Perfect Crystal Brittleness: Fact or Myth?

[bchl85]

My physics teacher taught me that, however, when I asked my chemistry teacher, he said NO. And now ... I'm confused. Plz anyone can help me to make me clear about this with explanation.?

 

[Gonzolo]

All ionic cristals will break rather than bend, but some are extremely hard to break. Diamond is a cristal, but it cannot be broken easily. It is brittle in the sense that it would still break before bending.

 

[PerennialII]

Perfect crystals have extreme strength but poor toughness (like fracture toughness or related measures of material "ductility") due to bond & lattice structure as said above, and as such are extremely brittle. Can't grasp the chemistry part of the question ...

 

[Gokul43201]

No.

A perfect crystal does not have to be brittle. In fact it is easier to embrittle a metal by adding impurities (steel is more brittle than iron), or dopants which cause embrittlement mostly due to grain boundary weakening.

Among perfect crystals, the brittleness (or inversely, the ductility) is a function of the crystal structure and the elements occupying the lattice. For instance, the FCC crystal has slip planes along the {111} family, which makes most FCC metals ductile. On the other hand, the absence of such slip planes in a BCC crystal makes metals with that structure less ductile.

So, it should come as no surprise that copper, silver and gold (all FCC - sometimes referred to as CCP - metals) are very ductile, while iron and tungsten (BCC) are much less so.

 

[PerennialII]

We'd have to be sure whether we're talking about "crystalline" materials or crystal structures, i.e. link it to bond type. Sure, if the latter the above is correct, structure itself does not turn it one way or the other in this case. Might actually be the reason for the two opposite interpretations of ductility.

 

[Gokul43201]

We'd have to be sure whether we're talking about "crystalline" materials or crystal structures, i.e. link it to bond type. Sure, if the latter the above is correct, structure itself does not turn it one way or the other in this case. Might actually be the reason for the two opposite interpretations of ductility.

You're comparing apples and sweetness.

Crystalline materials have crystal structures. Crystal structures are not things; they are a description of the arrangement of atoms in crystalline materials.

 

[PerennialII]

Thats why the quotations, not too clear I got to admit... however, material scientists, in terms of fracture mechanics in particular, many times tend to affiliate the word crystalline to materials where the description applies best, like ceramics e.g. with full covalent bonding the model example being diamond... in which case the analogy to brittleness becomes quite straightforward. When talking about structure of metallic materials I'd rather use the expression "lattice" to characterize atomic structure and also give an implication of the underlaying properties and deformation behavior.

 

[ZapperZ]

Thats why the quotations, not too clear I got to admit... however, material scientists, in terms of fracture mechanics in particular, many times tend to affiliate the word crystalline to materials where the description applies best, like ceramics e.g. with full covalent bonding the model example being diamond... in which case the analogy to brittleness becomes quite straightforward. When talking about structure of metallic materials I'd rather use the expression "lattice" to characterize atomic structure and also give an implication of the underlaying properties and deformation behavior.

However, if we go by your definition of "crystalline", then the question about something having a "perfect" crystal doesn't make any sense.

I agree with Gokul's answer. A "perfect crystal" here would tend to imply a material with only a single crystal orientation, rather than a polycrystal or a single crystal with major impurities and other defects. If we go by this, then it is incorrect to deduce that a perfect crystal is more brittle due to the fact that this properties is not a function of the "perfectness" of the crystal allignment, but rather the nature of the crystal structure itself. Some of the most brittle materials are amorphous which don't even have a well-defined crystal structure, like chalk.

Zz.

 

[Integral]

Having broken my share of Si wafers, I can testify that a Si cyrstal is brittle.

 

[PerennialII]

However, if we go by your definition of "crystalline", then the question about something having a "perfect" crystal doesn't make any sense.

I agree with Gokul's answer. A "perfect crystal" here would tend to imply a material with only a single crystal orientation, rather than a polycrystal or a single crystal with major impurities and other defects. If we go by this, then it is incorrect to deduce that a perfect crystal is more brittle due to the fact that this properties is not a function of the "perfectness" of the crystal allignment, but rather the nature of the crystal structure itself. Some of the most brittle materials are amorphous which don't even have a well-defined crystal structure, like chalk.

Yeah, I think the point I'm trying to figure out what "perfect" is the initial question aiming at (since it appears that people, like us for one, can interpret different aspects of it being "perfect")? I didn't see that being quite as straightforward, whether we're aiming at the crystal order, a specific property or a feature of the crystal, or what is contained within the crystal and its constituents. Since we're talking about crystals that limits the materials whose basic brittleness we're discussing. I agree that if we stick with single crystal orientation what you're saying is correct, however, that makes the deduction of brittleness / ductility dubious, suggesting a possibility that another angle should be looked at.

