Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 77
Precipitation hardening
Thermal treatment can be used to control the size and distribution of secondphase
particles in any alloy which undergoes a phase transformation in the
solid state. In many alloy systems, the solid solubility changes with temperature
in the way illustrated in Fig. 1.20. Above temperature T 1 , a single phase (α)
solid solution exists and, if the material is quenched rapidly from this
temperature range, a supersaturated α solid solution is formed, the degree of
supersaturation increasing with increasing supercooling below the solvus
line, ab. If the temperature is then raised again in order to allow solid state
diffusion to proceed, the supersaturation will be relieved by the nucleation
and growth of a precipitated second phase.
In alloys of relatively low melting-point (in aluminium alloys, for example),
there will be an appreciable diffusion rate of solute atoms at room temperature,
so that over a period of time a second phase will precipitate out in a very
finely divided form. This effect is known as ‘ageing’, but, in most alloys, the
temperature has to be raised in order to cause precipitation to occur and the
material is said to be ‘artificially aged’. The ageing temperature affects the
precipitate size in the manner illustrated schematically in Fig. 3.4. Low
α
Temperature
α + β
Composition
3.4 Variation of precipitate size with ageing temperature.