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ISRO Mechanical Engineering Material Science Questions, Answers and Explanation

ISRO Mechanical Engineering Material Science Questions, Answers and Explanation

Crystal Structure

2016.54. Which of the following order of crystal structure will match with metals Iron - Copper - Zinc in that order?
a) BCC - FCC - HCP
b) FCC - BCC - HCP
c) HCP - FCC - BCC
d) BCC - FCC - HCP

 Answer
d) BCC - FCC - HCP
2018.8. Which of the following order of crystal structure will match with metals Iron-Nickel-Titanium in that order at room temperature
a) BCC - HCP - FCC
b) FCC - BCC - HCP
c) HCP - FCC - BCC
d) BCC - FCC - HCP

Answer
d) BCC - FCC - HCP

Explanation
BCC - Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Barium (Ba), Vanadium (V), Chromium (Cr), Iron (Fe).
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure. 
FCC - Aluminium (Al), Copper(Cu), Gold (Au), Iridium (Ir), Lead(Pb), Nickel (Ni), Platinum (Pt), Silver (Ag).
HCP - Beryllium(Be), Cadmium (Cd), Magnesium (Mg), Titanium (Ti), Zinc (Zn), Zirconium (Zr)

2018.76. Which of the following is an amorphous material?
a) mica
b) lead
c) rubber
d) glass

Answer
c) rubber and d) glass

Explanation
In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal.
Mica - The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms. So it is not amorphous
Rubber - It is amorphous. But under stretched conditions it is partly amorphous and partly crystalline
Glass - Glass is a non-crystalline amorphous solid

Types of Iron

2006. 1. In nodular iron, graphite is in the form of
a) Cementite
b) Free Carbon
c) Flakes
d) Spheroids

Answer
d) Spheroids

Explanation
Nodular cast iron is also called ductile cast iron or spheroidal graphite iron. As graphite is in the form of nodules they inhibit formation of cracks and hence more ductile.
In grey cast iron where graphite is in the form of flakes exhibit more stiffness. On extension they create concentration points and hence are less ductile.
Free carbon that is present in an elemental form is often an impurity which could be removed by smelting.
Cementite is iron carbide with 6.67% Carbon by weight.

Alloys of Iron

2006. 2. Hardness of steel depends on
a) Amount of carbon it contains
b) The shape and distribution of of the carbides in the iron
c) Method of fabrication
d) Contents of alloying elements

Answer
a) Amount of carbon it contains

Explanation
As the carbon content in steel increases, its hardness increases.
Alloying elements other than Carbon are primarily used to reduce corrosion rate(Nickel, Copper).

2017.2.71. Element which makes steel stainless
a) Mg
b) Ni
c) C
d) Cr

Answer
d) Cr

Explanation
Chromium is the dominant other metal present in stainless steel (> 10%). It helps forming a very thin stable film of its oxide and hydroxide which limits additional corrosion and is not visible contrary to Iron oxides and hydroxides.
Nickel - Increases strength and hardness without sacrificing ductility and toughness. It also increases resistance to corrosion and scaling at elevated temperatures when introduced in suitable quantities in high-chromium (stainless) steels.
Carbon - The most important constituent of steel. It raises tensile strength, hardness, and resistance to wear and abrasion. It lowers ductility, toughness and machinability.

 Magnesium - Magnesium is not used in stainless steel. Magnesium - iron alloys are used to make electrochemical heat sources.

Material Properties

2017.2.27. Arrange the metals Aluminium, Copper, Iron & Silver in increasing order of thermal conductivity
a) Iron, Aluminium, Copper, Silver
b) Aluminium, Iron, Copper, Silver
c) Copper, Iron, Silver, Aluminium
d) Iron, Copper, Aluminium, Silver

Answer
a) Iron Aluminium, Copper, Silver

Explanation
Thermal conductivity of Silver - 406 W/mK
Thermal conductivity of Copper - 385 W/mK
Thermal conductivity of Aluminium - 205 W/mK
Thermal conductivity of Iron - 79.5 W/mK

Phase Transformation


2017.2.11.The reaction in which a liquid phase transform into two different solid phases is called
a) Eutectoid reaction
b) Peritectic reaction
c) Eutectic reaction
d) Peritectoid reaction

Answer
c) Eutectic reaction

Explanation
Eutectoid reaction - This describes the phase transformation of one solid into 2 different solids. In the Fe-C system there is a eutectoid point at approximately 0.8%wt C, 723oC.
Solid γ gives Solid α + Solid β 
or Austenite gives Ferrite + Cementite
Peritectic reaction - A peritectic reaction is a reaction where a solid phase and liquid phase will together form a second solid phase at a particular temperature and composition
Eutectic reaction - A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time.
Peritectoid reaction - A type of isothermal reversible reaction in the solid state that results in a new n-1 solid phase from n phases. 

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