Elsevier

Materials Characterization

Volume 153, July 2019, Pages 339-347
Materials Characterization

Effect of post annealing on microstructure and mechanical properties in Ni-free N-containing ODS steel

https://doi.org/10.1016/j.matchar.2019.05.008Get rights and content

Highlights

  • Thermal stability was evaluated using a combination of techniques: XRD, SEM/TEM, ACV.

  • The grain size is stable up to annealing temperature of 900 °C.

  • Annealing at up to 800 °C brings about nitrogen dissolution and precipitation of Y2O3.

  • At higher temperature, grain growth, Y2O3 coarsening and Cr2N precipitation take place.

  • The maximum in mechanical properties was for the sample annealed at 800 °C.

Abstract

The precipitation behavior of Y2O3 and MnO nanoparticles after isothermal treatment, in a Ni-free N-containing oxide dispersion strengthened (ODS) steel was studied by wide-angle X-ray scattering (WAXS), ultra-small-angle and small-angle X-ray scattering (USAXS/SAXS), small angle neutron scattering (SANS) and scanning transmission electron microscopy (STEM). Mechanical properties were investigated via tensile and micro-hardness testing. Additional precipitation occurred during annealing, in addition to the precipitates formed during sintering. The highest volume distribution of Y2O3, with average size of 6 nm, was determined by alloy contrast variation (ACV) analysis of SAXS and SANS for the sample annealed at 800 °C; this sample shows the highest hardness and ultimate tensile strength. Above that temperature, coarsening of both matrix and precipitates and depletion of nitrogen from matrix degraded the mechanical properties. The very fine nanoparticles and thermally induced dissolution of nitrogen influenced mechanical properties.

Introduction

Oxide dispersion strengthened (ODS) steels are candidates for structural materials of fusion reactors and core components of advanced fission reactors. The introduction of yttrium oxide (Y2O3) is considered as an effective way to improve both mechanical properties [1,2] and radiation resistance [3]. The dispersed oxide particles in the matrix prevent the movement of dislocations, thus improving creep resistance and elevating high temperature performance. Moreover, the interface between the oxide particles and matrix is an effective sink for point defects as well as helium atoms that are generated as transmutation-induced elements during neutron irradiation.

In this context, the ferritic and ferritic–martensitic ODS steels have been studied widely [[4], [5], [6]], while the austenitic ones far less so. ODS austenitic steels are characterized by their superior phase stability at high-temperatures, their excellent oxidation resistance as well as corrosion and high-temperature creep resistance properties, in comparison to their ferritic–martensitic counterparts [1,[7], [8], [9]]. This is why ODS austenitic steels have recently attracted significant interest as candidates for large-scale structural materials in new-generation nuclear power plants. One of the drawbacks of conventional austenitic steels is the relatively high content of nickel, which is a highly activated element not suitable in a high flux environment.

In our recent work [10], we have demonstrated that ODS strengthened Ni-free austenitic steel can be successfully manufactured by mechanical alloying and the Spark plasma sintering (SPS) techniques, which effectively eliminate the activation problem. However, one of the crucial requirements for materials suitable for high temperature environments is their sufficient thermal stability; consequently, the investigation of the thermal stability of this newly developed ODS strengthened Ni-free austenitic steel is of great importance.

In the case of ODS materials, the overall microstructure is, in general, reasonably stable as long as nanoparticles remain constant in terms of their size and spatial distribution. It has been demonstrated that, for short time annealing, ODS ferritic steels are highly thermally stable, as the oxide nanoparticles are stable up to 1250 °C [[11], [12], [13], [14]]. However, long-term annealing at 1200 °C causes the coarsening of nanoparticles and deterioration of mechanical properties [15]. Regarding austenitic ODS steels, a recent study demonstrated that the microstructure is stable at 1150 °C even up to 1000 h [16]. At higher temperatures, grain growth occurred with the decrease in the number density of nano-oxides, coupled with coarsening of the particles at the grain boundaries. The characterization of oxide nanoparticles is therefore crucial in predicting the stability of ODS steels.

Nano-sized oxides in ODS steels (mostly ferritic and ferritic-martensitic) have been characterized by numerous studies at the nanoscale, including transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) with energy-dispersive X-ray spectroscopy (EDX) [8,14,15,[17], [18], [19], [20], [21]] and atom probe analysis [22,23], which provide information on chemical composition, size and spatial distribution of nanoparticles, but, from a very small volume. As a consequence, the statistics are rather poor and the precise evaluation of average size and number density is limited.

Contrast variation small-angle scattering (SAS) [24] is a solution to the above. The complementary techniques are based on X-ray or neutron scattering, where the information comes from probing a relatively large volume; however, the information is limited to the number density and size distribution. In addition, the stoichiometry of scattering particles can be obtained from an ‘alloy contrast variation’ (ACV) analysis [24,25]. The ACV method is based on the difference of X-ray and neutron scattering length density ΔρX2ρN2 of each element, which is described as a ratio of intensity between SANS and SAXS curves. This approach guides the determination of the chemical composition, size and number density of the nanoparticles over a wide size range, while probing a large sample volume. However, we do not have information about individual nanoparticles nor their precise location in the microstructure.

Therefore, the aim of this study is to characterize the thermal stability of an austenitic stainless steel strengthened with oxide nanoparticles – in this case, a newly developed ODS Ni-free austenitic alloy - using a combination of imaging and analytical techniques, such as small angle X-ray scattering (SAXS), ultra-small angle X-ray scattering (USAXS), neutron small angle scattering (SANS), STEM together with mechanical measurements. We believe that such a combination of techniques is required for a comprehensive understanding of nanoparticles' behavior, grain growth phenomena and resulting changes in mechanical properties upon annealing

Section snippets

Manufacturing and annealing

Ni-free ODS austenitic alloy with compositions of Fe-13Cr-20Mn-0.35Y2O3 (in wt%) was manufactured by mechanical alloying under a nitrogen atmosphere. The consolidation of the powder was performed by SPS for 5 min at 1000 °C under a pressure of 50 MPa. The chemical composition of the samples after consolidation is presented in Table 1. The details of manufacturing procedure and description of samples quality, microstructure and properties can be found elsewhere [10]. To study the effect of post

WAXS

The WAXS of all samples indicates a mostly austenitic structure, as shown in Fig. 1, together with the small amount of MnO, Cr2N and ε-martensite. The process of annealing causes changes in the lattice parameter d of austenite, as austenite peaks are shifted from their original position, in comparison to the as-sintered sample. The shift of the austenite peaks depends on the annealing temperature (Fig. 1b)). For lower annealing temperatures, the d parameter increases with the greatest value

Summary

  • 1.

    XRD indicates an increase in the dissolution of nitrogen in the matrix during annealing up to 800 °C. Above this temperature, precipitation of Cr2N occurs.

  • 2.

    TEM revealed the stability of grain size up to 900 °C. The results of the ACV analysis indicate the presence of one population of Y2O3, and MnO nanoparticles for as-sintered and samples annealed in temperature range 700–900 °C, and an additionally enlarged Y2O3 population for sample annealed at 1000 °C. The highest volume distribution of very

Acknowledgements

This work was carried out within the statutory funds of the Faculty of Materials Science and Engineering of Warsaw University of Technology. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The neutron experiment was held at the Australian Nuclear Science and Technology Organization at the Bragg Institute Neutron

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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