Alloys (Ni, Ti, Ai) and Strengthening Mechanisms

Question 1

Whats the difference between elastic and plastic deformation?

Answer 1

Elastic deformation - reversible, object returns to original shape after loading removed, bonds are stretched/bent but not broken, follows hookes law.

Plastic deformation - irreversable, permanent change in geometry after loading removed (doesn’t return to original shape), bonds are broken and rearranged.

Question 2

Describe the main characteristics of the alpha phase for alloys?

Answer 2

• 0% Beta composition, alpha phase forms in solid during solidification (hcp), alpha phase diffuses into vacancies of the lattice as temperature falls
• Grains and grain boundaries form between the alpha phases of the microstructure due to different atom array orientations

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Question 3

Describe the main characteristics of the Beta phase for alloys?

Answer 3

0% Alpha composition, Beta lattice and beta (bcc) form in solution vacancies, during solidification, beta phase diffuses into vacancies of the lattice as temperature falls

• Grains and grain boundaries form between the beta phases of the microstructure due to different atom array orientations

Question 4

Why does alpha+beta (50/50) phases form optimum alloy properties?

Answer 4

Both alpha and beta phases diffuse into vacancies as solidification occurs known as a EUTECTIC COMPOSITION. This forms a eutectic point where the solid has a lower melting temperature than that of the individual alpha and beta phases separately, leading to more desirable properties.

Question 5

Describe the main characteristics of the Beta (25%) and alpha (75%) phase for alloys?

Answer 5

Primary alpha forms in vacancies (pro eutectic), as it further solidifies, secondary alpha (eutectic) and primary beta forms in-between the primary beta phase, operated by grain boundaries.

Question 6

Describe the main characteristics of the Beta (75%) and alpha (25%) phase for alloys?

Answer 6

Primary Beta forms in vacancies (pro eutectic), as it further solidifies, secondary beta (eutectic) and primary alpha forms in-between the primary beta phase, operated by grain boundaries.

Question 7

Describe the deformation mechanism “creep”

Answer 7

Over time, under heat, loading stays the same (constant stress) whilst deformation continues.

Question 8

What is a dislocation?

Answer 8

1D line defect in crystal, moves through the lattice by breaking old bonds and forming new bonds with neighbouring atoms.

Due to dislocations, not all bonds between lattice planes need to be broken in order to shear them, it is enough to overcome only one binding series at a time.

Question 9

What is the logic of strengthening when considering dislocations?

Answer 9

• Strengthening means reducing/avoiding plastic deformation
• Hence reduce dislocation movement
• Harder for dislocation to break old bonds and form new bonds with neighbouring atoms

Question 10

Explain the process of work hardening

Answer 10

1. Upon solidification, atoms arrange themselves to form crystals
2. Atom arrays with different orientations then form grains (with grain boundaries in-between them)
3. As dislocation movement deforms the crystal structure, the grain boundary impedes the dislocation movement.
4. The longer and the more grain boundaries, the more difficult for the dislocation to move.

• By deforming the metal (working), the metal grains get deformed and grain boundaries are longer and closer together, increasing dislocation density (more difficult for dislocations to move).

Question 11

Explain the process of Solid solution hardening

Answer 11

1. Dissolve another metal into the metal to create an alloy.
2. Solid solution has defects (alloying solute) making it harder for dislocations to move under stress.

• “Large/small substitutional solid solution strengthening” occurs when the solute atom is large enough that it can replace solvent atoms in their lattice positions (large solute atoms stretch out nearby bonds, small solute atoms pull in nearby bonds)
• “interstitial solid solutions” occur when the solute atom is small enough to fit at interstitial sites between the solvent atoms

Question 12

Explain the process of precipitation hardening

Answer 12

• Form fine particles of a second phase within the metal matrix to increase strength of metal alloy.

1. Heat the metal to dissolve the alloying elements (solutionising) then cool rapidly (quenching) to create a supersaturated solution.
2. Metal is aged at a lower temperature, causing alloying elements to precipitate out of the solution (coarsening) and form tiny particles that hinder movement of dislocations.

Question 13

What are the advantages/ Disadvantages of Nickel Alloy?

Answer 13

• Highest yield strength, and good creep resistance up to ~1000°C
• low corrosion resistance

Question 14

What are the advantages/ Disadvantages of Aluminium Alloy?

Answer 14

• High yield strength, low density and corrosion resistance
• lowest melting temperature

Question 15

What are the advantages/ Disadvantages of Titanium Alloy?

Answer 15

• High yield strength, low density and corrosion resistance, highest melting temp
• High cost