fcc and bcc - important differences?

Hi NG.
I'm preparing myself for the coming examination and I think I've heard that bcc-materials (steels) have a fatigue stress limit whilst
fcc-materials (aluminium) doesn't.
But I can't find any documentation for that!
A lot of places on google says that carbon steels have a fatigue limit and aluminium alloys doesn't. But does it have anything to do with the crystal structues?
I know that the crystal structure has an effect on the impact strength at lower temperatures (fcc has good properties, measured with Charpy V-Notch impact energy - whilst bcc shows worser results and bigger temperature dependence).
Anything else that is good to remember, when comparing fcc and bcc (or hcp, sc etc)?
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

That's an old-fashioned view.
'giga cycle fatigue' should be a useful google-string.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

Sorry about that, but I only see a lot of homepages saying that steel has a fatigue stress limit whilst aluminium doesn't.
So, your answer to the question is that it's incorrect to claim that bcc has fatigue-limit and fcc doesn't have one?
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

The so-called fatigue limit in bcc materials is a curve with a much smaller slope than the descending curve in fcc materials. In railway technology, it is important to know that there is no true fatigue-limit.
The reasons for the different behaviour is unclear to me, but I once learned that work hardening (increase of dislocation density) in fcc and bcc is different. The formation of fatigue-cracks is related to localized dislocation movement.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

in mild steel, carbon diffuses to dislocations locking them in place. the same mechanism is responsible for strain aging in mild steel too. a similar mechanism exists for oxygen atoms in the titanium alloys that exhibit an endurance limit.
the term "fatigue limit" is not the same as "endurance limit". "endurance limit" is where the s-n curve flattens out. "fatigue limit" is the % of yield needed to survive n cycles where there is no flattening of the s-n curve.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

It looks like it's a general rule of thumb that the curve for bcc materials flattens out whilst the fcc-curves doesn't. Ok, that should answer my question. I just didn't found any documentation for that anywhere so perhaps it's not a very good rule of thumb?
For instance, how about copper: Cu (fcc). That would look like the aluminium curve, perhaps except for some different s-n values?

I would definately like to hear if anyone can explain how the work hardening is different for fcc and bcc materials, with respect to dislocation movement... I've also heard a guy mention something about that.
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

no rule of thumb at all. afaik, the only alloy systems to exhibit any endurance limits are steel & ti, but hey, i've been out of the metallurgy game for a while.

basically, yes.

basically, there are many more slip planes available in fcc than bcc, hence more scope for dislocation movement.

Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jim beam wrote:

That's only true, if you believe that dislocation movement in bcc is on (110)-planes only. But that's not true.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

4 slip planes in fcc (Cu, Al, Ni, Pb, Au, Ag) according to my book. Do you agree?

No, it's not true. Slip along -planes should give 6 slip planes according to my book.

I can see that fcc has 4 close packed planes with 3 close packed slip directions in each. This gives 12 slip systems.
bcc has much more slip planes with much fever slip directions according to my book: 6, 12 or 24 closepacked slip planes and only 1 or 2 close-packed slip directions...
Cd, Zn, Mg, Ti, Be for hcp-structures has only 1 close-packed slip plane with 3 close-packed slip directions, which means that these materials should be very ductile - is that correct?
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

There is a famous paper by G. I. Taylor on the number of independent slip systems needed for general plasticity of a polycrystalline aggregate....
Taylor, G.I. (1938) "Plastic Strain in Metals," J. Inst. Metals, 62, p. 307.
He stated that five independent slip systems were needed for grain to grain displacement compatibility conditions because metal plasticity involves five independent tensoral componnents (would be siz, except that there is no volume change associated with classical metal plasticity).
This is only a necessary condition for ductility in a general sense, it is not a sufficient condition.
There has been a real pile of work on this in the literature in the last two decades inncluding tons of numerical methods.
Don't jump to easy conclusions.
Plasticity of materials has defeated or challenged some really good minds for a long time.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jbuch wrote:

-snip-
Ok, thanks for that. I can see that a lot of interesting references will appear when you google for that title.

Ok.
Ok.
Ok... Yep, it looks rather complicated.
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jbuch wrote:

That's it!
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

That's it.

Dutility is limited, since plastice deformation cross the close-packed slip plane ist difficult.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

I should look like Al.

In fcc, we have glide of dislocations on (111)-planes in <110>-directions. In bcc, no specific glide plane is defined. 'pencil glide' is predominant, where any glide plane containing <111> is possible.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

-snip-
I don't think I've ever heard about this "pencil glide" before. Does that mean that the slip systems for bcc materials are not close-packed and therefore bcc materials are not as ductile as fcc?
Or what does pencil glide mean?
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

The slip-planes are not close-packed but they all contain the <111>-direction. The critical resolved shear-stress is higher than in fcc materials.
Michael Dahms
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Michael Dahms wrote:

Ok, that's a little too complicated for me right now at this moment, but thanks anyway for all the comments... Perhaps I'll get back to this later in my life :-)
Med venlig hilsen / Best regards Martin Jørgensen
--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin Jørgensen wrote:

heard
limit
the
strength
(or
---------------------------------------------------------------------------
Martin,
See pages 415 to 419 (12-16 Effects of MetallurgicalVaraibles on Fatigue ) of G. Dieter MECHANICAL METALLURGY Third edition, McGraw-Hill ISBN 0-07-016893-8
Page 419 answers your question.
Both internet and books are tools.
Ed
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.