it is not mandatory to introduce a separate step of solution heat treating in the fabricating sequence; magnesium silicide stays in solution by quenching (usually by air) immediately after extrusion. This unstable state is maintained until the extrusion is ready for artificial ageing at a suitable tempareture, typically around 200
The mechanism for strengthening has been studied at great length. It was found that:
1. Appreciable strengthening occurs at room temperature over an extended period of time.
2. Zones are formed; short ageing times at around 200 C produce fine needle shaped zones approximately 6 nm (60A) in diameter and 20-100 nm (200-1000 A) in length.
3. Further ageing causes 3-dimensional growth to rod shaped particles with a structure corresponding to a highly ordered magnesium silicide.
4. At higher temperatures, this transition phase, designated P , undergoes diffusionless transformation to the equilibrium MG
5. The normal precipitation sequence may be designated: