The Mechanism of Zr and Hf in Reducing Radiation-Induced Segregation in 316 Stainless Steel

Abstract

The addition of oversized solutes has the potential to reduce the effects of radiation-induced segregation (RIS) in austenitic alloys. This RIS has been implicated as one of several factors in enhancing stress corrosion cracking (SCC) under irradiation, so oversized solute additions could promote SCC resistance. Either Zr or Hf was added to 316-type stainless steel, at levels between 0.05 and 0.37 at. pct. Samples were irradiated with 3 MeV protons to 3 dpa at 400°C and analyzed using high-resolution-scanning transmission electron microscopy (HR-STEM) with energy-dispersive X-ray spectroscopy (EDS), to measure the grain-boundary (GB) composition. The Zr additions substantially reduced the amount of RIS, while the Hf was much less effective. Despite similar sizes, first-principles calculations using the Vienna Ab Initio Simulation Package (VASP) demonstrate that solute-vacancy binding for Zr is 1.05 eV vs 0.69 eV for Hf. This difference results in the greater effectiveness of Zr in reducing RIS, as determined by kinetic rate theory calculations, in agreement with experimentally-measured results.

A systematic approach for the study of radiation-induced segregation/depletion at grain boundaries in steels

“…Alternatively, the present work clearly demonstrates a straightforward and systematic method to analyzing grain boundaries. The observed segregation widths are consistent with the highest resolution STEM-EDX data reported in the literature [19,20,37,38]. While STEM-EDX suffers from possible loss of resolution due to beam broadening, the spatial resolution in APT data is also limited by trajectory aberrations associated with non-uniform evaporation near grain boundaries and possible surface diffusion and retention effects.…”

Influence of oversized elements (Hf, Zr, Ti and Nb) on the thermal stability of vacancies in type 316L stainless steels

“…We therefore assume that the high-temperature water SCC can be controlled by passivating monovacancies at the nuclear reactor operating temperature. It has been reported that radiation-induced grain boundary segregation is inhibited by doping oversized elements, such as Hf and Zr, to stainless steels [8][9][10][11][12][13]. This effect is thought to be due to the binding of mobile vacancies to oversized elements.…”

Modeling solute-vacancy trapping at oversized solutes and its effect on radiation-induced segregation in Fe–Cr–Ni alloys

“…First principles calculations of solute-vacancy binding energy for Zr or Hf in fcc Fe with no spin polarization have been described in detail in reference [19] using the direct method. The direct method compares two energies, that of the solute and vacancy sitting on first nearest-neighbor sites and that of the solute and vacancy as far apart as possible in the cell considering the periodic boundary conditions.…”

“…Zero-temperature ab initio calculations of the lattice parameters for Fe, Zr, Hf, Ti and Pt were performed previously in order to calculate the linear size factors for the oversized solutes in fcc Fe [19]. Values for the calculated lattice parameters are shown in Table 4.…”

“…Calculated lattice parameters for fcc Fe, Pt, Ti, Hf and Zr and the oversized solute linear size factors in fcc Fe[19].…”