Material Science - short answer questions from AMIE exams (Summer 2020)

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Magnetic moment 
The magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets), permanent magnets, elementary particles (such as electrons), various molecules, and many astronomical objects (such as many planets, some moons, stars, etc).

Grain growth 
  • Grain growth refers to an increase in the size of crystallites (grains) in a material at high temperatures. 
  • Grain growth is due to an alteration of the internal energy of the molecular structure of a given metal, ceramic, or mineral. 
  • Grain growth is typical in metals that have undergone annealing and other processes of surface modification. 
  • Due to the destabilization of the molecular structures of metals due to grain growth, the probability of crevice, intergranular, and grain boundary corrosion is increased.
Age hardening 
  • Age hardening, also known as precipitation hardening, is a type of heat treatment that is used to impart strength to metals and their alloys. 
  • It is called precipitation hardening as it makes use of solid impurities or precipitates for the strengthening process. 
  • The metal is aged by either heating it or keeping it stored at lower temperatures so that precipitates are formed. 
  • The metals and alloys need to be maintained at high temperatures for many hours for the precipitation to occur; hence this process is called age hardening. 

Critical cooling rate 
  • It is a rate that just bypasses the nose of the TIT diagram OR the Rate of cooling necessary to just suppress the diffusion transformation
  • The critical cooling rate depends upon the amount of carbon and alloying elements
  • With an increase in carbon content and alloying elements, the critical cooling rate decreases i.e. a shift of the nose of the TTT curve to the right
  • A shift of nose to the right gives an idea about hardenability; the lower the cooling rate higher is the hardenability
  • A lower cooling rate reduces the tendency of warping and cracking
Iron-carbon system 
The iron-carbon phase diagram is widely used to understand the different phases of steel and cast iron. Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a small number of trace elements. See full answer

Work hardening
Work hardening, in metallurgy, increases in hardness of a metal-induced, deliberately or accidentally, by hammering, rolling, drawing, or other physical processes. Although the first few deformations imposed on metal by such treatment weaken it, its strength is increased by continued deformations.

Season cracking 
  • Season cracking occurs in copper-based alloys subjected to a residual or applied tensile stress and exposed to a specific environment, such as moist air containing traces of ammonia.
  • Season cracking is also known as stress corrosion cracking or environmental cracking.
  • Season cracking occurs most often in copper alloys containing more than 15% zinc. This cracking occurs due to a reaction between ammonia and copper, forming the cuprammonium ion, a chemical complex that is water-soluble, and hence washed from the growing cracks. Such cracking can also occur in copper and any other copper alloy, such as bronze.
  • Season cracking is characterized by deep, brittle cracks which penetrate into affected components. 
  • If the cracks reach a critical size, the component can suddenly fracture, sometimes with disastrous results. If the concentration of ammonia is high, then the attack is much more severe, and all exposed surfaces are subject to cracking.
Yield strength 
Yield strength refers to an indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of the elastic limit.

Nanomaterial
  • ISO (2015) defines a nanomaterial as a: ‘material with any external dimension in the nanoscale (size range from approximately 1 – 100 nm) or having an internal structure or surface structure in the nanoscale.
  • Nanomaterials are chemical substances or materials that are manufactured and used at a very small scale. Nanomaterials are developed to exhibit novel characteristics compared to the same material without nanoscale features, such as increased strength, chemical reactivity, or conductivity.
Susceptibility of paramagnetic magnetic material
Magnetic susceptibility, quantitative measure of the extent to which a material may be magnetized in relation to a given applied magnetic field. The magnetic susceptibility of material, commonly symbolized by Xm, is equal to the ratio of the magnetization M within the material to the applied magnetic field strength H, or Xm = M/H.

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