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Solidification of metals by Hari prasad



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Solidifiacatio of the metals and cast structures

Solidification of metals by Hari prasad

  1. 1. Hari Prasad-Assistant Professor
  2. 2. Learning Objectives • To know how does solidification affect casting and welding processes. • Differentiate homogeneous and heterogeneous nucleation. Hari Prasad-Assistant Professor
  3. 3. What is solidification? • Solidification is the process where liquid metal transforms into solid upon cooling • The structure produced by solidification, particularly the grain size and grain shape, affects to a large extent the properties of the products • At any temp, the thermodynamically stable state is the one which has the lowest free energy and consequently, any other state tends to change the stable form. Hari Prasad-Assistant Professor
  4. 4. Latent heat Super heat The heat that is added to convert all the solid into liquid at the constant temperature The heat is further added for the metal to remain in molten state Entropy Is a thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work The terms must be known Hari Prasad-Assistant Professor
  5. 5. • Gibbs free energy (G) of any system said to be minimum when the same is at equilibrium. G = H-TS • ‘G’ is a function of ‘H’ (enthalpy) and ‘S’ (entropy) • Important parameter is change in free energy ‘𝞓G’ • A transformation will occur spontaneously only when G has a negative value Hari Prasad-Assistant Professor
  6. 6. Ice melting in a warm room is a common example of increasing entropy Hari Prasad-Assistant Professor
  7. 7. • A crystalline solid has lower internal energy and high degree of order, or lower entropy as compared to the liquid-phase i.e., • Liquid has higher internal energy (equal to the heat of fusion) and higher entropy due to the more random structure Hari Prasad-Assistant Professor
  8. 8. • Transformation from liquid metal to solid metal is accompanied by a shrinkage in the volume • This volume shrinkage takes place in three stages: 1. Liquid – Liquid 2. Liquid – Solid 3. Solid – Solid Hari Prasad-Assistant Professor
  9. 9. Melting of Metals Time, Enthalpy Temp Tm Latent Heat Super Heat Solid + Liquid Hari Prasad-Assistant Professor
  10. 10. Time Temp Super Heat Latent Heat Solid + Liquid Freezing of Metals Hari Prasad-Assistant Professor
  11. 11. ∆𝐆 ∆𝑻 Freeenergy(G) Temp Free energy curve for solid (Gx ) Free energy curve for liquid(Gl) Melting Solidification Hari Prasad-Assistant Professor
  12. 12. • If we take a simple case of pure metal transforming to solid crystal of pure metal X as: L  X (Solid) • A crystalline solid has the lower internal energy and high degree of order, or low entropy as compared to the liquid phase i.e., • Liquid has higher internal energy (equal to the heat of fusion) and higher entropy Hari Prasad-Assistant Professor
  13. 13. ∆𝐆 ∆𝑻 Freeenergy(G) Temp Free energy curve for solid (Gx ) Free energy curve for liquid(Gl) Melting Solidification Hari Prasad-Assistant Professor • With the increase of temperature, the free-energy curve of the liquid phase falls more steeply than the solid-phase • At Tm, the equilibrium melting point, the free energies of both the phases are equal • Above Tm, the liquid has a lower free energy than the crystalline solid ‘X’, i.e., liquid is more stable The solidification reaction will not occur under such conditions as the free energy change, ∆𝑮 for the reaction is positive At the melting temperature, where the two curves cross, the solid and liquid phases are in equilibrium. Below Tm, the free energy of the crystalline solid X, is less than the liquid phase. The free energy change for the reaction is negative
  14. 14. • In alloys, commencement of solidification is easy since the foreign atoms act as source of nucleation • But pure metals experience difficulties in commencing solidification. (there are no foreign atoms to form nuclei) • In such cases the metal cools below its freezing temperature and actual solidification begins at the same point (shown in pic in the next slide) Undercooling (or) Supercooling in pure metals Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid Hari Prasad-Assistant Professor
  15. 15. Undercooling (or) Supercooling in pure metals Hari Prasad-Assistant Professor
  16. 16. Hari Prasad-Assistant Professor
  17. 17. Solidification of alloys • Solidification in alloys takes place in the same manner but with exceptions • They solidify over a range of temp rather than at a constant temp i. Begin solidification at one temp and end at another temp (Solid solution) ii. Begin and end solidification at a constant temp just like in pure metals (pure eutectics) iii. Begin solidification like a solid-solution and end it like a eutectic The local solidification time can be calculated using Chvorinov's rule, which is: 𝒕 = 𝑩 𝑽 𝑨 𝒏 Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mould, n is a constant, and B is the mould constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless Hari Prasad-Assistant Professor
  18. 18. a b c d Solid solution Time Temp Hari Prasad-Assistant Professor
