Precipitation hardening is achieved by

1. Solution heat treatment where all the solute atoms are dissolved to form a single-phase solution.

2.  rapid  cooling  across  the  solvus  line  to  exceed  the  solubility  limit.  This  leads  to  a supersaturated solid solution that remains stable (metastable) due to the low temperatures, which prevent diffusion.

3.  Precipitation heat treatment where the supersaturated solution is heated to an intermediate temperature to induce precipitation and kept there for some time (aging). If the process is continued for a very long time, eventually the hardness decreases. This is called over aging.

The requirements for precipitation hardening are:

•    appreciable maximum solubility

•    solubility curve that falls fast with temperature

•    composition of the alloy that is less than the maximum solubility

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