Age hardening
From GAWDAwiki
Hardness in metal alloys occurs as a result of "resistance to dislocation motion" in the alloy. If the alloy is held, or "aged," at a suitable temperature, the hardness may increase. Age hardening may occur gradually at room temperature (natural aging) and more rapidly at elevated temperatures (artificial aging). It will be greater with increasing aging treatment (i.e. an increase in temperature and/or time), until a point is reached when softening, or "over-aging," occurs. [1]
