High temperature materials

Question 1

Properties of materials required to operate at high temperatures

Answer 1

resistance to creep microstructural stability resistance to corrosion resistance to oxidation

Question 2

Nickel based alloys

Answer 2

density = 8900kgm^-3 high melting point - 1455 degrees Young’s modulus = 207GPa specific modulus = normal FCC up to the melting point alloying for precipitation forming, solution hardening, carbide formation and grain boundary strengthening

Question 3

Ni-based alloys microstructure

Answer 3

microstructure is FCC gamma matrix with gamma prime precipitates the precipitates are cubiodal in shape and occupy about 70% of the metal by volume

Question 4

Define coherency

Answer 4

small boundary energy between the gamma and gamma prime so the structure is stable

Question 5

What is the basic requirement for an alloy to be amenable to age hardening?

Answer 5

The solid solubility of the alloying elements decreases with temperature. This allows precipitates to form as the alloys cool

Question 6

Ageing time

Answer 6

• coherent precipitates
• interface dislocations
  
  • semi-coherent precipitates
• incoherent precipitates
  
  • coherency in the precipitate decreases as the ageing time increases

Question 7

Precipitate strengthening mechanisms

Answer 7

When precipitates nucleate and grow they intersect with slip planes and dislocation on those slip planes A dislocation will either shear or bypass the precipitate

Question 8

Orowan looping

Answer 8

Question 9

Cast Al alloys

Answer 9

high levels of alloying elements such as Si and Cu coarse, sharp and brittle constituent(Al-Si) can create harmful internal notches and nucleate cracks fatigue properties are very sensitive to large heterogeneties

Question 10

Benefits of rapid cooling

Answer 10

microstructure is greatly refined this improves ductility and tensile strength

Question 11

Refining(primary and eutectic) Si

Answer 11

inoculation modification

Question 12

Inoculation

Answer 12

increased nucleation-finer more evenly distributed primary Si additions of P used Aluminium phosphide(AIP) particles act as nucleation sites for Si. the crystal structure and lattice parameter of AIP are close to that of silicon and the melting point is higher than the Al-Si eutectic temperature

Question 13

Modification

Answer 13

changed growth mechanism of eutectic Si additions of Sr or Na used