Important Short Questions and Answers: Modern Engineering Materials

SHORT QUESTIONS WITH ANSWERS

1.  What are metallic glasses?

Metallic glasses are amorphous metals (Glass metals). It is a non-crystalline alloy with molten liquid state it has the short range of atomic order. Metallic glasses share the properties of both glasses and metals, these are strong, ductile, malleable, opaque and brittle.

2.Define Glass transition temperature

It is the temperature at which liquid like structure is frozen into the solid. The glass transition temperature for metallic alloys in about 20^OC to 30^OC

3.How the metallic glasses are prepared? Mention the different methods.

Metallic glasses are usually prepared by rapid quenching, ie., cooling a metallic liquid so rapidly such that there is no enough time for ordered crystalline structure to develop. By this way we retained the liquid like structure into the solid. The cooling rate is in the order of 2 x 10 Kelvin per second

Methods of preparation

         Twin roller system

         Melt spinning system

         Melt Extraction system

Melt spinning system is the most commonly used method.

4. What are the types of metallic glasses?

Metallic glasses are classified into two types

         Metal – Metal metallic glasses

These are combination of metals and metals Example: Ni-Nb, Mg-Zn, Cu-Zr, and Hf-V

         Metal – Metalloid metallic glasses

These are combinations of metals and metalloids Example: Fe, Co, Ni and B, Si, C, P

5.  What are the advantages of the metallic glasses used as core material?

         Transformer core is used to get maximum magnetic flux (energy). Transformer core should have low hysteresis loss and low eddy current loss.

         Metallic glasses are available in thin sheets therefore the size and weight of the transformer is reduced.

         Hysteresis loss is directly proportional to the area of the hysteresis loop. The loop area of the metallic glasses is very small and also has high initial permeability. So the hysteresis loss is almost zero.

         The eddy current in the core inversely proportional to the resistivity of the core material and directly proportional to the thickness of the lamination of the core. Since the resistivity of the metallic glasses is high and the thickness of the core laminated core is less. Therefore the eddy current loss is very less.

         Thus metallic glasses used as core material due to small thickness, smaller area, less weight high resistivity soft magnetic with low hysteresis and eddy current losses.

6. What are the applications of metallic glasses?

Metallic glasses are used in

         Reinforcing elements in concrete, plastic, rubber

         Reinforcing filament in pressure vessels

         Construction of flywheels for energy storage

         Razor blades and different kinds of springs

         Tape recorder heads, etc,

7.What are SMAs (or) Shape Memory Alloys? What are its types?

The ability of some metallic alloys to retain their original shape when heating or cooling is called as Shape Memory Alloys (SMA). It is also called as smart materials (or) intelligent materials or Active materials. There are two types of shape memory alloys.

         One way shape memory : It returns to its memory only when heating

Two way shape memory: It returns to its memory on both heating and Cooling.

8.How the SMAs are classified? Give Examples of SMAs.

         Piezo electric SMA materials.

         Electrostrictive SMA materials.

         Magnetostrictive SMA materials.

         Thermo elastic SMA materials.

Examples: Ni-Ti alloy, Cu-Zn –A1 alloy, Cu – A1 – Ni alloy, etc.,

9.Give the principle of Shape memory Alloys

The shape memory effect occurs between two temperatures states, ie., Martensite and Austenite. The SMA in the Marteniste is attained the temporary deformed shape when cooling. But when heating the martensite state is changed into Austenite state. During deformation the resistivity, thermal conductivity, Young modulus and yield strength are decreased by more than 40%.

10.What are the martensite and austenite phases?

Martensite is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. It has two molecular structures 1. Twinned martensite and 2. Deformed martensite.

Austenite is the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic.

11.Define Shape Memory Effect?

Some metallic alloys exhibit plastic nature when they are cooled to very low temperature and they return to their original nature when it is heated, this effect is known as Shape Memory Effect.

12.Name the materials exhibit Shape memory effect?

Most effective and widely used alloys are

         Ni Ti (Nickel – Titanium)

         Cu Zn A1,

         Cu A1 Ni,

         Au – Cd,

         Ni-Mn-Ga and,

         Fe based alloys.

13.What is pseudo elasticity?

When a metallic material is cooled from a temperatureT₂ to a lower temperatureT₁, it deforms and changes its shape. On reheating the material to Temperature T₂ , the shape change is recovered so that the material returns to its original state. This effect is known as pseudo elasticity or thermo elastic property.

14.Define Hysteresis of a SMA?

Hysteresis of a SMA is defined as the difference between the temperatures at which the material is 50% transformed to austenite when heating and 50% transformed to martensite when cooling.

15.What is Super elasticity?

Super elasticity is a property of SMA. When a material is deformed at a temperature slightly greater than its transformation temperature super elasticity property appears. (Rubber like property)

16.What are the advantages and disadvantages of SMA?

Advantages

         SMAs are Simple, compactness and safety

         SMAs have Good biocompatibility

         SMAs are Strong and have high corrosion resistance

         They have High power to weight ratio.

Disadvantages

         SMAs have Poor fatigue

         They are Expensive

         SMAs have Low energy efficiency

         SMAs have Complex control

         They Limited bandwidth.

19.What are the applications of Shape memory alloys?

         It is used as a thermostat valve in cooling system.

         It is used as a fire safety valve.

         It is used for cryofit hydraulic couplings.

         It is used for eye glass frame, toys.

         It is used to make microsurgical instruments, orthopedic implants.

         It is used as blood clot filter and for fracture pulling.

         It is used as antenna wires in cell phones.

20.What is Nano Technology or Nano Science?

Nanotechnology is a field of applied science and technology which deals with the matter on the atomic and molecular scale, normally 1 to 100 nm, and the fabrication of devices with critical dimensions that lie within that size range.

21.What are Nanomaterials? (AU., June 2007)

Nanomaterials are the materials with atoms arranged in nano sized clusters which become the building block of the material. Any Material with a size between and 100 nm[ 10^–9 m to 10^–7m] is called nanomaterials.

22.What is Top – down approach? Give its methods.

Top – down approach

The removal or division of bulk material or the miniaturization of bulk fabrication processes to produce the desired nanostructure is known as top-down approach. It is the process of breaking down bulk material to Nanosize

Methods : Top – down Methods, Milling, Light graphics and Machining

23.What is Bottom-up approach? Give its Methods

Bottom-up approach : Molecules and even nano particles can be used as the building block for producing complex nanostructures. This is known as Bottom-up approach. The Nano particles are made by building atom by tom.

Bottom up Methods: Vapour phase deposition, Molecular Beam epitaxy, Plasma assisted deposition, Metal Organic vapour phase epitaxy (MOVPE) Liquid phase process [Colloidal method and Sol-Gel method]