bu66er, wish I had not said that.
How much do you know about crystal structure?
Do face centred cubic and body centered cubic (FCC and BCC) unit cells mean anything to you?
Brain dump:I’m going to make some simplifications and generalisations now.
Deformation in metals happens in two ways, elastic and plastic.
Elastic: You can think of elastic as just springs stretching/compressing, which is the “bonds” between the atoms stretching, or squishing.
Plastic: is permanent deformation, so (simplified) when the “bonds” get stretched too far, they “break” and the atoms start to move relative to each other.
This movement manifests itself as something called slip, or dislocation movement, think of it like a ruck in a carpet.
Now these dislocations move along planes in the crystal structure, and the number of available planes depends on the crystal structure.
In FCC there are 12 planes (called slip systems)which the slip can occur along easily.
In BCC there are none, well there are 12 but it is not a truely “close packed” system so it needs some heat to help the dislocations “jump” from one plane to the next.
See wikipedia here
So that explains why FCC metals stay ductile at low temps, and BCC metals do not.
Next is why the ductility of FCC metals can “increase” at lower temps.
This is a bit harder to explain as it relates to competing processes that are thermally activated.
In all metals at higher temps the ductility increases. Purely due to available thermal energy making the atoms “looser” in their lattice so dislocations can move more easily. But this does not happen until higher temps have been reached (remember competing process).
Another process that happens with temperature is vacancy generation and diffusion in the crystal lattice. What this means is that some atoms jump out of position and leave a hole, this is a vacancy, and it causes a high energy lattice strain (an incongruence) due to the imbalance of the attractive/repulsive forces of the atoms no longer being ordered. The jumped atom squeezes in elsewhere (called an interstitial) and has a similar strain effect. These lattice strains can slow down dislocation movement and hence make the material less ductile.
BTW Spontaneous vacancy generation is modelled by an arrhenius equation which is exponential with temperature. (There is also some interesting stuff in semiconductors that is related to this, but I won’t go in to that now) The vacancy generation is limitied by an activation energy which is required to kick start it.
So thermal lattice vibrations increase ductility, but vacancy and interstitial generation can decrease ductility. These processes compete.
As temperatures decrease so do the generation of vacancies/interstitals, which in FCC metals means that disclocations can move about more freely without bumping into holes/vacancies.
But in BCC as temps decrease, there is no thermal energy to activate slip onto other planes, so ductility decreases.
So competing processes: with 6061 (an FCC metal) if you cool from say 200c, then ductility decreases as thermal mobilty decreases, until around 0c then ductility starts to increase (slowly) as the vacancy/interstitial generation tails off. I can’t quite remember but I think this is around the temp of the activation energy, below which there is no vacancy generation. (room temp is considered warm/high energy in these terms)
I’ll bet you wish you hadn’t asked now…
(Caveat: All simplified and generalized, so if you are a metallurgist too, please do not shoot me down, I’m sensitive..)
