MISRIMAL NAVAJEE MUNOTH JAIN ENGINEERING COLLEGE ME6403 ENGINEERING MATERIALS AND METALLURGY UNIT – I - ALLOYS AND PHASE DIAGRAMS PART A 1. What is an alloy? A metal alloy, or simply an alloy, is a mixture of two or more metals or a metal (metals) and a non- metal (non-metals). 2. How many components are found in an alloy? Two or more components are found in an alloy. 3. What is meant by base metal? In an alloy, the element which is present in the largest proportion is called the base metal. 4. What are alloying elements? In an alloy, all elements other than the base metal are called the alloying elements. 5. Define solid solution. [Nov/Dec 2011] A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state. In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions. 6. Differentiate between substitutional and interstitial solid solution. In substitutional solid solution, the solute atoms (impurities) substitute for parent solvent atoms in a crystal lattice. In interstitial solid solution, the solute atoms fit into the space between the solvent or parent atoms. 7. State Hume Rothery’s rules for formation of substitutional solid solutions. [May/Jun 2014] 1. Size factor: The atoms must be of similar size, with less than a 15% difference in atomic radius (in order to minimize the lattice strain). 2. Crystal structure: The materials must have the same crystal structure. 3. Valence: The atoms must have the same valence. 4. Electro negativity: The atoms must have approximately the same electro negativity. 8. What are intermediate phases? If an alloying element is added in excess of the limit of solid solubility, a second phase appears along with the primary solution. If the second phase differs in both crystal structure and properties from primary solid solution, then it is known as an ‘intermediate’ phase. 9. What are intermetallic compounds? The compound formed by two or more metals in apparently stoichiometric proportion is called intermetallic compounds. 10. What are electron compounds? If two metals consist of atoms more or less similar size but different valency, then the compounds formed are called electron compounds.
11. Define ‘phase’. What different kinds of phases are possible? A phase is defined as any physically distinct, homogeneous and mechanically separable portion of a substance. Three different kinds of phases are solid, liquid and vapour. 12. What is an equilibrium phase diagram? A phase diagram can be defined as a plot of the composition of phases as a function of temperature in any alloy system under equilibrium condition. 13. What are the advantages of the equilibrium diagrams? 1. To show what phases are present at different compositions and temperature under equilibrium conditions. 2. To indicate the equilibrium solid solubility of one element in other element. 3. To indicate the temperature range over which solidification of a material occurs. 4. To indicate the temperature at which different phases start to melt. 14. State Gibb’s phase rule. [May/Jun 2014] Gibb’s phase rule is given by F=C-P+2 where, F=degrees of freedom of system or number of variables (such as temperature, pressure or composition) that may be changed independently without altering the equilibrium; C=number of components (usually elements or compounds) forming the system; and P=no of phases present in the system. 15. What are cooling curves? Cooling curves are obtained by plotting the measured temperatures at equal intervals during the cooling period of a melt to a solid. 16. What is a liquidus line, a solidus line and a solvus line? In a phase diagram, liquidus line is the line or boundary that separates liquid and liquid + solid phase regions. A Solidus line is a line or boundary that separates solid and solid + liquid phase regions. A Solvus line separates single-phase solid regions from two-phase solid regions. 17. What is the information that can be obtained from each point in a phase diagram? Using a phase diagram, one can obtain at least the following three informations. 1. The phases that are present, 2. The composition of each phase, and 3. The amount of each phase present. 18. Explain the lever-law calculation and what information does it provide? Phase fraction = (Opposite arm of lever / Total Length of Tie line) Phase percentage = (Opposite arm of lever /Total length of Tie line) X 100 Using the lever law calculations, one can compute the phase fraction and the phase percentage. 19. What is meant by invariant reaction? [Nov/Dec 2013] The eutectic reaction is also called an invariant reaction since it occurs under equilibrium conditions at a specific temperature and alloy composition that cannot be varied. 20. Define the Eutectoid reaction. Give examples. [May/Jun 2012] In the eutectoid reaction a solid phase decompose into two other solid phases upon isothermal cooling. Austenite a solid solution of carbon and gamma iron, decomposes in to pearlite a mixture of two solids alpha iron and iron carbide
