Ferro-electricity Ferro-electricity is defined as the spontaneous alignment

of electric dipoles by their mutual interaction in theabsence of an applied electric field.

This arises from the fact that the local field increases inproportion to the polarization. Thus, ferro-electricmaterials must posses permanent dipoles. Ex.: BaTiO3,Rochelle salt (NaKC4H4O6.4H2O), potassium dihydrogenphosphate (KH2PO4), potassium niobate (KNbO3).

These materials have extremely high dielectric constants atrelatively low applied field frequencies.

Thus, capacitors made from ferro-electric materials aresmaller than capacitors made of other dielectric materials.

Piezo-electricity Piezo-electricity, or pressure electricity, is defined as

polarization induced by the application of external force. Hence, by reversing the direction of external force, direction

of the field can be reversed i.e. the application of anexternal electric field alters the net dipole length and causesa dimensional change.

This property is characteristic of materials havingcomplicated structures with a low degree of symmetry.

Thus, piezoelectric materials can be used as transducers –devices that convert mechanical stress into electrical energyand vice versa.

Application for these materials includes microphones,ultrasonic generators, sonar detectors, and mechanicalstrain gauges. Ex.: Barium titanate, lead titanate, leadzirconate (PbZrO3), ammoinium dihydrogen phosphate(NH4H2PO4), and quartz.

IES-1993Quartz is a

(a) Ferroelectric material

(b) Ferromagnetic material

(c) Piezoelectric material

(d) Diamagnetic material

Welding cracks Cracks may occur at the following locations :-

(i) In the weld (Fuse) Metal Zone

(ii) In the Base metal zone

(iii) Sometimes, the cracks originate in one Zone and then spread to the other Zone.

Contd…

Cracks in the base metal usually occur at low temperatures (around 200°C) and are known as Cold cracks.

On the other hand, the cracks in the weld metal zone occur while the metal is still very hot. Such cracks are called Hot cracks.

Cracks in the weld may be due to structural stresses in the metal (for example, the formation of martensite), heavy shrinkage, extra high amount of sulphur, phosphorous or carbon in the metal, excessively rigid clamping of the parts being welded or the presence of gases in the weld metal.

Contd…

Cold cracking can occur due any to the following several factors :- improper welding conditions, the presence of gas and other impurities in the weld, wrong choice of filler rod and metallurgical factors such as excessive cooling rate resulting in the formation of martensite and formation of brittle phases in the weld when cold or the formation of phases which are brittle at high temperatures [Allotropic transformations].

Cracks in the base metal can occur due to the following reasons :- Corrosion, Base metal defects, Base metal composition variations, hydrogen embrittlement and internal stresses set up due to restrained shrinking after welding.

IES-2005Hot cracks occur in the weld and fusion zone as the metal solidifies. Which of the following are the causes for hot cracks?

1. Presence of sulphur and phosphorus in the base metal

2. High carbon or alloy content of the base metal

3. Moisture in the joint or electrode

4. Joint restraint

Select the correct answer using the code given below:

(a) 1, 2 and 4 (b) 1, 2 and 3

(c) 3 and 4 (d) 1, 2, 3 and 4

H-embrittlement and Passivity Hydrogen embrittlement is form of failure than

corrosion,but occurs as a result of hydrogen produced during corrosion. Atomic hydrogen diffuses into crystals and inhibits dislocation motion, causing failure.

Counter measures: heat treatment; removal of hydrogen source.

Passivity: It is form protection against corrosion. It results from thin, strong adherent oxide layer formed over the surface. Usually observed in Al, Cr, Fe, Ni, Ti and their alloys. Passive layers may get damaged during mechanical vibrations, and so these metals are prone to erosion corrosion.

IES-1992Presence of hydrogen in steel causes

(a) Reduced neutron absorption cross-section

(b) improved weldability

(c) Embrittlement

(d) corrosion resistance

Other questions

IES-2006In case of power screws, what is the combination of materials used for the screw and the nut?

(a) Cast iron screw and mild steel nut

(b) Carbon steel screw and phosphor bronze nut

(c) Cast iron screw and cast iron nut

(d) Aluminium screw and alloy steel nut

IES-1992For the pipe fitting like elbow, tee, union etc. which of the following is preferred?

(a) Pig iron

(b) Malleable iron

(c) Spheroidal graphite cast iron

(d) High carbon steel

IES-1992Which of the following display properties similar to that of steel

1. Black-heart cast iron

2. White-heart cast iron

3. Gray cast iron

4. Pig iron

(a) 1 and 2 only (b) 3 and 4 only

(c) 2 and 4 only (d) 1 and 3 only

IES-1992Addition of which of the following improves machining of copper?