 

[Gokul43201]

Having broken my share of Si wafers, I can testify that a Si cyrstal is brittle.

Which can be deduced from the fact that Si has a diamond-cubic structure, with no slip planes. I've broken my share of GaAs wafers - intentionally. GaAs, like Si has cleavage planes, which make it easy to cut out chips (but in GaAs the cleavage planes are the {100} planes - good for rectangular chips) by simply breaking it from a scribed mark.

Pereniall, I'm not sure I follow your argument. Yes, it is uncertain what the OP means by a perfect crystal. Typically, however, it means a defect-free single-crystal : no impurities, no dislocations, no grain boundaries, and infinitely large (no edge effects).

 

[Gokul43201]

... however, material scientists, in terms of fracture mechanics in particular, many times tend to affiliate the word crystalline to materials where the description applies best, like ceramics e.g. with full covalent bonding the model example being diamond... in which case the analogy to brittleness becomes quite straightforward. When talking about structure of metallic materials I'd rather use the expression "lattice" to characterize atomic structure and also give an implication of the underlaying properties and deformation behavior.

I am a materials scientist, and we use the word 'crystalline' to describe all materials that have a periodic arrangement of atoms, irrespective of whether they are covalent (diamond) or ionic (NaCl), ceramic (Al2O3) or metallic (Cu), semiconductor (Si) or superconductor (YBa2Cu3O7) - that just doesn't matter.

A lattice is simply a periodic array of points. When you put atoms in these points, you get a crystal (or a crystal lattice). All crystalline materials have a lattice. The arrangement of these lattice points (along with the basis) is called the lattice structure or crystal structure.

What gives you the impression that the term 'crystalline' is used only to refer to ceramics or covalently bonded materials ? I hope they don't teach that in school ?

 

[PerennialII]

I am a materials scientist, and we use the word 'crystalline' to describe all materials that have a periodic arrangement of atoms, irrespective of whether they are covalent (diamond) or ionic (NaCl), ceramic (Al2O3) or metallic (Cu), semiconductor (Si) or superconductor (YBa2Cu3O7) - that just doesn't matter.

A lattice is simply a periodic array of points. When you put atoms in these points, you get a crystal (or a crystal lattice). All crystalline materials have a lattice. The arrangement of these lattice points (along with the basis) is called the lattice structure or crystal structure.

What gives you the impression that the term 'crystalline' is used only to refer to ceramics or covalently bonded materials ? I hope they don't teach that in school ?

Pereniall, I'm not sure I follow your argument. Yes, it is uncertain what the OP means by a perfect crystal. Typically, however, it means a defect-free single-crystal : no impurities, no dislocations, no grain boundaries, and infinitely large (no edge effects).

The issue was trying to answer the question on how different people infer different terms inherently different. I do agree very much with how you're using the term crystal and what a typical 100% perfect crystal were to be. In this case it just doesn't lead to any sort of information on brittleness, which to me suggests the person is thinking about something else and using a different branch of termilogy / definition.

edit :

So does this essentially end up that both teachs have a defect free perfect crystal, the chemistry one having a perfectly ordered one, the physics one adding also "perfect bonding" to the definition (a la covalent etc. essentially eliminating mechanisms of deformation)?

 

[bchl85]

The issue was trying to answer the question on how different people infer different terms inherently different. I do agree very much with how you're using the term crystal and what a typical 100% perfect crystal were to be. In this case it just doesn't lead to any sort of information on brittleness, which to me suggests the person is thinking about something else and using a different branch of termilogy / definition.

Thank you all. I'm studying solid material. And it's quite new things with me.
In my notes, I wrote: Almost perfect crystals of material contain few dislocations to cause plastic flow. Therefore, they are strong but brittle.
That's what physics teacher taught.

 

[PerennialII]

Thank you all. I'm studying solid material. And it's quite new things with me.
In my notes, I wrote: Almost perfect crystals of material contain few dislocations to cause plastic flow. Therefore, they are strong but brittle.
That's what physics teacher taught.

My prof. went the same way, adding other properties to the definition of a perfect crystal than its arrangement ... I'd say its a bit a school/teach/prof thing (in the process causing the commotion above ).

 

[bchl85]

Haha, yeah ... I totally want to sleep when my teacher is teaching... It seems the more he teaches, the more I don't understand