remaining part of chap-11

11.6 Influence of Quenching Medium, Specimen Size, and Geometry
The cooling rate depends on the cooling medium. Cooling is fastest using water, then oil, and then air. Fast cooling brings the danger of warping and formation of cracks, since it is usually accompanied by large thermal gradients.
The shape and size of the piece, together with the heat capacity and heat conductivity are important in determining the cooling rate for different parts of the metal piece. Heat capacity is the energy content of a heated mass, which needs to be removed for cooling. Heat conductivity measures how fast this energy is transported to the colder regions of the piece.
Precipitation Hardening
Hardening can be enhanced by extremely small precipitates that hinder dislocation motion. The precipitates form when the solubility limit is exceeded. Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.

11.7 Heat Treatments
Precipitation hardening is achieved by:
a) solution heat treatment where all the solute atoms are dissolved to form a single-phase solution.
b) 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.
c) 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 overaging.
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

11.8 Mechanism of Hardening
Strengthening involves the formation of a large number of microscopic nuclei, called zones. It is accelerated at high temperatures. Hardening occurs because the deformation of the lattice around the precipitates hinder slip. Aging that occurs at room temperature is called natural aging, to distinguish from the artificial aging caused by premeditated heating.

11.9 Miscellaneous Considerations
Since forming, machining, etc. uses more energy when the material is hard, the steps in the processing of alloys are usually:
solution heat treat and quench
do needed cold working before hardening
do precipitation hardening
Exposure of precipitation-hardened alloys to high temperatures may lead to loss of strength by overaging.