Quenching aluminium and aluminium alloy

[Falgata]

Hello.

I got two questions.

1. Why does aluminium become more ductile if one rapidly quench it.

2. Why does aluminium alloys(not heat treatable alloys) become more ductile when it is rapidly cooled from solubility temperature.

I have read through my notes from lectures, but could not find anything about it in them and it is not that well explained in the course source material

 

[GuillermoH]

This occurs because the micro structure doesn't change immediately after the cooling. The micro structure remains the same: just one phase. Through a natural process called "aging", which is a sort of diffusion, the micro structure begins to change gradually. Several phases appear in the form of grains and the structure becomes less ductile because of them. Normally, the smaller the grains, the more fragile the material becomes. The speed of the precipitation process and grain size are controlled with the temperature. That is why some aluminum alloys are stored in a freezer after quenching, so that the precipitation is retarded and they can still be machined a few days later.
Aluminum is a pure substance, and therefore it cannot be quenched. Only alloys (e.g. aluminum-copper) can be quenched, because they might form several types of phases.