  Identifying the operative mode of phase separation (spinodal decomposition
(SD) or nucleation-growth (NG)) remains a largely unexplored area of research
in spite of its importance. The present work examines this critically in Fe-Cr
system using atom probe tomography (APT) and small angle neutron scattering
(SANS), and establishes the framework to distinguish the two different modes of
alpha-prime phase separation in thermally aged Fe-35 at.% Cr and Fe-20 at.% Cr
alloys. Independent APT analysis determines the mode of phase separation on the
basis of: (i) presence / absence of periodic chemical fluctuation through
radial distribution function analysis; and (ii) inter-phase interface
characteristics (diffuse / sharp). SANS analysis, in contrast, yields virtually
indistinguishable correlation peaks for both the modes, which necessitates
further investigation of the several different aspects of SANS profiles in the
light of APT results. For the first time, key features of SANS profiles have
been identified that can unambiguously distinguish SD from NG in Fe-Cr system:
(i) nature of temporal evolution of FWHM of the correlation peak; and (ii)
appropriate value of 'gamma' for fitting with the dynamic scaling model
('gamma'= 6 for SD, Fe-35 at.% Cr alloy; 'gamma'= 4 for NG, Fe-20 at.% Cr
alloy).