  19. 19. Hari Prasad-Assistant Professor Solid solution
  20. 20. a b c d Pure eutectic Time Temp Hari Prasad-Assistant Professor
  21. 21. a b c d Partly solution and partly eutectic Time Temp e Hari Prasad-Assistant Professor
  22. 22. Understanding solidification Solidification Nucleation Growth Hari Prasad-Assistant Professor
  23. 23. • The basic solidification process involves nucleation and growth • Nucleation involves the appearance of very small particles, or nuclei of the new phase (often consisting of only a few hundred atoms), which are capable of growing. • During the growth stage these nuclei increase in size, which results in the disappearance of some (or all) of the parent phase. • The transformation reaches completion if the growth of these new phase particles is allowed to proceed until the equilibrium fraction is attained Hari Prasad-Assistant Professor
  24. 24. a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope. Hari Prasad-Assistant Professor
  25. 25. Types of Nucleation Nuclei of the new phase form uniformly throughout the parent phase Nuclei form preferentially at structural inhomogeneities, insoluble impurities, grain boundaries, dislocations, and so on. Homogeneous Nucleation Heterogeneous Nucleation Hari Prasad-Assistant Professor
  26. 26. Homogeneous nucleation • Prominent is pure metals • Nuclei of the solid phase form in the interior of the liquid as atoms cluster together Hari Prasad-Assistant Professor
  27. 27. • Each nucleus is spherical and has a radius ‘r’. • This situation is represented schematically Solid 𝐴𝑟𝑒𝑎 = 4𝜋𝑟2 𝑉𝑜𝑙𝑢𝑚𝑒 = 4 3 𝜋𝑟3 Solid-Liquid interface Hari Prasad-Assistant Professor
  28. 28. • There are two contributions to the total free energy change that accompany a solidification transformation. • The first is the free energy difference between the solid and liquid phases, or the volume free energy 𝞓Gv and the volume of spherical nucleus 𝟒 𝟑 𝝅𝒓 𝟑 • The second energy contribution results from the formation of the solid–liquid phase boundary during the solidification transformation. • Associated with this boundary is a surface free energy 𝜸 (positive) ∆𝑮𝒔 = 𝟒𝝅𝒓 𝟐 𝜸 • Latent heat released by atoms is: ∆𝑮𝒗 = − 𝟒 𝟑 𝝅𝒓 𝟑 ∆𝑮 *Negative value is taken since the temp is considered below the equilibrium solidification temperature Hari Prasad-Assistant Professor
  29. 29. • Finally, the total free energy change is equal to the sum of these two contributions—that is: ∆𝐺 ∗ = ∆𝑮 𝒗 + ∆𝑮 𝒔 = − 𝟒 𝟑 𝝅𝒓 𝟑 ∆𝑮 + 𝟒𝝅𝒓 𝟐 𝜸 These volume, surface, and total free energy contributions are plotted schematically as a function of nucleus radius in Figures Hari Prasad-Assistant Professor
  30. 30. • From the fig. it is clear that as the particle radius increases, the net free energy ∆ G also increases till the nucleus reaches a critical radius ‘r*’. • Further increase in particle radius the free energy decreases and even goes to negative. • In order for grain growth to take place around a particular nucleus, it should have reached the critical radius Hari Prasad-Assistant Professor
  31. 31. • The size of the critical radius can be estimated by differentiating ∆𝐺 ∗ with respect to ‘r’ and equating by zero 𝒅 𝒅𝒓 ∆𝑮 ∗ = 𝒅 𝒅𝒓 − 𝟒 𝟑 𝝅𝒓 𝟑 ∆𝑮 + 𝟒𝝅𝒓 𝟐 𝜸 = 𝟎 −𝟒𝝅𝒓 𝟐∆𝑮 + 𝟖𝝅𝒓𝜸 = 𝟎 r = r* = 𝟐𝜸 ∆𝑮 If we substitute r/r* in ∆𝑮 ∗ ∆𝑮 ∗ = 𝟏𝟔𝝅𝜸 𝟑 𝟑 ∆𝑮 ∗ 𝟐 Hari Prasad-Assistant Professor
  32. 32. Heterogeneous nucleation • It is easier for nucleation to occur at surfaces and interfaces than at other sites. • Nucleation occurs with the help of impurities or chemical inhomogeneities. • Impurities can be insoluble like sand particles or alloying elements • Nuclei are formed on the surfaces of the above possible surfaces often called the ‘substrate’ Hari Prasad-Assistant Professor
  33. 33. Nucleation of carbon dioxide bubbles around a finger Hari Prasad-Assistant Professor
  34. 34. Two essential things must happen: 1. The substrate must be wetted by the liquid metal 2. The contact angle/wetting angle (𝜽) of the cap- shaped nucleus should be less than 90o Substrate 𝜹 Liquid 𝜶 Cap 𝜽 Solid 𝜷 𝛾 𝑆𝐼 = 𝑆𝑜𝑙𝑖𝑑 𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 ( 𝜸 𝞫𝞭) 𝛾SL = Solid-liquid interfacial energy (𝜸 𝞪𝞫) 𝛾IL = Liquid interfacial energy (𝜸 𝞪𝞭) 𝜸𝑰𝑳 = 𝜸 𝑺𝑰 + 𝜸 𝑺𝑳 𝒄𝒐𝒔𝜽𝜸 𝞪𝞭 = 𝜸 𝞫𝞭 + 𝜸 𝞪𝞫 𝒄𝒐𝒔𝜽 or 𝜽 = 𝟑𝟔𝟎 𝒐 Hari Prasad-Assistant Professor
  35. 35. A typical cast metal structure Coarse grain structure can be converted into fine grain structure by grain refinement. This can be achieved by high cooling rates, low pouring temp, and addition of inoculating agent Hari Prasad-Assistant Professor
  36. 36. • The chill zone is named so because it occurs at the walls of the mould where the wall chills the material. • Here is where the nucleation phase of the solidification process takes place. • As more heat is removed the grains grow towards the centre of the casting. • These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. • Finally, in the centre the equiaxed zone contains spherical, randomly oriented crystals. • These are desirable because they have isotropic properties. • The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or inoculants Hari Prasad-Assistant Professor
