Ductile and Brittle Metals vs Crystal Structure

[Enthalpy]

Hello everybody!

We read again and again "Face-centered cubic metals are ductile, body-centered are not, while hexagonal close-packed are brittle". Some authors want even to justify it through numbers of slipping planes. Worse, a few ones would curb Nature to their reasoning if only they could, like "Ti-Al5V is brittle because it's a hexagonal alpha alloy" or "Zinc breaks upon bending because hexagonal" - no, I won't tell you who wrote that one.

About every metallurgy textbook has a crystallography chapter that reproduces this nonsense, and I'm getting tired of it, so I've just made a table with metals of mechanical use.



Most metals go against the claim. Observations :

Some very ductile metals like magnesium and zinc are hexagonal close packed.

Iron, which can be the origin of the false claim, is very ductile as a body-centered cubic lattice, if pure. More so than as some austenitic alloys.

Pure titanium is ductile and hexagonal, as are its alpha alloys. Its alpha+beta alloys are less ductile.

All alkaline metals (not for mechanical use) are body-centered and very ductile. Silicon, germanium (not exactly metals) are face-centered cubic and perfectly brittle, as polycrystals as well.

Many "brittle" metals become ductile if pure enough. Tantalum is extruded cold. Some demand a very high purity to be less brittle, like beryllium. Maybe some "brittle" metals are just not pure enough? Most metals can be embrittled by some alloying element, like phosphorus in iron of any phase.

A partial link can be seen with the fusion temperature, which logically relates with easy propagation of dislocations - but there are exceptions.

Marc Schaefer, aka Enthalpy

[studiot]

 

Many "brittle" metals become ductile if pure enough

 

Or you can turn this the other way round and observe that alloying materials harden a material eg the addition of carbon to iron to make steel.

 

Impurities are also known to be an impediment to dislocation spread.

 

But what was your question?

 

Edit : Have you also thought about the transition temperature?

Edited October 24, 2013 by studiot

[Enthalpy]

Meanwhile I've heard comments of disbelief, especially about magnesium and zinc.

 

I may have written too quickly about magnesium. The small ingot I had from then Péchiney (they told me "pure" with no more details) could be hammered thin (say 50% thickness reduction) without splitting. Though, most alloys guarantee only 2-5% elongation at break.

 

In their 2009 paper "A Highly Ductile Magnesium Alloy System", W Gao and H Liu from Auckland university report 18-22% elongation at break for magnesium alloyed with 0.3-3% of Sn and Pb, optionally Al, Zr, Zn. That would be a welcome improvement. The paper doesn't describe the crystal structure, but at 0.3% Sn plus 0.5% Pb it's probably hexagonal like pure Mg.

 

The zinc I took from batteries as a kid could be folded at 180° several times before breaking. At that time it contained 0.1% Hg or Pb. Sheets to cover roofs contain 0.1% Ti and can also be sharply bent at normal temperature. Some sources claim that this 0.1% makes zinc bendable; that would be an other case where alloying elements increase the ductility.

 

Could ductility through alloying be a key to beryllium use? Up to now, people try to control the tiny oxygen proportion, but maybe some element addition is the way instead. One that scavenges the oxygen traces?

[DrP]

Isn't it the case with chemistry and the periodic table that we have loads of historic 'rules' atoms follow.... which then nearly always have a list of 'exceptions' to the rules that follow?

 

Physics always seemed a lot more logical with rules that don't break.

[Enthalpy]

Thanks for your interest!

 

I have nothing against rules with exceptions, which is the common case in chemistry or metallurgy... Physics likes rules without exceptions, but this is possible in some domains only, and as it looks chemistry isn't such one. We could even formulate it the other way: the physicists are the people who chose a domain of expertise where rules apply neatly.

 

What bothers me is that any link between the crystal organisation and ductility has as many exceptions as followers, from the list I checked above. This looks like one "rule" that perpetuates only as a habit that no-one checks.

 

So much that we read false claims like "alpha titanium is fragile because hexagonal", "zinc breaks upon bending because hexagonal" or, more recently, "0.3% Sn and Pb make Mg alloys ductile by adding slipping planes". This idea seems hard-wired in the conception of metallurgy.

 

Anyway, I don't even see why more slipping planes should make an alloy more ductile. Three planes should suffice.

[Enthalpy]

I had put the body-centered cubic vanadium in the "brittle" box, but it's ductile

http://www.webelements.com/vanadium/

http://en.wikipedia.org/wiki/Vanadium

fine, this serves my purpose.

[Enthalpy]

When a wire is plastically elongated, its section shrinks, and the stress (force/area) increases by as much. If the material's proof stress increases faster than the section shrinks, a more elongated portion of the wire's length resists elastically a bigger force, so the less elongated portions get more elongated. If not, the striction diverges locally and the wire breaks early.

