Microstructure and mechanical properties of ultrafine-grained Fe–14Cr and ODS Fe–14Cr model alloys

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Abstract

Reduced activation ferritic Fe–14 wt%Cr and Fe–14 wt%Cr–0.3 wt%Y2O3 alloys were produced by mechanical alloying and hot isostatic pressing followed by forging and heat treating. The alloy containing Y2O3 developed a submicron-grained structure with homogeneous dispersion of oxide nanoparticles that enhanced the tensile properties in comparison to the Y2O3 free alloy. Strengthening induced by the Y2O3 dispersion appears to be effective up to 873 K, at least. A uniform distribution of Cr-rich precipitates, stable upon a heat treatment at 1123 K for 2 h, was also found in both alloys.

Introduction

Fe–Cr binary alloys are nowadays the most promising base to fabricate reduced-activation ferritic/martensitic (RAFM) and ferritic (RAF) steels for structural applications in future fusion reactors as well as in generation IV fission reactors [1], [2], [3]. The reasons are their high resistance to swelling, helium embrittlement and irradiation creep, in combination with their mechanical properties at elevated temperatures and corrosion resistance. However, conventional RAFM steels have limited the upper operating temperature to ∼823 K because of their low thermal creep rupture strength above this temperature. A goal of fusion reactor designs is to enhance efficiency and safety by increasing the operating temperatures using fusion devices cooled by helium or liquid metals. This would require the development of RAFM/RAF steels with upper operating temperatures of ∼923 K, at least. Oxide dispersion strengthening appears to have the capability to improve the elevated–temperature strength of high-chromium steels, thus increasing the operating temperatures up to ∼950 K [4], [5], [6], [7]. Consequently, the development of oxide dispersion strengthened (ODS) steels forces to investigate the effect of the fabrication and processing techniques on the mechanical behavior of ODS model alloys as well as to understand their strengthening mechanisms. The aims of the present work are: (1) to fabricate non-ODS and ODS Fe–14 wt%Cr alloys, via powder metallurgy methods, and (2) to investigate the effect of the processing parameters on their microstructure and mechanical behavior, in order to elucidate the oxide dispersion capacity for strengthening ferritic steels and extending their operating temperature window.

Section snippets

Experimental

Powder blends with target compositions: Fe–14%Cr and Fe–14%Cr–0.3%Y2O3 (wt%), hereafter designed respectively as reference Fe–14Cr and ODS Fe–14Cr, were mechanically alloyed at 300 rpm for 60 h inside a chromium steel vessel (11–12% Cr) under a He atmosphere in a highenergy planetary mill (Fritsch Pulverisette 6). Chromium steel balls (1–1.7% Cr, 10 mm diam.) with a balltopowder mass ratio of 10:1 were used as grinding media. The alloyed powders were packed in 304 stainless steel cans (42 mm diam. × 

Chemical composition and density

Table 1 shows the chemical composition of the starting elemental powders and the HIP consolidated alloys after forging and heat treating. The compositional analyses indicated a significant Fe enrichment in the alloys, attributable to the observed Cr sticking on the grinding media surface. Under the present processing conditions, no significant intake of O and C appears to occur. The high oxygen content in the Fe and Cr starting powders is responsible of the oxygen content measured in the

Conclusions

The addition of Y2O3 to the Fe–14Cr blend appears to promote particle size refinement of the mechanically alloyed powder and favors its densification. The fabrication and processing route herein applied produced an ODS ferritic Fe–14Cr alloy with a submicron-grained structure and a dispersion of Y–O rich nanoparticles and Cr-rich precipitates.

The Y–O rich dispersion is responsible for the enhancement of the tensile properties, grain refinement and stability of the induced structure. This

Acknowledgements

This investigation was supported by the Spanish Ministry of Science and Innovation (project No ENE 2008-06403-C06-04 and Juan de la Cierva program), the Comunidad de Madrid through the program ESTRUMAT-CM (grant S0505/MAT/0077), and the European Commission through the European Fusion Development Agreement (contract No. 09-240), the IP3 FP6 ESTEEM project (contract No. 026019) and the Fusion Energy Materials Science (FEMaS) FP7 coordination action. All the fundings are gratefully acknowledged.

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