Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
ageing temperatures correspond to high supersaturation and prolific nucleation
of precipitates occurs, whereas at higher ageing temperatures (lower
supersaturation) fewer, coarser particles are formed. The kinetics of precipitation
can be represented by a temperature–time–transformation (TTT) diagram,
Fig. 3.5. At small undercoolings, there is a long incubation period, due to the
low probability of formation of the (large) critical nucleus (equation [1.6]).
As the supercooling increases, the nucleation rate will increase, since the
critical nucleus size is smaller. The lower the transformation temperature,
therefore, the more prolific the nucleation and the finer the dispersion of
particles. However, the lower the temperature the more sluggish the solid
state diffusion becomes and the TTT curve has a ‘C’ shape indicating a more
sluggish transformation at low temperature.
Quenching and ageing therefore constitute a very powerful means of
controlling the distribution of a precipitate of second phase in an alloy. After
quenching the alloy from the single-phase region of the phase diagram, a
high ageing temperature is selected if a coarse, widely spaced dispersion of
particles is required and a lower ageing temperature is used to produce the
second phase in a more finely divided form.
These precipitates can have a profound effect upon the mobility of
dislocations and it is possible to produce large changes in the yield strength
of such alloys by suitable heat-treatment. A great advantage is that the required
strength can be induced in a product at the most convenient stage in its
manufacture. For example, the alloy may be retained in a soft form throughout
the period when it is being shaped by forging and it is finally hardened by
precipitation in order to give it good strength in service.
T I
Temperature
3.5 A TTT diagram
Nucleation time (log scale)