All 6xxx alloys contain a quantity of soluble magnesium silicide which exceeds the equilibrium solid solubility limit at room temperature.
however, it does not exceeds the maximum solubility limit. it is schematically represented by line x-x in Fig.6.1 a
Fig 6.1 indicates that as the temperature is increased from room tempareture into the alpha region and held for sufficient time, equilibrium is attained and magnesium silicide goes completely into solid solution. If the temperature is subsequently reduced below the solves line (curved line which separates the alpha region from the a+b region),