Thermodynamics and kinetics of core-shell versus appendage co-precipitation morphologies: An example in the Fe-Cu-Mn-Ni-Si system

Abstract

What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys of two precipitating phases, with the fast-precipitating phase acting as heterogeneous nucleation sites for a second phase manifesting slower kinetics. Kinetic lattice Monte Carlo simulations show that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly affect the selection of core-shell verses appendage morphologies. We study a FeCuMnNiSi alloy using the combination of atom probe tomography and simulations, and show that the ordering energy reduction of the MnNiSi phase heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the energy penalty of a predominantly Fe/Cu interface, leading to initial appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around and through the Cu core towards the ordered phase results in subsequent appendage growth. We further show that in cases with higher primary precipitate interface energies and/or suppressed ordering, the coreshell morphology is favored.3

On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions

“…However, MNSPs are now widely observed [25][26][27][28][29][30][31][32][33][34][35]. Further, recent models [36][37][38][39] support the early predictions of their evolutions as intermetallic phases. While MNSPs form in low Cu steels, primarily by heterogeneous nucleation [36,37], they also emerge as separate appendage co-precipitates, attached to CRP core shell structures at high t [4,31,33,39,40].…”

The mechanistic implications of the high temperature, long time thermal stability of nanoscale Mn-Ni-Si precipitates in irradiated reactor pressure vessel steels

“…The central question relates to the driving force for MNSP formation. Thermodynamic models predict that: a) MNSPs are nm-scale variants of equilibrium G and 2 phases in typical low alloy Mn (0.8 to 1.6 at.%), Ni (0.2 to 1.6 at.%) and Si (0.3 to 1.2 at.%) bearing RPV steels; and, b) MNSP formation is accelerated at low service temperatures, around 290°C, by the excess defect concentrations under irradiation, and corresponding radiation enhanced diffusion (RED) rates [4,14,[26][27][28]. Others propose that MNSPs are not thermodynamic phases, but rather are formed, grown and sustained by radiation induced solute segregation (RIS) to defect sinks [23][24][25]29], such as dislocation loops, vacancy clusters [23,24,30] and network dislocations [25,31].…”

High-resolution Transmission Electron Microscopy Characterization of the Structure of Cu Precipitate in a Thermal-aged Multicomponent Steel

“…In addition, computational modeling also used to understand the structural and compositional evolution in Fe-Cu-X (X denotes Mn and/or Ni, etc.) systems [26][27][28][29][30][31][32], and indicates that the structures of Cu precipitates are greatly influenced by alloying elements.…”