So one condition for a ductile material should be that the proof stress increases faster than the section shrinks. I'd have written "necessarily" if this weren't an experimental science. At least the well bendable Al-Mg alloys and austenitic stainless steel exhibit this behaviour when a compression reduces the section.

[Area54]

You may have a point, but so far, as presented, it is not convincing. Specifically:

• You have claimed that "About every metallurgy textbook has a crystallography chapter that reproduces this nonsense", but have offered zero evidence that this is the case. How many metallurgy textbooks are currently extant? How many of them have you examined to reach this conclusion? How definitively is the "nonsense" repeated in each instance.
• Your table of Brittle and Ductile metals in the OP is far from comprehensive. As such it is not clear that the exceptions will actually be almost as numerous as those following the rule.
• Much of your material is anecdotal and includes more than one expression of personal disbelief. (Hardly scientific.)
• While you mention the significant effect that impurities and alloying may have on the metal properties, you appear to ignore the fact that this may be largely responsible for the percevied exceptions.
• A comparatively minor point - you have failed to structure your argument, but rather presented it as a series of apparently "off the cuff" comments. That tends to create the impression that your belief lacks appropriate objectivity.

As I noted, you may very well be correct, but I believe you need to address the points above if you are to convince this skeptic.

[Bender]

It is not unheard of that one author comes to the wrong conclusion, or oversimplifies a conclusion and then others copy it without further thought. I have encountered other examples of such (such as plain wrong examples on Newton's third law in several mechanics textbooks), and these seem to get more common as the level gets more basic and more simplification occurs.

A second effect is that one oversimplification might have caught your attention, and now you put additional weight to secondary or out of context comments on the subject whenever you encounter them. It can be tempting to miss the nuance of the text when you already made up your mind.

Now I can't remember reading about such rules. I'm also no expert on material science. But I tend to agree with you that ductility or brittleness, which can be rather vague concepts to begin with, depend on too much factors to be cast in such simple rules.

[Enthalpy]

Having read recently "refractory metals are hard and wear-resistant", I've made a table of refractory metals - because Nb, Hf and Zr are knowingly soft, and because Ta and Nb gall so brutally that wear performance is irrelevant.

W and Nb may not be exactly cubic but slightly elongated. The yield strength of ever purer Re may well drop below 280MPa, so it would be soft.

No relationship in this list between refractory, hard, brittle, nor the crystal form.

[John Cuthber]

Last time I checked, the word "ductile" meant being able to be drawn into thin wire.

My light bulb filaments say that you have put tungsten in the wrong column.

So does my roll of Molybdenum wire (not to mention the Mo wire and tape used in lamps as connectors for the W filaments).

[Sensei]

22 minutes ago, John Cuthber said:

My light bulb filaments say that you have put tungsten in the wrong column.

"Polycrystalline tungsten is an intrinsically brittle"

Just in second paragraph of

https://en.wikipedia.org/wiki/Tungsten

 

Try bending tungsten electrode (used in TIG welding).

 

Edited January 6, 2018 by Sensei

[studiot]

My understanding of the distinction between ductile and brittle materials is that a ductile material has a significant plastic region in its stress / strain curve, whereas a brittle material does not.

This means that a brittle material is unable to redistribute its maximum stress plastically and so cracks at the point of maximum tensile stress.

This can lead to what is known as fast fracture or just standard brittle fracture depending upon the nature of the load and the material.

This behaviour is important in structural engineering and strongly temperature dependent, which is important in geology and metallurgy.

Edited January 6, 2018 by studiot

[John Cuthber]

2 hours ago, Sensei said:

"Polycrystalline tungsten is an intrinsically brittle"

Just in second paragraph of

https://en.wikipedia.org/wiki/Tungsten

 

Try bending tungsten electrode (used in TIG welding).

 

Do you mean the paragraph that then says "However, pure single-crystalline tungsten is more ductile, and can be cut with a hard-steel hacksaw"?

Cherry picking wiki like that doesn't make you look good.
Later on that same page it explains " If made very pure, tungsten retains its hardness (which exceeds that of many steels), and becomes  malleable enough that it can be worked easily.^[13] It is worked by forging, drawing, or extruding. ".

When you have finished reading wiki, go look at a tungsten light bulb filament, drawn into fine wire, then coiled - typically twice- and explain how that's possible if it's brittle.
Did it occur to you that "polycrystalline" practically speaking, means "with cracks already built in"?

The interfaces between the crystals make it brittle, but they don't relate to the topic which is about the crystal structure, not the interfaces.

Edited January 6, 2018 by John Cuthber