Strengthening, crystalline alloys, and yield strength questions?

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Discussion Overview

The discussion revolves around questions related to the mechanical behavior of materials, specifically focusing on the strengthening mechanisms of crystalline alloys, the properties of single crystalline alloys with fine grains, and the yield strength variations of aluminum alloys during aging. The scope includes theoretical aspects and applications relevant to materials science and engineering.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that carbon (C) is more effective than nickel (Ni) in strengthening iron (Fe) due to its role as an interstitial element and its ability to form carbides, contributing to solid solution and precipitation strengthening.
  • There is a discussion about the desirability of single crystalline alloys with fine grains for low temperature applications, with some participants noting that fine grain alloys exhibit higher strength and toughness but may suffer from creep issues due to grain boundaries facilitating dislocation movement.
  • One participant questions the definition of a single crystal with fine grains and discusses the effects of over-aging in aluminum alloys, suggesting that grain growth can negate the benefits of precipitation hardening.
  • Another participant references a Wikipedia source for information on the yield strength variation of aluminum alloys with aging time and the interaction of dislocations with precipitates.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of carbon versus nickel in strengthening iron, the implications of grain size on alloy performance, and the specifics of aluminum alloy aging. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

There are limitations in the discussion regarding the definitions of terms like "single crystal" and "fine grains," as well as the assumptions underlying the explanations of strengthening mechanisms and aging processes. Some participants reference external sources without fully integrating those ideas into the discussion.

Who May Find This Useful

This discussion may be useful for students and professionals interested in materials science, particularly those studying mechanical behavior, alloy strengthening mechanisms, and the properties of crystalline materials.

hermtm2
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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.
 
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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.
 
I've the idea from the wiki source.

Thanks,
Ryan.
 
hermtm2 said:
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).


hermtm2 said:
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.

hermtm2 said:
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.

Other people answered this.
 

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