(a) Sulphur

(b) Vanadium

(c) Tin

(d) Zinc

IES-2006Disruptive strength is the maximum strength of a metal, when

(a) Subjected to 3 principal tensile stresses at right angles to one another and of equal magnitude

(b) Loaded in tension

(c) Loaded in compression

(d) Loaded in shear

IAS-1996Match List I with List II and select the correct answer using the codes given below the lists:

List I (Cutting tools) List II (Major constituent)

A. Stellite l. Tungsten

B. H.S.S. 2. Cobalt

C. Ceramic 3. Alumina

D. DCON 4. Columbium

5. Titanium

Codes: A B C D A B C D

(a) 5 1 3 4 (b) 2 1 4 3

(c) 2 1 3 4 (d) 2 5 3 4

IAS-1998Which of the following methods are suitable for the production of super alloys?

1. Atomization from molten state using inert gas.

2. Atomization using plasma arc and rotating electrode.

3. Reduction and crushing.

Select the correct answer using the codes given below:

Codes:

(a) 1 and 2 (b) 2 and 3

(c) 1 and 3 (d) 1, 2 and 3

IES-1996The limit to the maximum hardness of a work material which can be machined with HSS tools even at low speeds is set by which one of the following tool failure mechanisms?

(a) Attrition

(b) Abrasion

(c) Diffusion

(d) Plastic deformation under compression.

GATE-2002The ductility of a material with work hardening

(a) Increases (b) Decreases

(c) Remains unaffected (d) Unpredictable

IES-2009Nano composite materials are highly preferable in design consideration for their

(a) High resistance to crack propagation

(b) Vibration resistance

(c) Impact resistance

(d) High resilience

IES-1992Machine tool frame should have

(a) High rigidity to weight ratio

(b) Graphite in the form of nodules

(c) Low hardness

(d) High work hardness

GATE-1995Machine tool structures are made …………..for high process capability. (tough/strong/rigid)

IES-2009Which one of the following possesses the property of nonsparking character?

(a) Hadfield's manganese steel (b) Spring steel

(c) Stellite (d) Invar

JWM 2010Consider the following properties of engineering materials

1. Density

2. Conductivity

3. Elasticity

4. Hardness

5. Ductility

Which of these are mechanical properties of a material ?

(a) 1, 2 and 4 only (b) 1, 2, 3 and 4

(c) 3, 4 and 5 (d) 2, 4 and 5

IES 2011Materials which show direction dependent properties are called:

(a) Homogeneous

(b) Visco-elastic

(c) Isotropic

(d) Anisotropic

IES 2011Lead is widely used in:

(a) Transformers

(b) Switch gear

(c) Galvanized pipes

(d) Batteries

IES 2011Match List –I with List –II and select the correct answer usingthe code given below the lists :

Codes

A B C D A B C D

(a) 2 1 4 3 (b) 3 1 4 2

(c) 2 4 1 3 (d) 3 4 1 2

List –I List –II

A. System 1. Free energy is a minimum

B. Phase 2. Chemical elements or chemical compounds

C. Phase equilibrium

3. Consists of solids, liquids or gasses or their combination

D. Components 4. Homogeneous portion of a system that has uniform physical characteristics

GATE-2005When the temperature of a solid metal increases

(a) Strength of the metal decreases but ductilityincreases

(b) Both strength and ductility of the metal decrease

(c) Both strength and ductility of the metal increase

(d) Strength of the metal increases but ductilitydecreases

IAS-1998Magnesium is extruded and not rolled because

(a) It has a low melting point

(b) It has a low density

(c) Its reactivity with roll material is high

(d) It has a dose-packed hexagonal structure

IES 2011Match List –I with List –II and select the correct answer usingthe code given below the lists :

Codes

A B C D A B C D

(a) 1 2 3 4 (b) 4 2 3 1

(c) 1 3 2 4 (d) 4 3 2 1

List –I List –II

A. Elasticity 1. Deform non elastically without fracture

B. Malleability 2. Undergo plastic deformation under tensile load

C. Ductility 3. Undergo plastic deformation under compressive load

D. Plasticity 4. Return to its original shape on unloading

IES 2011Injection moulding process used to producethermoplastic matrix composites with fibrereinforcement normally gives:

(a) Short fibre composites

(b) Two layer structure composites

(c) Continuous fibre composites

(d) Single layer composites

IES 2011Assertion (A) : Solid solutions of metal arecrystal whose properties are close to those ofthe solvent.

Reason (R) : They retain the same crystal latticeand type of bond.

(a) Both A and R are individually true and R is thecorrect explanation of A

(b) Both A and R are individually true but R is NOTthe correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

IES 2010The correct statement is

(a) Characteristic of any series of alloys cannot befound by phase diagram.

(b) Phase diagram does not give amount ofphases which are a function of composition,temperature and pressure.

(c) The phase may be liquid or vapour withordered crystal structure.

(d) Phase diagram provides the information onhow rapidly equilibrium is reached.

JWM 2010Assertion (A) : Electric arc furnace can be used for acid and basic method of steel making.

Reason (R) : Impurities are eliminated extensively in acid are process.

IES 2010Consider the following:

1. Water

2. Ice

3. Brine solution

4. Oil

Which of these is/are used as quenchingmedia in case of Alloy steels?

(a) 1, 2 and 3 only (b) 2, 3 and 4 only

(c) 1 and 4 only (d) Only

The End