Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
At T p , the liquid of composition b and the solid α phase of composition a
react to form the solid solution β phase. Having for simplicity chosen an
alloy composition corresponding to the peritectic point, the reactants are
fully consumed so that β is the only structure observed below the temperature
T p .
Considering now an alloy of composition c 1 , this will likewise freeze
initially at temperature T 2 to form α in the liquid phase, but, at temperature
T p , although the reactants for the peritectic reaction are present (i.e. α phase
of composition a and liquid of composition b), by the application of the lever
rule it is seen that the fraction of solid phase present eb/ab is greater than that
required for the peritectic reaction to proceed to completion, so that the β
phase will be produced (with the disappearance of all the liquid phase and
part of the α phase), and microstructure will consist of ‘walls’ of the
peritectically produced β phase enveloping the unconsumed parts of the
original α dendrites.
Non-equilibrium conditions
When a peritectic reaction begins, the α phase and the liquid are in contact
and the β phase is formed at the solid–liquid interface, as illustrated in
Fig. 1.19. When the β phase has formed an envelope about the α phase, the
rate of reaction will depend upon the rate of diffusion of the reactants through
the wall of β phase that separates them and, since this may be a sluggish
process, it is quite commonly observed in cast alloys which have not been
cooled extremely slowly, so that the peritectic reaction has not gone to
completion and other ‘metastable’ phases are seen.
1.19 Microstructure of a specimen showing a peritectic reaction (× 500).