Please explain the meaning of this statement with close reference to Precipitation hardening,"The relative rates of nucleation and growth are controlled by temperature. The size and dispersion of the secondary phase is controlled by time, with coarsening occurring as time proceeds."What I do not understand is the difference between the usage of "growth" and "size" as different parameters. They say they are dependent on temperature and time respectively.Arent they basically the two sides of the same coin.Please explain me with close reference the the precipitation hardening in Al-4.5wt%Cu alloy.
More precipitation sites initiate the higher the temperature. So you want a higher temp to have many sites. The precipitates grow faster ( become coarse) at higher temperature. But the objective is many precipitates ( high temp.) with small size ( low temp). So the temperature selection is a compromise. I think some newer precipitation stainless steels have a two temperature age : start high , then a second lower temperature. Digressing ; one of my favorite metallurgical terms "anti-phase domain" is the undefined zone ( several atoms wide) between the matrix and the precipitate.