Alloys and phase diagrams

Alloys and Phase Diagrams, Grain structure, System, Phases, Equilibrium, Degree of Freedom, Structural Constituents, Cooling Curves, Phase Diagrams, Interpretation of Phase Diagrams, Gibbs Phase Rule, Eutectic System, Peritectic Reaction, Eutectoid Reaction, Peritectoid Reaction, Allotropy of Iron, Iron - Carbon Diagram, TTT Diagram

1. Alloys and Phase Diagrams By: Ratnadeepsinh Jadeja

2. Introduction The “Grain Structure” of a material shows shape and size of the grains (crystals) which form the bulk material. It is characterized by grain boundaries, grain shape and grain size. There are different types of grains such as columnar, dendritic, equiaxed or a combination of these types. Grain type can be controlled by controlling nucleation and growth phenomena which occur during solidification of the liquid metal. To understand the microstructure of single and multiphase solids and conditions required to improve their properties by changing micro-structure thermodynamically, one needs to understand the phase diagrams and phase transformations.

3. Introduction System Thermodynamically, a system is an isolated body of matter which is under study. More precisely, a system can be defined as a substance or a group of substances so isolated from its surroundings that it is totally unaffected by the surroundings but changes in its overall composition, temperature, pressure or total volume can be allowed as per the desire of the person who investigates it. A system may contain solids, liquids, gases or their combination. It may have metals or non-metals separately or in combined form.

4. Introduction Phase A phase is a substance or a portion of matter which is homogenous, physically distinct and mechanically separable. It is homogenous in the sense that its two smallest parts cannot be distinguished (identified differently) from one another. Physically distinct and mechanically separable means that the phase will have a definite boundary surface. A phase can exist in three different states of vapour , liquid or solid depending upon pressure and temperature. Different phases are given different names or symbols like α (alpha),ß (Beta), γ (Gamma), etc.

5. Introduction Equilibrium : Equilibrium in a system is the state of minimum free energy under any specified combination of overall composition, temperature, pressure and overall volume. Once equilibrium is achieved, even a minor change in these parameters of composition, temperature, pressure, volume or state of any substance within the system means an increase in free energy. Degrees of Freedom: It is also known as variance of system. It is defined as number of external or internal factors of the system (temperature, pressure and concentration) that can be independently changed without altering equilibrium i.e. without causing disappearance of a phase or formation of a new phase in the system.

6. Introduction Structural Constituent Phase distribution in a system is not necessarily uniform throughout the structure. These phases are associated in different ways to form the structure. This association of phases in a recognizably distinct fashion is referred to as “structural constituent” of the alloy.

7. Cooling Curves A method, to determine the temperatures at which phase changes (Liquid to Solid or Solid to Liquid) occur in an alloy system, consists of following the temperature as a function of time as different alloys in the system are very slowly cooled. The data obtained in this manner from a cooling curve for each of the alloys. Cooling curves for (a) Pure metal or compound (b) Binary solid solution (c) Binary eutectic system

8. Phase Diagram

9. Interpretation of Phase Diagrams The following three conclusions are the rules necessary for the interpreting phase diagrams: The phases that are present. The chemical composition of each phase. The lever Rule (the amount of each phase).

10. Gibbs Phase Rule Gibbs Phase Rule establishes the relationship between the number of degree of freedom (F), the number of components (C), and the number of phases (P). It is expressed mathematically as follows: P + F = C + 2

11. Classification of Equilibrium Diagrams Equilibrium diagrams may be classified according to the relation of the components in the liquid and solid states as follows: Components completely soluble in the liquid state, and also completely soluble in the solid state, but partly soluble in the solid state (Eutectic Reaction). but insoluble in the solid state (Eutectic Reaction). the Peritectic reaction Components partially soluble in the liquid state, but completely soluble in the solid state. and partly soluble in the solid state. Components completely insoluble in the liquid state and completely insoluble in the solid state.

12. Two metals completely soluble in the liquid and solid states

13. Eutectic System In an eutectic reaction, when a liquid solution of fixed composition, solidifies at a constant temperature, forms a mixture of two or more solid phases without an intermediate pasty stage. This process reverses on heating. Liquid Solid 1 + Solid 2 Cooling Heating Solid 1 + Solid 2 Liquid

14. Eutectic System In eutectic system, there is always a specific alloy, known as eutectic composition, that freezes at a lower temperature than all other compositions.

15. The Bismuth-Cadmium Equilibrium Diagram Two metals completely soluble in the liquid state but completely insoluble in the solid state. In eutectic system, there is always a specific alloy, known as eutectic composition, that freezes at a lower temperature than all other compositions.

16. The Tin-Lead Equilibrium Diagram Two metals completely soluble in the liquid state, but only partly soluble in the solid state

17. The Tin-Lead Equilibrium Diagram Tin will dissolve up to maximum of 2.6 % Pb at eutectic temperature, forming the solid solution α . Lead will dissolve up to a maximum of (100 – 80.5), i.e., 19.5 % tin at the eutectic temperature, giving the solid solution β . Slope of BA and CD indicate that the solubility of Pb in Sn ( α ) and that of Sn in Pb ( β ) decreases as temperature falls.

