Elsevier

Materials Letters

Volume 285, 15 February 2021, 129088
Materials Letters

Kinetics of the α-α′ phase separation in a 14%Cr oxide dispersion steel at intermediate temperatures

https://doi.org/10.1016/j.matlet.2020.129088Get rights and content

Highlights

  • 14%Cr steel was studied after heat treatments at 400, 450 °C for 5000, 10,000hr.

  • α′ precipitates after 5,000 hr at 400 °C, while at 450 °C only after 10,000 hr.

  • Amount of α′ after 10,000 hr at 450 °C was larger, than after 10,000 hr at 400 °C.

  • Comparison of experimental results and calculations was performed.

  • It was concluded that in this steel separation occurs by the nucleation and growth.

Abstract

Current paper presents a study of the α–α′ phase separation during prolonged heat treatments at 400 and 450 °C of 14%Cr oxide dispersion strengthened steel. Vickers hardness, thermo-electric power and differential scanning calorimetry measurements enabled to conclude that α′ precipitates already after 5,000 hr at 400 °C, while at 450 °C precipitation is detected only after 10,000 hr. Yet, the amount of α′ after 10,000 hr at 450 °C was larger. The results, combined with thermodynamic and diffusion length calculations, affirm that in 14%Cr ODS steel the separation occurs by the nucleation and growth mechanism rather than spinodal decomposition.

Section snippets

Introduction:

The superior mechanical properties of oxide dispersion strengthened (ODS) steels make these alloys promising candidates for structural components in the next generation of nuclear power plants. However, when aged at temperatures between 300 and 500 °C, the steel may embrittle due to the α-α' phase separation phenomenon [1], [2], [3], [4], occurring either by spinodal decomposition or nucleation and growth.

The degree of embrittlement is the highest at 475 °C [2], thus, most studies focused on

Materials and methods

14 wt%Cr ODS steel was provided by CEA/DEN/DANS, France. Fabrication process and chemical analysis was presented in [7], [12]. For current study, heat treatments (HT) were performed at 400 and 450 °C for 5,000 and 10,000 hr.

Vickers hardness test was conducted using a Zwick Roell Indent hardness apparatus under 10 kg loading for 10 sec. As received (AR) and all HT samples were measured, 10 times each, with load perpendicular to extrusion direction.

TEP values were obtained from voltage

Results

As can be seen at Fig. 1, hardness and TEP values increased following HT in all samples, indicating the formation of α′. However, the extent of α′ precipitation was higher following 5,000 hr in 400 °C, as compared to same time at 450 °C. Increasing the dwell time to 10,000 hr hardly induced further precipitation in the sample treated at 400 °C, while in sample heated at 450 °C the precipitation was noticeable.

The heat flow history for the samples treated for 5,000 and 10,000 hr at 400 and

Thermodynamic calculations

In order to facilitate the understanding of the experimental results, we conducted thermodynamic calculations of the Fe-Cr system (using [14]) applying Thermo-Calc software [15]. The phase diagram, including the chemical spinodal of the α + α′ phases, and the composition dependence of the Curie temperature is shown in Fig. 3. This phase diagram reproduces all known experimental features of the Fe-Cr system. Furthermore, anomalous decrease in Cr-solubility at a point of intersection of the Curie

Discussion

Comparison of the experimental results and calculations, presented in Table 1, points on an agreement on order of magnitude of the heat absorbed during the dissolution of α′. However, the calculation predicts a smaller ΔHexcess after treatment at 450 °C, as compared to the corresponding value for 400 °C HT which is in correlation to the prediction for mol% of the α′, but contrary to the experimental measurements. Significant ΔHexcess is was measured after ageing at 400 °C for 5000 hr, while for

Conclusions

Our experiments show that at prolonged dwell of 10,000 hr, the amount of α′ formed at 450 °C is much higher than at 450 °C, which seems at a first glam as contradiction with thermodynamic calculations. However, results fit perfectly to the nucleation and growth mechanism which governs the α-α′ phase separation. At 400 °C, nucleation is faster and α′ precipitated already after 5,000 hr, while at 450 °C diffusion (governing the growth) is more dominant, leading to a higher growth rate, resulting

CRediT authorship contribution statement

Yael Templeman: Writing - original draft, Formal analysis, Investigation, Visualization. Malki Pinkas: Writing - review & editing, Data curation, Supervision. Eli Brosh: Methodology, Software. Einat Strumza: Formal analysis, Investigation. Shmuel Hayun: Data curation, Resources. Louisa Meshi: Writing - review & editing, Conceptualization, Data curation, Resources, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank CEA for providing the ODS alloy. Prof. Gelbstein and Mr. George are acknowledged for their help with TEP.

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