Strengthening, crystalline alloys, and yield strength questions?

[hermtm2]

Hi, these are questions from my exam from the mechanical behavior of materials. I seriously screwed the exam up. Can you guys help me out?

a) Provide two reasons why C is more effective than Ni in strenghtening Fe.

b) Explain why single crystalline alloys with fine grains are more desirable for low temperature applications.

c) Show schematically how the yield strength of an aluminum alloy may vary with aging time and explain how dislocations interact with the precipitates in different parts of the aging curve.


Thanks,
Ryan.

 

[mah65]

The answer to question C can be found here:

http://en.wikipedia.org/wiki/Precipitation_hardening

 

[Enthalpy]

What is a single crystal with fine grains? Did I get this wrongly?

C makes precipitates with Fe while Ni is soluble.

An over-aged aluminium alloy (one the responds to heat treatment) softens again, for instance T73 is softer than T6. Do I remember that grains grow, cancelling the benefit of precipitation?

More generally, metallurgists have so many explanations ready that they can explain everything, and predict, err, everything as well, but afterwards.

 

[hermtm2]

I've the idea from the wiki source.

Thanks,
Ryan.

 

[streeters]

a) Provide two reasons why C is more effective than Ni in strenghtening Fe.

All strengthening comes from Grain refinement, Phase manipulation, Solid solution strengthening, Work hardening, or Precipitate strengthening.

I think it is because C stays as an interstitial element and acts as a Solid Solution Strengthening agent, and also C can form carbides with other alloying elements and act as a Precipitation Strenghting agent.

If there is enough Ni, it can retain austenitic structure (fcc) rather than the normal ferrite structure (bcc) which is more ductive due to the close packed lattice planes (which allows elimination of dislocations due to different stacking fault energies).

b) Explain why single crystalline alloys with fine grains are more desirable for low temperature applications.

fine grain alloys have higher strength, due to grain refinement, and higher toughness, etc. They have superior qualities in most field, really, EXCEPT creep. Creep is elongation at high temperatures and is made a lot worse by grain boundaries as these allow pathways for dislocation movement giving greater elongation.

c) Show schematically how the yield strength of an aluminum alloy may vary with aging time and explain how dislocations interact with the precipitates in different parts of the aging curve.

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