18. Peritectic Reaction Platinum-Silver Equilibrium Diagram

19. Eutectoid Reaction

20. Peritectoid Reaction

21. Allotropy Iron is a relatively soft and ductile material. Iron has melting point of 1539 °C. Iron is allotropic metal, which means that it exists in more than one type of lattice structure (e.g., BCC/FCC) depending upon temperature. At room temperature iron is BCC lattice arrangement, where as 908 °C it changes to FCC and then at 1403 °C back to BCC again and vice versa. At 770 °C called the curie point

22. 1539 1404 910 768 ( O C) CURIE TEMP. (AUSTENITE) (FERRITE) a = 2.93 A O a = 3.63 A O a = 2.87 A O Allotropy of Iron

23. Micro – Constitutes of Iron and Steel When steel is heated above the austenitic temperature and is allowed to cool under different conditions, the austenite in steel transforms into variety of microconstituents. The study of these microconstituents is essential in order to understand Fe-C equilibrium diagram and T. T. T. diagrams. Various micro – constituents are: Austenite Cementite Pearlite Martensite Sorbite Ferrite Ledeburite Bainite Troostite

24. Micro – Constitutes of Iron and Steel Austenite Austenite is the solid solution of carbon and/or other alloying elements (e.g., Mn, Ni, etc.) in gamma iron. Carbon is in interstitial solid solution whereas Mn, Ni, Cr, etc., are in substitutional solid solution with iron. Austenite can dissolve maximum 2% carbon at 1130 °C. Austenite has: Tensile strength 10500 . Elongation 10% in 50 mm. Hardness Rockwell C 40 (Approx.) Austenite is normally not stable at room temperature. Austenite is non-magnetic and soft.  

25. Micro – Constitutes of Iron and Steel Ferrite Ferrite is B.C.C. iron phase with very limited solubility for carbon. The maximum solubility is 0.025 % carbon at 723 °C and it dissolves only 0.008% carbon at room temperature. Ferrite has: Tensile strength 2800 (Approx.) Elongation 40% in 50 mm. Hardness less than Rockwell C 0 or Rockwell B 90  

26. Micro – Constitutes of Iron and Steel Cementite Cementite or iron carbide, chemical formula Fe 3 C, contains 6.67% carbon by weight. It is typical hard and brittle interstitial compound of low tensile strength (approx. 350 ) but high compressive strength. Cementite is the hardest structure that appears on the iron-carbon equilibrium diagram. Its crystal structure is orthorhombic. Ledeburite Ledeburite is the eutectic mixture of austenite and cementite. It contains 4.3% carbon. It is formed at about 1130 °C (2065 °F)  

27. Micro – Constitutes of Iron and Steel Pearlite The perlite microstructure consists of alternate lamellae of ferrite and cementite. Pearlite is the product of austenite decomposition by eutectoid reaction; thus pearlite is eutectoid mixture containing about 0.8% carbon and is formed at 1333 °F (723 °C). Perlite has Elongation 20% in 50 mm. Hardness Rockwell C 20. Bainite Bainite is the constituent produced in a steel when austenite transforms at a temperature below that at which perlite is produced and above that at which martensite is formed. Bainite is an isothermal transformation product and cannot be produced by continuous cooling.

28. Micro – Constitutes of Iron and Steel Martensite Martensite is a metastable phase of steel, formed by transformation of austenite below the M s temperature. Martensite is an international supersaturated solid solution of carbon in alpha iron and has a Body Centered Tetragonal Lattice. Martensite, normally, is a product of quenching. Martensite possesses an acicular or needle-like structure. Troostite Troostite (Nodular) is a mixture of radial lamellae of ferrite and cementite. In steel heat treatment, the troostite, i.e. the microstructure, consisting of ferrite and finely divided cementite is produced on tempering martensite below approximately 450 °C.

29. Micro – Constitutes of Iron and Steel Sorbite Sorbite is the microstructure consisting of ferrite and finely divided cementite, produced on tempering martensite above approximately 450 °C. The constituent also known as Sorbite Pearlite, is produced by the decomposition of austenite when cooled at a rate slower than that which will yield a troostitic structure and faster than that which will produce a pearlitic structure.

30. Micro – Constitutes of Iron and Steel α or Ferrite γ or Austenite Ferrite + Cementite or a+ Fe 3 C or Pearlite Fe 3 C or Cementite Austenite + Cementite or γ + Fe 3 C or Ledeburite

31. T. T. T. Diagram T.T.T. (Time – Temperature – Transformation) diagram is also known as S Curve, C Curve, Bain’s Curve or Isothermal Transformation diagram. T.T.T. diagram is used more particularly in the assessment of decomposition of austenite in a heat treatable steel. The principal source of transformation on the actual process of austenite decomposition under non equilibrium conditions is the T.T.T. diagram, which related the transformation of austenite to the time and temperature conditions to which it is subjected.

32. Molten Salt Bath

33. T. T. T. Diagram

34. T. T. T. Diagram

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