__Last time__

__Growth with moving interfaces--Stefan conditions__

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__Solidification of Pure Substances__

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__Diffusional Growth during Precipitation__

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__Solidification with and without Undercooling__

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__Shape Stability of moving interfaces; constitutional undercooling__

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__Johnson-Mehl-Avrami: Evolution of Transformation Fraction__

Consider the growth stage for the case where a parent phase will undergo a compete transformation to a new equilibrium phase. The amount of new equilibrium phase will depend on time, growth rate and the number of nuclei.

For simplicity, suppose that all the nuclei grow as spheres with constant velocity, , and are nucleated at some different time .

The nucleus which nucleated at some time will have a volume at time :

(34-1) |

*If the nuclei do not interfere (collide during growth)*, then the total
volume of transformed material is the sum of all the nuclei which have
created at times

(34-2) |

The volume fraction, , of transformed material is:

(34-3) |

*If the nuclei do interfere with each other* (a condition which
would apply at either
high density of nucleation sites or long times), then the above equation must
be modified to account for volumes of particles which are multiply counted.
If the particles are nucleated independently, then the volume fraction
is given by the *Avrami* equation:

(34-4) |

In general, depending on whether all the nuclei appear at one time, or the dimensionality of growth, the general equation for volume transformed looks like:

(34-5) |

This equation summarizes transformation kinetics and is called generally,

** Cahn's Time-Cone Analysis of JMA Kinetics**
The method is first described for the one-dimension case:
nucleation and growth on a wire.
Extensions to higher dimensional nucleation domains is straightforward.

The first step is to add a time axis to the nucleation domain as follows:

The small circles in the figure represent random nucleation positions
and times. The lines which emanate from the nucleation positions indicate
the growth fronts for the particles as a function of time as if the
particles do not interact--these are called *growth time-cones*.
If the growth rates are constant, the growth cones are truly circular conical
shapes.
At later times, some regions are covered by one or more growth cones.
Since a particlar region cannot transform twice, it is incorrect to
count the overlapping area more than once.

The actual fraction of transformed material can be determined by
turning the problem upside-down. Consider an arbitrary test point
at some time , that point will *not*
have been transformed if and only there is no nucleation event which
can influence that point.
The nucleation events which can influence that point lie within
a *cone of influence* which extends backwards in time from
that point:

It is reasonable to assume that the nucleation events are random in time--in this case, Poisson statistics apply. In Poisson statistics, the probability of exactly events occurring in a volume is given by:

(34-6) |

where is the mean number events and is the average density of events.

In particular, the probability that *no* event occurs is
; so the probability that *at least one* event occurs is
.
For the case illustrated in the figure above, one needs to calculate
the fraction of points which are influenced by at least one event:

events in influence cone | (34-7) |

For larger dimensionality nucleation domains, the calculation is quite similar:

The final frame shows the cones from below (looking in direction of increasing time). Since the growth velocity is constant, the resulting microstructure after full impingement is a Voronoi tessellation.

__Example of Time-Cone: Edge Effects__

Consider transformation kinetics in one dimension, such as recrystallization in a narrow wire. For a finite wire of length , you might expect that probability that a region will have transformed will depend on the proximity to the end of the wire. Investigate the end effects on transformation kinetics on a finite length of wire , with constant transformation growth rate (length/time) and a constant, uniform, nucleation rate (number/(length time)). Calculate the probability that a point will have transformed at time .

This problem is ideally suited for the `time-cone' method: for each point on
the wire, there an `area' in `length-time' in which if *any* nucleation
occurs, then will have transformed.

If the nucleation events are independent, then Poisson statistics apply (it
may be useful to look in an elementary statistics book) and the key to the
solution will be to find the probability that *no* nucleation event
occurs in the time-cone for a point .

Poisson statistics apply when events are random and mutually independent,
which is assumed to be the case both in time and along the wire.^{1}Therefore, the problem depends only on the area of the time cone
illustrated
for the three distinct cases for
.

**Very short times or effectively infinite**There is no interference from the boundaries. The condition for this case is . The area of the time cone is . The probability that a point will have transformed is independent of when :**Near the end of a finite wire**There is interference from only the boundary at The condition for this case is and (or in a slightly different form, and ). The area of the time cone, , is minus the area where .**Very short wire or long times**There is interference from both boundaries. The condition for this case is (or, and ). The area of the time cone, , is minus the area where .