2 Marks Question & Answers

Two Marks Questions

UNIT - 1: CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS

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. Differentiate between substitutional and interstitial solid solutions.

✔ In a 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.

6. State Hume Rothery's rules for formation of substitutional solid solutions.

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. Electronegativity: The atoms must have approximately the same electronegativity.

7. 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.

8. What are intermetallic compounds?

The compounds formed by two or more metals in apparently stoichiometric proportion is called intermetallic compounds. 

9. What are electron compounds?

If two metals consist of atoms of more or less similar size but different valency, then the compounds formed are called electron compounds.

10. 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.

11. 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 conditions.

12. What are the advantages of the equilibrium diagrams? 

1. To show what phases are present are present at different compositions and temperatures under equilibrium conditions.

2. To indicate the equilibrium solid solubility of one element in another 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.

13. Stag Gibb's phase rule?

Gibb's phase rule is given by

F = C - P + 2

where

F = Degrees of freedom of system or the 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 = Number of phases present in the system.

14. 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.

15. What is liquidus line? A solidus line? A solvus linė?

In a phase diagram, liquidus line is the line or boundary that separates liquid and liquid + solid phase regions. Solidus line. is the line or boundary that separates solid a solid + liquid phase regions. Solvus lines separate single-phase solid regions from two-phase solid regions.

16. What pieces of information can be obtained for 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.

17. What is tie-line?

A tie line is simply an isothermal line drawn through the point of consideration, extending across the two-phase region and terminating at the phase boundary lines on either side.

18. What is a lever-law calculation and what information can it provide?

Phase fraction = Opposite arm of lever / Total length of tie line

and Phase percentage = Opposite arm of lever / Total length of tie line

× 100

Using the lever law calculations, one can compute the phase fraction and the phase percentage.

19. What do you mean by invariant reaction?

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. Differentiate between eutectic and eutectoid reactions.

21. Distinguish between peritectic and peritectoid reactions.

22. 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.

23. Define: Ferrite and Austenite.

✔ Ferrite is a primary solid solution based on a 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 on y iron having FCC structure. The maximum solubility of carbon in FCC iron is about 2% at 1140°C.

24. 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.

25. Define: Martensite, and Bainite.

✔ Martensite is the super saturated solid solution of carbon in a iron. It is formed when a steel is very rapidly cooled from the austenitic state.

✔ Bainite is a decomposition product of autenite, consisting of an aggregate of ferrite and carbide. Bainite has hardness in between the hardness of pearlite and martensite.

26. Why is the iron-carbon equilibrium diagram usually not shown beyond 6.67% carbon?

This is because in practice, all steels and cast irons have carbon contents less than 6.67 wt % C.

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?

✓ 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 contain more than 4.3% C are termed as hypereutectic.