Re : Dislocation versus Microfissure

Is this homework? sounds like homework.

Read through this link, and this one.

all a microfissure is is a microscopic crack, read about what a dislocation is and you will have your answer.

Dislocation is no homework for me. The mathematical treatment of Dislocation (at Wiki-site) is way beyond me.

Umpteen years ago, the initial contact with Dislocation in school was free from maths-heavy.

Well now, going thru NDT stuff, the underlying things in a metal substrate do not include any mention of Dislocation ;

So, I reminisce : Where would it be that Dislocation might fit into the overall picture of Discontinuity ?

Does Dislocation concur with grain boundary ?

Otherwise, Is Dislocation Intra-granular ? Trans-granular ?

Please share your experience. Regards.

The dislocation is intragranular defect.

It is at the crystal level and is created when a crystal (or may be more) do not wish to follow the orderly arrangement (and thereby disturb the others too). This is strictly limited to the grains and do not cross the grain boundaries.

The microfissures are most of the time not intragranular and are larger. And in any case the fissure will split the grain (when it is intragranular) unlike the dislocation (which will create stress and may eventually develop into microfissure but till then..)

A regular NDT text would not mention Dislocation as the "Primary Factor of Leading to Cracks" ; but, Why ?

Regards.

Been a long time. You raise an interesting question.

Think of dislocations as explaining the PLASTIC deformation of metals.

Cracking is not plastic.

End of short answer.

Dislocation is a two dimensional or line defect. Briefly, it is the position where the long range crystal structure breaks down. I think of it as a region of local lattice disturbance between slipped and unslipped regions of the crystal.

The number of dislocations in a crystal is a very large number, they are of normal occurrence. The way that I choose to think about these dislocations is that they are rsponsible for SLIP, not cracking, and thus explain the differences between the theoretical and actual shear strength of metals.

The closest it comes to practical interest is if one were to do a single crystal Yield strngth test, in that case the critical resolved shear stress is essentially the yield stress of the material.

But of more practical interest this is the aspect of the material which demonstrates the role of small amounts of impurities and alloying elements in raising the critical resolved shear stress. These impurities prevent/ impede/inhibit slip. this is why electric furnace high residual steels of an otherwise identical carbon manganese content and thermomechanical history will have such different mechanical properties. the role of impurity atoms in inhibiting slip, and thus plastic deformation. This is why the cold headers want BOF low residual steels.

The other extreme to make this point would be single crystal whiskers which can be grown dislocation free. These have tensile strength that approximate the theoretical for the perfect crystal.

Hope this helps. Dislocation is about plastic deformation effects-not cracking.

Final thought, you mentioned NDT- These are at the crystal lattice scale- requires X ray diffraction to even sense, let alone resolve.

Available tools are far to crude to identify and model / count in commercial use.

Milo

Milo, Esq.,

Thank you very much indeed for the dissertation. I feel blessed.

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Have perused your article several times. Please explain,

1. High/Low Residual Steels ; Are these steels containing "Residual Elements" such as Cr, Cu and Ni ?

2. SLIP and hence the "slip planes" due to shear stress ;

Does a material creep due to SLIP (operating at relatively high temperature, subject to stress and chemical attack) ?

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There were school days umpteen years ago. Watched a video clip (black & white, long before the debut of computer graphics as of nowadays).

On-screen, Dislocations were modeled in the midst of a (rectangular) 2-D matrix of glass beads (or, were they bubbles of sort ?).

A Dislocation was shown as a line demarcating 2 adjacent areas of well aligned glass beads. That line was in fact a void separating the 2 areas.

When the beads are disturbed (such as being pushed on one side of the rectangle), the dislocation moved (as a line of void) to separate a different set of beads.

The video clip was indeed helpful to manifest that aspect of Dislocation (that it moved to separate different matrices in a material).

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As Dislocations would move (so many of them, all at once) in a material under stress, they entangle in a cluster eventually.

The "Spaghetti Model of Dislocations" explained that the material was then work-hardened (becoming less elastic but higher strength and hence more liable to brittle failure than plastic deformation).

Please authenticate that metallography would depict Dislocations as described above.

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In the case of a didactic text on NDT, despite that it does not preach Dislocation :-

Are Dislocations ubiquitous in a material (such as steels in general) down to the scale of individual Crystals or Grains ?

Are the observable defects (such as cracks) traceable to Dislocations in a material ?

Please share your experience to enlighten the Rest of Us.

