at the surfaces, imperfections and severely damaged regions.
  20. 20. • As the presence of impurities in molten metal lowers the liquid-solid interface energy, the amount of supercooling (undercooling) required to start nucleation will be less. • Hence, nucleation process takes place easily. • Heterogeneous nucleation requires the ability of liquid metal to wet the foreign particle
  21. 21. Solidification of Pure Metals and Alloy •A pure metal solidifies at a constant temperature equal to its freezing point i.e. the same as its melting point. •The grain structure of pure metal cast structure solidified in a square mould as shown in fig . •At mould walls metal cools rapidly due to exposure to ambient temperature. This result in shell of fine equiaxed grain in the chill zone.
  22. 22. • The grain growth through the mould away from wall is columner. This is known as columner zone. Solidification process Pure metals (cooling curve) Alloy - copper-nickel alloy system
  23. 23. • The actual freezing time is called as local solidification time in casting. During this time the latent heat of fusion of metal is released into the surrounding mould. • Due to chilling action of the mould wall, a thin skin of solid metal is initially formed at the interface. • This thickness of skin increases to form a shell around the molten metal as the solidification progresses towards the centre of the cavity.
  24. 24. • Alloys • Most of the alloys freeze over a temperature range rather than at a single temperature. • The start of freezing is similar to that of pure metal. A thin skin is formed at the mould wall due to large temperature gradient at the surface. After this, the freezing progresses similar to pure metal. • During the solidification of metal alloy where both liquid and solid phases exist known as mushy zone. It is an important factor during solidification. • This zone present in freezing temperature range. The columner dendritic structure of grain growth is seen in this zone.
  25. 25. • Chill Zone • Chill zone contains narrow band of randomly oriented grains. • The metal at the mould wall cools to the freezing temperature at first due to higher heat dissipation. • The mould wall provides the surfaces at which heterogeneous nucleation takes place.
  26. 26. • Columnar Zone • Columnar zone contains elongated grains oriented in specific crystallographic directions. • The grain growth in this region takes place in the direction opposite to heat flow. • Heat is removed from casting by the mould material and grain grows perpendicular the mould wall. • This columnar zone has anisotropic properties and formation of this zone is a growth controlled process
  27. 27. • Equiaxed Zone • This zone forms in the center of the casting or ingot. • It contains randomly oriented grains that are relatively round or equiaxed. • The formation of equiaxed zone is a nucleation controlled process and has isotropic properties. • The grain structure resulting from the solidification of molten metal decides the strength, hardness, toughness, and etc. properties of a component •
  28. 28. Gibb’s Phase Rule The Gibb’s phase rule states that under equilibrium conditions, the following relation must be satisfied. 𝑃 +𝐹=𝐶 +2 ……. (1.1) Where, P = Number of phases existing in a system under consideration. F = Degree of freedom i.e. the number of variables such as temperature, pressure or concentration (i.e. composition) that can be changed independently without changing the number of phases existing in the system. C = Number of components (i.e. elements) in the system, and 2 represents any two variables out of the above three i.e. temperature, pressure and concentration. Most of the studies are done at constant pressure i.e. one atmospheric pressure and hence pressure is no more a variable. For such cases, Gibb’s phase rule becomes: 𝑃 +𝐹=𝐶 +1 ……. (1.2) In the above rule, 1 represents any one variable out of the remaining two i.e. temperature and concentration.
  29. 29. Cooling curve of a pure metal Cooling curve of solid solution alloy According to phase rule applied at different regions. Region AB: P + F = C + 1 1 + F = 1 + 1 F = 1 Region BC: P + F = C + 1 2 + F = 1 + 1 F = 0 The meaning of F = 1 is that the temperature will be different without changing the liquid phase existing in the system. The meaning of F = 0 is that the temperature will not be different without changing the liquid and solid phases existing in the system. If the temperature increased, the metal goes in the liquid state and if decreased, it goes in the solid state. Hence, pure metals solidify at constant temperature.
  30. 30. Hume-Rothery’s Rules for Solid Solubility • Atomic Size • Alloying elements having similar atomic size as that of the base metal matrix have better solid solubility. • The greater the difference in size between the atoms of the two metals involved, the smaller will be the range over which they are soluble. • If atom diameter differ by more than 15% of that of the solvent metal then solid solubility is generally extremely small. But if diameter differ by less than 7% then, other things being equal, they will be likely to form solid solution in all proportions. • Any difference in atomic size will of course produce some strain in the resultant crystal structure.
  31. 31. • Electrochemical Properties/ Chemical Affinity • The greater the chemical affinity of two metals, the more restricted is their solid solubility and greater is the tendency of formation of compound . Generally , wider the separation of element in the periodic table, greater is their chemical affinity.
  32. 32. • Relative Valency • A metal of lower valency is more likely to dissolve one of higher valency than vice versa always assuming that other conditions are favourable. • This holds true particularly for alloys of the monovalent metals copper, silver and gold with many metals of higher valency
  33. 33. • Crystal Structure • Metal having same crystal structure will have greater solubility. • This gives rise to the formation of crystals in the solid state. The atoms oscillate about fixed locations and are in dynamic equilibrium rather than statically fixed. • From the above, it can be concluded that two metals are most likely to form substitutional solid solutions over a wide range of compositions. If their atomic sizes are about equal and if their electrochemical properties are similar.
  34. 34. • These conditions are fulfilled when two metals are very close together in the periodic classification of the elements. Usually side by side in the same period (nickel and copper) or one above the other in the same group (silver and gold). • Also if singly they crystallize in the same pattern this will assist simple disordered substitution. • Pairs of metals which fulfil these conditions and dissolve in each other in all proportions in the solid state.
  35. 35. LEVER RULE
  36. 36. Phase Diagram/Equilibrium Diagram • Isomorphous System – CSS and Liquid (L+S) • Eutectic System - CSL and complete insoluble in solid • Partial Eutectic System -two metals have complete solubility in liquid state and partial solubility in the solid state. • Layer Type system- Complete insolubility in L and solid.