 T_{0}: T_{0} temperature 
Definition:  Temperature (or
composition) at
which the Gibbs
energies of two
phases are equal. 
Explanation:  Indentifying the
temperature (or
composition) where
the Gibbs energies
of two phases are
equal is important
because
it shows the limit
where a
diffusionless
transformation
(martensitic or
massive) from one
phase to another is
possible. Once
T_{0}
is known, the
modeling of the
martensitic
transformation
temperature
(M_{s})
is a matter of
modeling the
necessary driving
force to initiate
(or nucleate) the
transformation. This
is obviously of
importance for a
wide range of
steels. For high Mn
steels, it offers a
possibility to
predict if plastic
deformation can take
place by
transformation
induced plasticity
(→TRIP)
or twinning induced
plasticity (→TWIP). 
Picture / Figure / Diagram: 

(a) Gibbs energies in FeMn. The crossing of two curves marks the T_{0} for that phase pair. (b) T_{0}, M_{s} and A_{s} (reverse transformation) for the fcc/hcp pair and the Neel temperature (T_{N}) for the fcc phase.


SFBLink:  The T_{0} for the fcc/hcp phase pair is the starting point for the thermodynamic calculation of stacking fault energy (→SFE) and M_{s} (for εmartensite). 