This set of Mechanical Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Strengthening Mechanisms – Yield Point Phenomena”.
1. Yield point phenomena is observed in ____________
a) pure metal
b) low carbon steel
c) high carbon steel
Explanation: Yield point phenomena is observed in low carbon steel or in the system where the alloy solute content is low.
Explanation: The yield point phenomena are observed after the proportional limit is crossed, and material is the plastic region.
3. In steel, the responsible alloying element to cause yield point phenomena?
Explanation: The yield point phenomena is caused by the interstitial type alloying element so that they can diffuse and pin down the dislocation. So only carbon sits in interstitial site, rest all sit in the substitutional site or form compound.
4. Find the strain rate (in /sec) of testing of metal with dislocation density 109 m-2 with burger’s vector equal to 5 Ao? velocity of dislocation is 1 micron/sec.
Explanation: Strain rate is given as:
Strain rate = b*ρ*v; where v is the average dislocation velocity.
Strain rate=5*10-10*109*1*10-6 = 5*10-7/sec.
5. After the strain ageing of the material, the _________
a) strength increases and ductility decreases
b) strength decreases and ductility increases
c) strength and ductility both increases
d) strength and ductility both decreases
Explanation: The strain aging is delayed hardening of metal. It increases the required stress to deform the material but decrease the elongation of the material.
6. The reappearance of yield point in strain aging is due to the diffusion of carbon and nitrogen to the dislocation atmosphere.
Explanation: The yield point occurs due to pinning down of dislocation by carbon and nitrogen atoms. If the material once crosses the yield point, the dislocation is ripped out from this atmosphere. But if the material is kept for some day at room temperature, the carbon and nitrogen will again diffuse to the dislocation and yield point will reappear.
6. The nitrogen plays a higher role in strain aging than carbon in case of iron.
Explanation: Because the nitrogen has a higher solubility and higher diffusion coefficient. So the movement of nitrogen is much higher than carbon in iron lattice.
7. The stretcher strain marks are generally observed in which of the following metal working process?
c) Deep drawing
Explanation: The stretcher strain marks associated with yield point phenomena are generally observed in deep drawing. This is because of localized heterogeneous deformation of metal in deep drawing.
8. Aluminum, vanadium, titanium are generally added in steel to eliminate the yield point because they ___________
a) occupy the place of carbon in lattice
b) increase the required value of stress to cause yield point
c) they are strong carbide former to catch the carbon
d) they act as a deoxidant
Explanation: The given metals are strong carbide former, so they form a compound with carbon. This limit the diffusivity of free carbon in the lattice and eliminate the yield point phenomena.
9. Which of the following method helps in avoiding stretcher strain marks in deep drawing?
a) Shot Peening
c) Skin pass rolling
Explanation: The skin pass rolling is the method in which the material is given slight deformation to produce the sufficient number of dislocation and material crosses the point A. So during deep drawing, the yield point is never encountered.
Explanation: The serration in the stress-strain curve are always observed in the temperature range where diffusion is sufficient to catch the torn dislocation. If the temperature is very low, yield point will not appear. If the temperature is very high, the carbon atom will diffuse so fast that it will never leave the dislocation, so yield point will not appear.
11. The dynamic strain aging is not associated with _______
a) Diffusion of carbon
b) Serration in the stress-strain curve
c) Portevin-LeChatelier effect
d) Dislocation multiplication
Explanation: In simple terms, dynamic strain aging increases the required stress to cause the deformation of the material. The diffusion of carbon which causes serration in the curve, is termed as Portevin-LeChatelier effect. But the dislocation multiplication is not associated with dynamic strain aging.
Sanfoundry Global Education & Learning Series – Mechanical Metallurgy.
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