Regards.

Well I wouldn't exactly call it a dissertation, but your question was well enough constructed to merit a well thought out response.

Have perused your article several times. Please explain, 1. High/Low Residual Steels ; Are these steels containing "Residual Elements" such as Cr, Cu and Ni ?

Milo: YES, Especially Electric furnace scrap fed steels. These are 95% + scrap melt, and thus the incidental elements such as copper, cr, ni, etc. Plus They pick up more nitrogen (a ferrite strengthener) than BOF steels (Basic oxygen furnace is saturated with Oxygen, minimizes pickup of atmospheric nitrogen.Also 25% max scrap charge in BOF or else the chill factor freezes the melt)

2. SLIP and hence the "slip planes" due to shear stress ; Does a material creep due to SLIP (operating at relatively high temperature, subject to stress and chemical attack) ?

Milo: Yes Creep is plastic (non Cracking) deformation of ductile metals. Raising temperature increases energy available of those "beads you mention below" to "slip" or roll over each other. I'm not sure about the role of chemical attack in creep. Sorry.

_______________________________________________________________________ There were school days umpteen years ago. Watched a video clip (black & white, long before the debut of computer graphics as of nowadays). On-screen, Dislocations were modeled in the midst of a (rectangular) 2-D matrix of glass beads (or, were they bubbles of sort ?). A Dislocation was shown as a line demarcating 2 adjacent areas of well aligned glass beads. That line was in fact a void separating the 2 areas. When the beads are disturbed (such as being pushed on one side of the rectangle), the dislocation moved (as a line of void) to separate a different set of beads. Milo: This is part of it. Now add a handful of different size beads (residual elements have different atomic radii) and you can see how it requires more energy to roll the beads over themselves. Like wedging a tire with a block of wood repeated millions of times.

The video clip was indeed helpful to manifest that aspect of Dislocation (that it moved to separate different matrices in a material). _______________________________________________________________________ As Dislocations would move (so many of them, all at once) in a material under stress, they entangle in a cluster eventually. Milo: I'm saying that the dislocation might be the absence of an expected atom in the matrix. By moving, eventually that "absence" will somehow be filled. The entanglement would be another atom from another row filling that void space. Because there is no longer any room at this atomic arrangement level to move, the material is work hardened- ie it has no more internal opportunity to slip (ductile mode).

The "Spaghetti Model of Dislocations" explained that the material was then work-hardened (becoming less elastic but higher strength and hence more liable to brittle failure than plastic deformation). Please authenticate that metallography would depict Dislocations as described above.

Milo: I don't see this as metallographic scale. This is crystal lattice scale. Could X-ray diffraction show this, yes.

_______________________________________________________________________ In the case of a didactic text on NDT, despite that it does not preach Dislocation :- Are Dislocations ubiquitous in a material (such as steels in general) down to the scale of individual Crystals or Grains ?

Milo: YES! That is exactly my point they are of such a number that they affect (lower) the tensile strength of the material from it's theoretical to its actual. My point above about the high purity whiskers getting closer to theoretical was provided to make that point. (I think I talked about that above, I'm on my macbook and can't easily go back to see.)

Are the observable defects (such as cracks) traceable to Dislocations in a material ?

Milo: This is a trick question of a sort- If I say that dislocations are ubiquitous in metals, and that metals crack, than it becomes an easy to make conclusion that therefore these ubiquitous dislocations must be at least associated with the cracks. (Fallacy of causation, Non Causa Pro Causa). It is my point that cracking is a non plastic failure, and that the dislocations explain how the material behaves PLASTICALLY. I would suggest that it is the elimination of those dislocations by cold work that eliminate any internal ability to adjust to an applied force, which causes the material to embrittle, and thus crack. I think of the dislocations as "slack" allowing crystal lattices to adjust, rather than as nucleation sites for cracks.

I have no recollection of any scholarly or practical work which traces cracking ( a brittle phenomenon) with dislocations, whose very presence is the currently held explanation for plasticity.

Please share your experience to enlighten the Rest of Us.

Milo: Hope this helps

The didactic text on NDT doesn't preach about dislocations because they are not themselves the causa sine qua non of cracks which would be found by NDT methods.

PS- If you think of the atoms of the residual elements of being of such varyng sizes that they effectively lock the matrices and prevent slip, this easily explains how such high residuals (and deliberate alloying additions) can alter properties from this atomic matrix movement point of view.

Milo Regards.