Engineering Materials

Phase Diagrams

Fundamentals of phase diagrams for Mechanical Engineering and GATE & IES preparation.

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9 Active Questions

Answers & Explanations Included

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Question 1 of 9

Original Exam: GATE (2018)

In a binary phase diagram, which reaction represents a eutectoid transformation?

A. M1 → M2 + M3 B. L1 → M1 + M2 C. L1 + M1 → M2 D. M1 + M2 → M3

✓ Correct Answer: A

Explanation:
A eutectoid reaction is a solid-state invariant reaction in which one solid phase transforms into two different solid phases. Therefore M1 → M2 + M3 is the correct representation. Traceable to GATE ME 2018 Set-2, Q12.

Difficulty: basic Type: conceptual

Question 2 of 9

Original Exam: GATE (2020)

For an equilibrium binary phase diagram, which statement is INCORRECT?

A. It identifies temperatures at which melting/solidification begins or ends. B. The relative amounts of coexisting phases can be determined using the lever rule. C. It provides information about the rates of phase transformations. D. It indicates limits of solid solubility.

✓ Correct Answer: C

Explanation:
Equilibrium phase diagrams describe stable phases, phase boundaries and equilibrium compositions/amounts. They do not provide transformation kinetics or rates; TTT/CCT diagrams are used for that purpose. Traceable to GATE ME 2020 Set-2, Q7.

Difficulty: basic Type: conceptual

Question 3 of 9

Original Exam: IES (2020)

A binary alloy is in a two-phase liquid + solid region. Which rule is used to determine the relative proportions of the two phases at a specified temperature?

A. Hume-Rothery rule B. Lever rule C. Gibbs phase rule D. Empirical rule

✓ Correct Answer: B

Explanation:
The lever rule determines the relative mass or mole fractions of the coexisting phases along a tie line in a binary equilibrium phase diagram. Traceable to ESE Mechanical 2020 official paper.

Difficulty: basic Type: conceptual

Question 4 of 9

Original Exam: IES (2014)

Consider these statements about binary phase diagrams: (1) Additional alloying elements can change a binary phase diagram. (2) Tie-line construction and the lever rule are used to determine phase compositions/amounts. (3) Phase diagrams provide transformation-rate information. Which statements are correct?

A. 1 and 2 only B. 1 and 3 only C. 2 and 3 only D. 1, 2 and 3

✓ Correct Answer: A

Explanation:
Statements 1 and 2 are correct. Equilibrium phase diagrams do not give transformation rates; kinetics are treated using TTT/CCT diagrams. Therefore the correct choice is 1 and 2 only. Traceable to ESE Mechanical 2014 Paper-II.

Difficulty: intermediate Type: conceptual

Question 5 of 9

Original Exam: IES (2014)

In the Fe–C equilibrium system, the eutectoid transformation is represented by:

A. Austenite → ferrite + cementite B. Austenite → ferrite + martensite C. Austenite → ferrite + pearlite D. Austenite + martensite → bainite

✓ Correct Answer: A

Explanation:
At the eutectoid composition and temperature, austenite transforms into ferrite and cementite; the resulting microstructure is pearlite. Traceable to ESE Mechanical 2014.

Difficulty: basic Type: conceptual

Question 6 of 9

Original Exam: IES (1995)

In the Fe–C equilibrium diagram, the eutectoid transformation occurs at approximately:

A. 600 °C B. 723 °C C. 114 °C D. 1493 °C

✓ Correct Answer: B

Explanation:
The eutectoid temperature of the Fe–Fe3C system is conventionally taken as about 723 °C in the cited examination question. Traceable to ESE Mechanical 1995.

Difficulty: basic Type: conceptual

Question 7 of 9

Original Exam: GATE (2016)

For a binary phase diagram, four samples of the same composition are heated to temperatures marked a, b, c and d in the supplied phase-diagram figure. At which marked temperature does the sample become solutionized fastest?

A. a B. b C. c D. d

✓ Correct Answer: C

Explanation:
For solution treatment, the alloy is heated into the appropriate single-phase solid-solution region; among the marked temperatures in the original figure, c gives the fastest solutionizing condition. Traceable to GATE ME 2016 Set-2, Q21. The original question is figure-dependent, so the original figure must be supplied if this item is imported with an image.

Difficulty: intermediate Type: conceptual

Question 8 of 9

Original Exam: GATE (2022)

A hypothetical binary alloy has a eutectic point at 30 wt% B and 400 °C. The liquidus from 0 wt% B at 700 °C to the eutectic is straight. For an alloy containing 15 wt% B, the liquidus temperature is 550 °C and solidification ends at 400 °C. What is the solidification range?

A. 400 °C B. 250 °C C. 800 °C D. 150 °C

✓ Correct Answer: D

Explanation:
At 15 wt% B, linear interpolation on the left liquidus gives TL = 550 °C. The solidification ends at the eutectic temperature, TS = 400 °C. Therefore the solidification range is TL − TS = 550 − 400 = 150 °C. Traceable to GATE ME 2022 Set-2, Q19. The original question contains a phase-diagram figure; the numerical information is retained here so the SSRAO version is self-contained.

Difficulty: intermediate Type: numerical

Question 9 of 9

Original Exam: GATE (2024)

In pearlite, the two phases present are:

A. Austenite and ferrite B. Cementite and austenite C. Ferrite and cementite D. Martensite and ferrite

✓ Correct Answer: C

Explanation:
Pearlite is the eutectoid microconstituent consisting of ferrite (α-iron) and cementite (Fe3C). Traceable to GATE ME 2024 Set-2, Q28.

Difficulty: basic Type: conceptual