Unit III Part-A Equilibrium Diagrams.pptx

It describes about different phases in materials

1. Dr.S.R.Thorat Assistant Professor Sanjivani College of Engineering, Kopargaon E-mail : thoratsandipmech @sanjivani.org.in Subject :-Material science & Metallurgy S.Y. B.Tech . Mechanical Unit III : Iron-iron Carbide Equilibrium Diagarm

2. Introduction 2 One of the most important objective of engineering metallurgy is to determine properties of material. The properties of material is a function of the microstructure which depend on the overall composition and variable such as pressure and temperature. Hence to determine the phase present in the material system , an equilibrium or phase diagram is plotted. Equilibrium diagram or phase diagram is a graphical representation of various phase present in material system at various temperature and composition point. All the phase diagrams have temperature as the ordinate(Y-axis) and percentage composition by weight as the abscissa(X-axis)

3. Uses of equilibrium or phase diagram: 3 The equilibrium diagram is used to obtain following information: It shows the various phases present at different composition and temperature. 2. It indicate solid solubility of one element in other. 3. It shows the temperature range over which solidification or liquification of material system occurs. 4. It indicate the temperature at which different phase start to melt.

4. Basic Terms: System: A part portion of the universe under the study is called as system. 2. Phase: It is a physically and chemically composition of a substance(system), separated from the other portion by a surface and an interface . Each portion have different composition and properties. In a equilibrium diagram, liquid is one phase and solid solution is another phase. 3. Variables: A particular phase exists under various condition of pressure and temperature and composition . These parameters are known as the variables of the phase.

5. 4. Component: These are the substances, element or chemical compound whose presence is necessary and sufficient to make a system. A pure metal is one component system whereas and alloy of metals is a two component (binary) system etc. Eg.Cu -Al System 5. Alloy: It is a mixture of two or more elements having metallic properties. In the mixture, metal is in the large proportion (Solvent) and the other can be metal and non-metals (Solute).

6. Solid solution and Compound The element present in the alloy in the largest portion is referred as base metal or parent metal or solvent and the other elements are referred as alloying element or solute. Solid solution is a type of alloy in which the atoms of alloying element are distributed in the base metal and both have similar crystal structure. The composition of alloying element may vary but the structure should be similar to base metal.

7. Thorat S.R. Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

8. 1) Substitutional solid solution In substitutional solid solution , atoms of alloying element occupy the atomic size of base metal. They are further classified as: (a) Regular or ordered substitutional solid solution: In this type, the substitution of atoms of alloying element is in definite order in the base metal matrix. Examples: Ni-Al solid solution below 400 C.

9. (b) Random or disordered substitutional solid solution: In this type, substitution of alloying elements is in any random order in the base metal matrix. Example: Alpha brass

10. In Interstitial solid solution, the atoms of alloying elements occupy the interstitial sites of base metal. This type of solution is formed when atomic size of alloying element is much smaller compared to that of the base metal. Example: Fe-C (2) Interstitial solid solution:

11. Hume - Rothery’s Rules for Solid Solubility Solid solution is an alloy of two or more element where in the atomic crystal structure of alloying element (solute) is same as that of the base metal matrix (solvent). The solubility limit of the solute in the solvent ( of the alloying element in base metal matrix) is governed by certain factors . These governing factors are known as Hume- Rothery’s rules for solid solubility. These governing factor are as follows.

12. Hume - Rothery’s Rules for Solid Solubility 1. Atomic size: Alloying elements having similar atomic size as that of the base metal matrix have better solid solubility. For a favorable solid solution formation, the difference of atomic size of solute and solvent should be less than 15 %. 2. Chemical affinity: Element having lower chemical affinity have greater solid solubility. Element having higher chemical affinity have the tendency of formation of compound and hence restrict formation of solid solution. In general, the alloy elements located closer in the periodic table have higher solid solubility.

13. Hume - Rothery’s Rules for Solid Solubility 3. Relative valency : Metals having lower valency have more solubility for metals having higher valency . Hence, for better solubility, the base metal selected should be one that has lower valency as compared to that of alloying elements. 4. Crystal structure: As mentioned earlier, solid solution is an alloy of element having similar crystal structure. Difference in crystal structure limits the solid solubility of elements.

14. GIBB’S PHASE RULE Gibbs phase rule establishes the relationship between the number of variable (F), the number of element (C), and the number of phases(P). It is expressed mathematically as follows: P + F = C + 2 ……….(I) Where, P = Number of phases in system F = Number of variables that can be change independently without effecting number of phases C = Number of elements 2 = It represent any two variables amongst temperature, pressure and composition

15. In general all equilibrium diagram studied at constant pressure , hence Gibb’s phase rule is modified to” P + F = C + 1 ……….( II) Phase rule helps to determine maximum number of phase present in an alloy system under equilibrium conditions at any point in phase diagram. The phase rule can also be used to determine the degree of freedom that can be changed GIBB’S PHASE RULE

16. Solidification of Pure Metals and Alloys Solidification of Pure Metal : Solidification is process of forming Grains (Nucleation) and its growth in the melt. In the first step, nuclei of a solid phase (crystallite) form from the liquid . In the second step these solid crystallites begins to grow as an atoms until complete liquid solidifies.

17. Solidification of Pure Metals and Alloys 58 Hence solidification is the process of forming grains (nucleation) and its growth in the melt. The metal exists in liquid form above the melting point. 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 supercooling and on the presence of impurities which mainly facilitate nucleation. 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.

18. Solidification

19. 60 Solidification of Pure Metals and Alloys Solidification of Metals Molten metal possesses high energy and it will lost when metal cools to form crystals. As the heat loss is more rapid near mould walls than its centre, the formation of nuclei (crystallites) starts here. The molten metal finds difficulty in starting nucleation process if there is no nuclei in the form of impurities are present to start the crystallisation. In such cases, undercooling is done to accelerate nucleation and thus nuclei or crystals are formed.

20. Solidification of Metals

21. 62 Solidification of Pure Metals and Alloys Solidification of Metals a) Shape of Crystals: The growth of the crystal varies in different crystallographic directions. Slow cooling rate gives growth of crystals uniformly in all the directions of growth. Also, it gives equiaxed crystals (the crystals with equal dimensions in all the directions). Similarly, rapid cooling rate gives crystals like tree which are called as dendrites . The exact shape of the crystals depend on the conditions that exist during solidification.

22. 63 Solidification of Pure Metals and Alloys b) Dendritic Growth: A dendrite is a crystal with a tree-like branching structure . In the current context, we are interested in metallic dendrites formed when a metal or an alloy of multiple metals, in liquid form freezes is called dendritic growth . This temperature exceeds the freezing temperature of a metal hence further growth of the crystal in this direction will be stopped. The liquid region will have a lower temperature in a perpendicular direction, because there is no crystallisation and no heat generation.

23. Solidification of Pure Metals and Alloys b) Dendritic Growth: 64 This sequence of growth of crystals leads finally to the structure characteristics of dendrites. This type of structures are commonly observed in cast components.

24. Solidification of Pure Metals 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. 68

25. Solidification of Alloys Most of the alloys freeze over a temperature range rather than at a single temperature. The exact range depends on the alloy system and the particular composition . 69