S. Cotes et al., FCC HCP MARTENSITIC-TRANSFORMATION IN THE FE-MN SYSTEM - EXPERIMENTAL-STUDY AND THERMODYNAMIC ANALYSIS OF PHASE-STABILITY/, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 26(8), 1995, pp. 1957-1969
A new experimental study of A(s) and M(s) in the Fe-Mn system has been
performed by using two complementary experimental techniques, viz., d
ilatometry and electrical resistivity measurements, which are applied
to the whole composition range where the transformation can be detecte
d, i.e., between 10 and 30 pet Mn. We used the A(s) and M(s) temperatu
res as input information in an analysis based on thermodynamic models
for the Gibbs energy of the face-centered cubic (fcc) and hexagonal cl
ose-packed (hcp) phases. In these models, the magnetic contribution to
Gibbs energy is accounted for, which allows us to study, by calculati
on, the influence of the entropy of magnetic ordering upon the relativ
e stability of the phases. The picture of magnetic effects upon the fc
c/hcp transformation that emerges from our work is as follows. At low
Mn contents, the martensitic transformation temperatures are larger th
an the Neel temperature of the fcc phase, and both A(s) and M(s) decre
ase linearly with increasing Mn. This encourages an extrapolation to z
ero Mn content, and we use that to critically discuss the available in
formation on the fcc/hcp equilibrium temperature for Fe at atmospheric
pressure. At sufficiently large Mn contents, we have M(s) < T-N(gamma
) which implies that the fcc phase orders antiferromagnetically before
transforming to the hcp phase. Since hcp remains paramagnetic down to
lower temperatures, the ordering reaction in fee leads to a relative
stabilization of this phase, which is reflected in a drastic, nonlinea
r decrease of M(s).