Nucleation-growth versus spinodal decomposition in Fe-Cr alloys: an experimental verification by atom probe tomography and small angle neutron scattering

TL;DR

This study compares spinodal decomposition and nucleation-growth in Fe-Cr alloys using atom probe tomography and small angle neutron scattering, establishing new criteria to distinguish the two phase separation modes.

Contribution

It introduces a framework combining APT and SANS to differentiate SD and NG in Fe-Cr alloys, with specific SANS profile features identified as key indicators.

Findings

APT analysis distinguishes SD from NG based on chemical fluctuations and interface sharpness.

SANS profiles alone are insufficient, but specific features like FWHM evolution and gamma values can differentiate modes.

First experimental verification of SANS features uniquely identifying phase separation mechanisms in Fe-Cr alloys.

Abstract

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).