Material Science - short answer questions from AMIE exams

If G = 130 GPa for a metal and Poisson's ratio = 0.3, calculate the value of E for the metal.
Modulus of elasticity (E) = 45 MPa, and Poisson's ratio (μ) = 0.31  
The relation between Elastic modulus and Poisson's ratio and Modulus of rigidity is given as:  
E = 2G(1 + μ)    
⇒ 130 = 2 × G (1 + 0.3)
From this, G = 50 GPa

Stainless steels (an alloy of iron, a = 0.2867 nm) always can contain a huge amount of chromium. (a = 0.2885 nm). Explain.
From the given lattice parameters (a) the corresponding radii for iron and chromium are 0.124 nm. and 0.125 nm which are nearly the same so can easily form the crystal bonding. Stainless steel contains about up to 11% chromium.

Define isomorphous systems with examples.
The term isomorphous system is related to phase diagram, isomorphic means having the same crystal structure or complete solubility for all compositions. The copper-nickel system is an example of an isomorphous system.

A pure copper wire has been drawn at a temperature of 750 °C. Is it hot or cold working by relevant parameter?
The melting point of copper is 1085°C. While the temperature at which it is drawn is 750°C which is 305°C less than the melting point. As for hot working the working temperature should be about 50°C less than the melting point, so the working temperature (750°C) is much less (about 250°C) as required for hot working, so the copper wire has been drawn by cold working.

Define 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 - e.g.

Liquid + alpha → beta

These reactions are rather sluggish as the product phase will form at the boundary between the two reacting phases thus separating them, and slowing down any further reaction. 

Define (a) remanence, and (b) coercivity.
  • Remanence: (remnant induction Br) For a ferromagnetic or ferrimagnetic material, the magnitude of residual flux density that remains when a magnetic field is removed is called remanence.
  • Coercivity: In materials science, the coercivity. also called the coercive field or coercive force, of a ferromagnetic material, is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized.
What is the angle between [111] and [121] direction of a cubic crystal?
Cos\,\phi \,\, = \,\,\frac{{{h_1}{h_2} + {k_1}{k_2} + {l_1}{l_2}}}{{\sqrt {h_1^2 + k_1^2 + l_1^2} \,\,x\,\,\sqrt {h_2^2 + k_2^2 + l_2^2} }}

= \frac{{1x1 + 1x2 + 1x1}}{{\sqrt {({1^2} + {1^2} + {1^2})} \sqrt {({1^2} + {2^2} + {1^2})} }} = 0.9428
Hence ⲫ = 19.47⁰

What are superalloys? Give suitable examples.
  • A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Several key characteristics of a superalloy are excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation. 
  • The crystal structure is typically face-centred cubic (FCC) austenitic. 
  • Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. 
Define the glass transition temperature (Tg).
  • The glass transition temperature is described as the temperature at which 30 – 50 carbon chains start to move. At the glass transition temperature, the amorphous regions experience the transition from a rigid state to a more flexible state making the temperature at the border of the solid-state to a rubbery state.
  • The glass transition temperature is represented by Tg and is a property of the amorphous materials.
What is TD nickel?
TD nickel: TD Nickel is a recently developed alloy containing 2 volume percent thoria and the balance nickel (Ni-2ThO₂).
The Thoria-dispersed nickel alloys (TD Nickel) contain thorium oxide additions (~2 wt%) for increased elevated temperature strength up to 1200°C


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