Material Science -- using materials without specifying crystal directions

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Discussion Overview

The discussion revolves around the representation of material properties in practical applications without specifying crystal directions. Participants explore whether this implies that materials are considered isotropic and the implications of crystal orientation on material behavior.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question if the lack of specification for crystal directions suggests that all materials are isotropic.
  • Others argue that there are indeed applications where crystal axes are significant, indicating that not all materials can be treated as isotropic.
  • One participant notes that many material properties are measured in both transverse and machine directions, highlighting the complexity of material behavior.
  • A later reply discusses polycrystalline materials, suggesting they can be approximately isotropic when the length scale of interest exceeds the grain size, although they may exhibit texture due to processing history.
  • Examples are provided, such as the stiffness of iron and low-alloy steels, which are presented as averages of the properties of randomly oriented grains.
  • Links to external resources are shared to illustrate specific applications, such as single-crystal turbine blade casting, which emphasize the importance of crystal orientation in certain contexts.

Areas of Agreement / Disagreement

Participants express differing views on the isotropy of materials and the relevance of crystal directions in practical applications. There is no consensus on whether the generalization of material properties without crystal direction is appropriate.

Contextual Notes

The discussion highlights the limitations of generalizing material properties, particularly in relation to the scale of observation and the potential influence of processing history on material texture.

Evilavatar2
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Why in most practical applications, most materials properties are given without specifying crystal directions. Are they trying to say all materials are isotropic?
 
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Evilavatar2 said:
Why in most practical applications, most materials properties are given without specifying crystal directions. Are they trying to say all materials are isotropic?
Can you give some examples of the applications you have in mind? Certainly there are applications where crystal axes do matter...
 
Evilavatar2 said:
Why in most practical applications, most materials properties are given without specifying crystal directions. Are they trying to say all materials are isotropic?

What are you talking about? Many material properties are measure in both the transverse and machine direction. Of course amorphous materials have no crystal structure.
 
Evilavatar2 said:
Why in most practical applications, most materials properties are given without specifying crystal directions. Are they trying to say all materials are isotropic?

Polycrystalline materials are approximately isotropic when the length scale of interest is much larger than the grain size and the grains are randomly oriented. (Subtlety: materials can have a so-called "texture," which in a materials science context refers not to roughness but to a predominant crystalline orientation, due to processing history.) For example, a handful of iron is going to have a bulk stiffness of 210 GPa, even though the (111) stiffness of each microscopic grain is 270 GPa and the (100) stiffness is 125 GPa. For that matter, the stiffness of low-alloy steels is generally also around 200 GPa because steel is predominantly polycrystalline iron. For certain bulk properties such as stiffness, what you're measuring at the macroscopic scale is essentially the average value of a huge number of randomly oriented microscopic grains.
 
berkeman said:
... Certainly there are applications where crystal axes do matter...

https://www.theengineer.co.uk/issues/june-2015-online/jewel-in-the-crown-rolls-royces-single-crystal-turbine-blade-casting-foundry/
 
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Nidum said:
https://www.theengineer.co.uk/issues/june-2015-online/jewel-in-the-crown-rolls-royces-single-crystal-turbine-blade-casting-foundry/
That's truly amazing! Thanks for sharing :smile:
 

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