Kellyanne_Conman: This is more ELI am a teenager. I’m sorry, but it’s the best I can do.
When a material fails, it can fail in two different ways. One way is for the material to deform which is caused by the atoms in that material moving past one another. This is called plastic flow. The other way, is for a crack in the material to propagate all the way through the material cleaving it into two different pieces. This is called crack propagation.
In general, failure of a material is heavily dependent on its atomic structure. In some types of materials their susceptibility to fail from plastic flow is heavily temperature dependent, while its susceptibility to failure through crack propagation is not. When these types of materials get cold they lose their ability to fail due to plastic flow (which is synonymous with their ability to deform), and so crack propagation takes over. When a material fails through crack propagation instead of through plastic flow, we call this a brittle fracture. Materials that show this change in susceptibility to plastically flow are said to go through a [*”ductile to brittle transition.”*](https://www.intechopen.com/source/html/50809/media/fig1.png)
What this means is that some materials when they are warm fail due to plastic flow but when they get cold they begin to fail in a brittle fashion.
As a disclaimer, I will add that almost all materials become more brittle as they get colder, but some materials still stay what we would consider ductile as their temperature drops. This is due to the fact that plastic flow becomes easier when atoms are further apart which is the case when a material is warmer. Atoms are vibrating more quickly, and so on average they are farther apart from one another in a warmer material. Because of this they can slide past one another more easily, which is essential for a material to deform. Conversely, a material that is brittle will fail without deforming much at all… It will simply crack all the way through.
Credentials: I am a PhD candidate in mechanical engineering, specializing in Mat. Sci. and I often guest lecture for the Mat. Sci. and Engineering course at my college.
FourDogsinaHorseSuit: You know how chocolate gets soft and flexible before it melts? Well that’s what happens when things get hot, when they get cold, the opposite happens. They get stiffer and stiffer so that anything will cause them to snap.
drewth12: Y’all need to chill about brittle transitions and glass transitions. You need to take it back to what’s causing all of this in the first place.
Energetics and kinetics.
Yes bcc metals will often be more ductile than hcp bc of slip planes in the system. Yes metals are more ductile bc of metallic bonding. Yes polymers experience a glass transition that allows for polymer flow to begin. Sure whatever fine.
Elit: But the reason materials fail in a brittle fashion in colder temperatures is because it’s more convenient to break bonds than it is to create more disorder in the system.
go look at a stress strain curve and take the integral. The area under the curve is the toughness of the material, how much work it can take before failure. Work and heat are the similar, and heat capacity is dependent on temperature. Hotter the material, more work can be done. How much the heat capacity varies in temperature determines how much more brittle it will be as it gets colder.
Then compare to what is favorable, adding entropy in forms of dislocation, or break a bond.
seicar: What doesn’t get brittle when it gets cold?
It is a matter of your “cold” scale.
My memory is incomplete, but I believe only Helium won’t “freeze” to a solid state (and it can under special conditions). I’m sure someone can fill in my gaps here.
A common solid you may be thinking of would be a metal. Metals are malleable (depending on metal/alloy) due to metallic bonding. Metallic bonding is like an old school stereotype of a hippie commune. Electrons are shared pretty easily, and the atoms slide between partners easily. When cooled further the atoms tend to form lattice structures, like crystals. For the analogy, the atoms are forming long term partnerships like marriage, children, and perhaps get a little selfish with their atoms (though some brittle materials like superconductors share electrons like madmen). These structure cannot slide or move (much). The colder a material gets, the more lattice forms until CRACK! An analogy marriage ends with a full divorce and they are arguing over who gets to keep the box set of Friends.
shtoots: In material science, this phenomenon is known as the ductile to brittle transition temperature (DBTT). Once a material is cooled below a certain temperature, it will exhibit brittle fracture characterized by
a lack of plastic deformation before failure (no yield).
There are several different mechanisms responsible for this behaviour depending on the material. The best understood mechanisms is in body-centered cubic (BCC) metals. BCC refers to the crystallographic alignment of the atoms (crystal structure), which varies from metal to metal.
So to answer your question, the mechanisms responsible for a material’s DBTT are not always well understood but in the case of metals it is strongly influenced by the material’s crystal structure.
KitKatBarMan: Ok, so everything will break if the rate at which you deform it is high enough. Think about silly puddy, if you pull it really quick it will snap. Let’s call this ‘brittle yield strength’ – how high of a strain rate something can have applied to it before it breaks.
It just so happens that some materials have interesting atomic and molecular properties which make then have lower brittle strain rates as they get cold, and others are not as sensitive.
A true ELI5 is difficult without a few basic chemical principles.
Knaite: I can give an explaination about Polymeric materials such as plastics, rubbers, etc. Every material has a Temperature called “Glass transition temperature”. It’s a temperature or small range of temperatures in which the material softens to rubbery state from a glassy state. For some materials it is 0 degree C, some 50 degrees C, some -35 degrees C while some 200 degrees C.
When a material is cooled below its glass transition temperature, it becomes brittle.
Why does this happen?
The molecule chains in the material are entangled with each other. At high temperatures, these chains loosen up. At low temperatures, they get more entangled with each other.
Less entanglement = the chains move effortlessly when pulled and pressed and return back to original position when released like a spring.
More entangled = more effort to pull, which means at a point the chains would break instead of disentangling.
Why does it depend on temperature?
At any temperature above zero K, molecules vibrate. The higher temperature, the higher vibrations. Vibrations enable disentangling of the chains from each other. Similar to shaking a pepper can to get pepper put.
At lower temperatures, this vibrations are less and therefore the chains do not get enough energy to disentangle.
Low temperatures -> Less vibrations -> more entangled molecule chains -> more brittle
Ser_Dunk_the_tall: Because different materials have different properties than other materials, which emerge at different temperatures as well. At low enough temperatures, I would imagine most materials would be quite brittle, so your definition of cold would be quite relevant
NAMYNNUF: Most materials have a “glass transition temperature”. A temperature below which it behave like glass. Bonds don’t have enough energy to bend or stretch, so they rupture.