In the Fe-Fe3C equilibrium diagram the eutectoid reaction takes place at the temperature of 727°C, at the composition of 0.8% of C, Austenite is converted into Pearlite mixture during cooling. 21. What is eutectic reaction? [Nov/Dec 2013] Upon cooling a liquid phase is transformed into the two solid phases at eutectic temperature. The opposite reaction occurs upon heating. This is called eutectic reaction. In general the eutectic mixture with in a simple binary alloy system is the lowest melting point mixture within that alloy system. 22. Distinguish between peritectic and peritectoid reactions. The peritectic reaction can be written as The peritectoid reaction can be written as 23. What do you understand by ‘allotropy of iron’? Allotropy refers to the possibility of existence of two or more different crystal structures for a substance depending upon temperature. 24. Define: ferrite and austenite. [May/Jun 2013] Ferrite is a primary solid solution based on iron having BCC structure. Maximum solubility of carbon in iron is 0.025% carbon at 723°C, while its solubility at room temperature is only about 0.008%. Austenite is a primary solid solution based oniron having FCC structure. The maximum solubility of carbon in FCC iron is about 2% at 1140°C. 25. Define: Cementite and Pearlite. Cementite is the name given to the carbide of iron (Fe3C). It is the hard, brittle, intermetallic compound of iron with 6.69% of carbon. Pearlite is the eutectoid mixture of ferrite (87.5%) and cementite (12.5%). It is formed when austenite decomposes during cooling. It contains 0.8% of carbon. 26. Define: martensite, and bainite. Martensite is the super saturated solid solution of carbon in iron. It is formed when a steel is very rapidly cooled from the austenitic state. Bainite is a decomposition product of austenite, consisting of an aggregate of ferrite and carbide. Bainite has hardness in between the hardness of pearlite and martensite. 27. What is steel? The ferrous alloy having the carbon composition ranging from 0.008 to 2% is known as steel. 28. What is meant by eutectoid, hypoeutectoid, hypereutectoid steels? [Nov/Dec 2013] Steels that contain 0.8% C (the eutectoid amount of carbon) are called eutectoid steels. Steels having less than 0.8% C are known as hypoeutectoid steels. Steels having more than 0.8% C are known as hypereutectoid steels.
29. How do cast irons differ from steels in terms of carbon content? Composition from 0.008% to 2% carbon represent steel and those above 2% carbon represent cast iron. 30. Distinguish between hypoeutectic and hypereutectic cast irons. Cast irons that contain less than 4.3% C are termed as hypoeutectic whereas cast irons that contains more than 4.3% C termed as hypereutectic. 31. What do you mean by substitutional solid solution? Briefly explain the rules governing the formation of substitutional solid solution. [May/Jun 2009] When the solute atoms (impurities) substitute for parent solvent atoms in a crystal lattice they are called substitutional atoms and the mixture of the two elements is called a substitutional solid solution. Hume Rothery’s rules govern the formation of substitutional solid solution. 1. Size factor 2.Crystal structure 3.Valence 4.Electronegativity 32. Why carbon solubility is more in an austenite? [May/Jun 2009] Austenite is a primary solid solution based on γ iron having FCC structure. Carbon solubility is more in austenite, because austenite is an interstitial solid structure of carbon in iron. 33. What is substitutional solid solution? Give two examples. [Nov/Dec 2007] When the solute atoms (impurities) substitute for parent solvent atoms in a crystal lattice, they are called substitutional atoms, and the mixture of the two elements is called a substitutional solid solution. E.g. Cu-Ni system, Cu-Zn system and Au-Cu system. 34. Distinguish between hypo-eutectoid steels and hyper-eutectoid steels.[May/Jun 2007] Hypoeutectoid steels: Steels having less than 0.8% C are known as hypoeutectoid steels. Hypereutectoid steels: Steels having more than 0.8% C are known as hypereutectoid steels. 35. State the conditions under which two metallic elements will exhibit solid solubility. To exhibit unlimited solid solubility, the solute and solvent elements should obey the general rules of Hume’s Rothery : 1.Size factor 2.Crystal structure 3.Valence 4.Electronegativity 36. Define polymorphism and Allotropy. 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 best known example for allotropy is iron. 37. Name the various micro structures of Fe-Fe3C phase diagram. Various micro structures of Iron carbon equilibrium diagram are Ferrite, Austenite, Pearlite, Cementite, Ledeburite, Martensite, Troostite, Sorbite and Bainite.