The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels

Abstract

Solute clustering and G-phase precipitation cause hardening phenomena observed in some low alloy and stainless steels, respectively. Density functional theory was used to investigate the energetic driving force for the formation of these precipitates, capturing temperature effects through analysis of the system's configurational and magnetic entropies. It is shown that enrichment of Mn, Ni and Si is thermodynamically favourable compared to the dilute ferrite matrix of a typical A508 low alloy steel. We predict the ordered G-phase to form preferentially rather than a structure with B2-type ordering when the Fe content of the system falls below 10-18 at. %. The B2 → G-phase transformation is predicted to occur spontaneously when vacancies are introduced into the B2 structure in the absence of Fe.

G-phase strengthened iron alloys by design

“…Therefore, it is likely that there is a solubility limit of Fe into the X6Ni16Si7 G-phase. In the Fex(Mn6Ni16Si7)100-x system it is predicted that this limit is x = ~18 at.% [23,40]. Comparing values in table 2 and table 3 it is predicted that, in the majority of cases (with the exceptions of Ti, V and Zr), when Fe replaces Ni in the G-phase, there is an increase in the magnitude of δ.…”

“…The site occupancies of the BCC supercell were kept consistent with the G-phase i.e. one sublattice was occupied by Ni and the other by Mn and Si as done in a previous study [23]. The occupancies of Fe were selected pseudo-randomly using a random number generator.…”

“…However, it is plausible that the lattice misfit argument holds true for this case, due to the similarities between a 4×4×4 BCC structure and the cubic G-phase i.e. 60 of 116 G-phase lattice sites are shared with the BCC structure and the remaining 56 atoms are only reconstructed slightly from the original positions by ≤0.1052 nm [3,23]. Therefore we continue our analysis assuming that dislocations can move from the matrix through the precipitate and δ could influence the ductility of the alloy.…”

High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system

“…Many single-phase HEAs are stabilised at elevated temperatures and may precipitate or phase separate when aged at lower temperatures (823 -1473 K)[40][41][42][43][44][45][46][47]. The calculated enthalpies of mixing for the solid-solution phases, , of Zr0.5HEA, Zr1HEA and Zr2HEA, are all positive and therefore require entropy to stabilise.…”

Understanding the importance of the energetics of Mn, Ni, Cu, Si and vacancy triplet clusters in bcc Fe

“…The most common features observed as a result of fast neutron radiation damage in RPV steels are voids, dislocation loops, and solute clusters composed primarily of Si, Ni, Mn and Cu [3][4][5][6]. The latter is the focus of a global research effort to gain a mechanistic understanding of solute clustering in RPV steels [7][8][9][10][11][12][13][14][15][16]. The aim of this global effort is two-fold, to improve the design of new steel compositions, especially their resistance to radiation-induced solute clustering, and improving current dose damage relationship models at high fluences beyond 40 years operation of a PWR.…”