# Mechanical Behaviour Questions and Answers – Precipitation Hardening

This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Precipitation Hardening”.

1. Precipitation hardening is also known as ____
a) Age hardening
b) Dispersion hardening
c) Grain boundary strengthening
d) Strain hardening

Explanation: Precipitation hardening is also known as age hardening. Precipitation hardening of an alloy occurs with time due to precipitation of coherent phase from supersaturated solid solution. The second step in precipitation hardening is known as ageing.

2. Which of the following is not an age hardening alloy?
a) Al-Cu
b) Al-Mg-Si
c) Cu-Ni
d) β titanium alloys

Explanation: Cu-Ni is an isomorphous binary system. While precipitation hardening alloys possess limited solid solubility. Al-Cu, Al-Mg-Si and β titanium alloys are age hardenable alloys.

3. What is ageing temperature of Cu-Be alloys?
a) 300-320°C
b) 430-650°C
c) 170°C
d) 850°C

Explanation: Cu-Be alloys have ageing temperature of 300-320°C. The alloy is firstly given solution treatment. In second step, it is given ageing treatment on 300-320°C temperature.

4. What is the ageing temperature of Al-4%Cu alloy?
a) 160°C
b) 130°C
c) 218°C
d) 850°C

Explanation: The ageing temperature for Al-Cu alloys is 130°C. 160°C, 218°C and 850°C are for Al-4%Cu, Al-Cu-Mn, Mg-Al-Zn and waspalloy respectively. Precipitation formation takes place when these alloys are heated to this temperature and holding for some time.

5. What are preferential planes for segregation of solute atoms in Al-Cu alloys?
a) {1 1 1}
b) {1 1 0}
c) {2 1 0}
d) {1 0 0}

Explanation: During ageing in first step, atoms segregate at preferential planes of the matrix. In Al-Cu alloys, these planes are {1 0 0}.
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6. the density of precipitated GP-zones is the order of ___
a) 1010 cm-3
b) 1012 cm-3
c) 1016 cm-3
d) 1018 cm-3

Explanation: The density of precipitated GP-zones is the number of zones per unit volume. The density is 1018 cm-3.

7. What is the spacing between GP-zones of Al-Cu alloy?
a) 25-50 Å
b) 50-75 Å
c) 75-100 Å
d) 125-150 Å

Explanation: The spacing between GP-zones of Al-Cu ranges between 75 to 100 Å. It depends on Ageing temperature and holding time. It changes with the size of GP-zones.

8. With an increase in the size of precipitate particle, magnitude of strengthening ______
a) Decreases
b) Increases
c) Remains constant
d) First increases then decrease

Explanation: With increasing size of the precipitate particle, magnitude of strengthening increases. It attains maximum value at the critical size. After which material begins to soften.

9. For precipitation hardening, the alloy should exhibit decreasing solid solubility with ______
a) Increasing temperature
b) Decreasing temperature
c) Increasing pressure
d) Decreasing pressure

Explanation: Alloy should exhibit decreasing solid solubility with decreasing temperature. Here decrease in solubility causes clustering of solute atoms. This causes precipitation hardening.

10. In the first step of precipitation hardening, alloy is heated to ____ temperature.
a) Recrystallization
b) Melting
c) Ageing
d) Homogenizing

Explanation: In the first step of precipitation hardening, solutionizing is done. In this step, alloy is heated to a temperature, where alloy has a single phase. This temperature is known as homogenizing temperature.

11. In the second step of precipitation hardening, alloy is heated to ____ temperature.
a) Melting
b) Homogenizing
c) Solutionizing
d) Ageing

Explanation: In second step, supersaturated solid solution is heated to make precipitation. This temperature is well above room temperature. This temperature is referred to as ageing temperature.

12. Coherency between the precipitating phase and the matrix phase is required for precipitation hardening.
a) True
b) False

Explanation: First requirement for precipitation hardening is decreasing solid solubility with decreasing temperature. Second requirement is coherency between precipitating and matrix phase. Second condition is fulfilled by only a few alloys.

13. In Al-Cu alloys, ϴ” phase is ____ coherent with matrix.
a) Fully
b) Semi
c) Non
d) In

Explanation: ϴ” phase in Al-Cu alloys is a metastable phase. It is fully coherent. ϴ’ on the other hand is partially coherent.

14. Alloy in overaged condition display _____
a) Lower yield strength
b) Less strain hardening rate
c) Higher hardness
d) Higher tensile strength

Explanation: Alloy when overaged, shows lower yield strength. The hardness of overaged alloy becomes lower. It shows higher strain hardening rate.

15. The hardening magnitude, in precipitation hardenable alloys, doesn’t depend on alloy composition.
a) True
b) False

Explanation: The magnitude of hardening in precipitation hardening alloys depends on alloy composition. Low density of particles causes large spacing between particles. It reduces hardening in alloy.

Sanfoundry Global Education & Learning Series – Mechanical Behaviour & Testing of Materials.

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