  37. 37. a) Columnar grains c) Equiaxed grains b) Partially columnar and partially equiaxed grains Hari Prasad-Assistant Professor
  38. 38. Coring • In thermal equilibrium diagram, it is assumed that cooling will be slow enough for equilibrium to be maintained. • However, during actual operating condition where rate of cooling is more rapid, e.g. the production of Cu-Ni alloy, there is insufficient time for complete diffusion to take place. • This leads to lack of uniformity in the structure of the metal. This is termed a cored structure, which give rise to less than the optimal properties. • As a casting having a cored structure is reheated, grain boundary regions will melt first in as much as they are richer in low-melting component. • This produces a sudden loss in mechanical integrity due to the thin liquid film that separates the grains. • Moreover, this melting may begin at a temperature below the equilibrium solidus temperature of the alloy. • Coring may be eliminated by a homogenization heat treatment carried out at a temperature below the solidus point for the particular alloy composition. • During this process, atomic diffusion occurs, which produces compositionally homogeneous grains. Hari Prasad-Assistant Professor
  39. 39. Solid solutions • A solid solution is a solid-state solution of one or more solutes in a solvent. • Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. Hari Prasad-Assistant Professor
  40. 40. • The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. Substitutional solid soln. (e.g., Cu in Ni) Interstitial solid soln. (e.g., C in Fe) Hari Prasad-Assistant Professor
  41. 41. • W. Hume – Rothery rule – 1. r (atomic radius) < 15% – 2. Proximity in periodic table • i.e., similar electronegativities – 3. Same crystal structure for pure metals – 4. Valency • Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency. Conditions for substitutional solid solution (S.S.) Hari Prasad-Assistant Professor
  42. 42. Hari Prasad-Assistant Professor
  43. 43. • A familiar example of substitutional solid solution is found for copper and nickel to form monel. • Polymorphous metals may possess unlimited solubility within a single modification of the space lattice. • For example, Fe 𝛼 can form a continuous series of solid solutions with Cr (BCC lattices) and Fe 𝛾, a continuous series of solid solutions with Ni (FCC lattices). • The formation of solid solutions is always associated with an increase of electric resistance and decrease of the temperature coefficient of electric resistance. • Solid solutions are usually less plastic (except for copper-based solid solutions) and always harder and stronger than pure metals. Hari Prasad-Assistant Professor
  44. 44. Intermediate phases • If a solid solution neither forms a substitutional type nor interstitial type, it certainly forms an intermediate compound. • And the compound is said to be “intermediate phase” or “intermediate compound” or “intermetallic” if it has metal in it. • If one element has more electropositivity and the other more electronegativity, then there is greater likelihood that they will form an intermetallic compound instead of a substitutional solid solution. Hari Prasad-Assistant Professor
  45. 45. Common intermediate compounds • Intermetallic or valency compounds • Interstitial compounds • Electron compounds Hari Prasad-Assistant Professor Crystals formed by various elements and having their own type of crystal lattice which differs from the crystal lattices of the component elements are called intermediate phases.
  46. 46. Intermediate phases Intermetallic/valency compounds (Ni3Al) Interstitial compounds (Fe3C) Electron compounds (Cu9Al4) Formed between chemically dissimilar metals. Follow the valence rules. Have complex crystal structure These are of variable composition and don’t obey valence rules Very hard in nature. Very similar to interstitial solid solutions except they have fixed compositions Hari Prasad-Assistant Professor
  47. 47. Intermetallic compound: • A compound formed of two or more metals that has its own unique composition, structure, and properties • Nonstoichiometric intermetallic compound A phase formed by the combination of two components • into a compound having a structure and properties different from either component. • The nonstoichiometric compound has a variable ratio of the components present in the compound Hari Prasad-Assistant Professor
  48. 48. Interstitial compounds • Fe3C (iron carbide), a common constituent of steels, is an example of intermediate phase (interstitial compound). • It has a complex crystal structure referred to an orthorhombic lattice and is hard and brittle. Hari Prasad-Assistant Professor
  49. 49. Electron compounds • The intermediate phases of variable composition which do not obey the valency law are called electron phases or electron compounds. • Hume Rothery has shown that electron phases occur at certain definite value of free electron to atom ratio in the alloy such as 3 : 2, 21 : 13 and 7 : 4. • Few typical examples of electron phases are CuZn (3 : 2), Cu5Zn8 (21 : 13) and CuZn3 (7 : 4). Hari Prasad-Assistant Professor