M has the units of length. Since the flux of Al into the surface is zero
during the heat treatment process, the value of the M is constant.
In stage I, the interdiffusion of FeAl and a-Fe(Al) phase is taken
as a diffusion couple considering a high Al phase (Fe2Al5 or FeAl2)
surface. Under this hypothesis, one concentration profile cðx=
ffiffi
t
p
Þ as
a function of the parameter x=
ffiffi
t
p
[6] of the FeAl and a-Fe(Al) phase
can be achieved with experimental concentration–distance profiles
of Al at special temperature, which is independent of time (as