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INTRODUCTION

OXIDATION is a major source of degradation and failure of structural components exposed to elevated temperature in thermal power plants including atomic power stations. Thermal power plants operated at higher temperature with water cooled environment in the supercritical state improves the energy conversion and fuel utilization efficiency and reduces environmental pollution.[1] Oxide dispersion strengthened (ODS) high Cr ferritic matrix alloys are considered more appropriate structural components in advanced nuclear energy reactors including fast breeder reactors, ultra-supercritical thermal power plants and solid oxide fuel cells due to their higher resistance to oxidation and creep, better thermal conductivity and lower coefficient of thermal expansion.[2–5] Conventional ferritic steels are considered unsuitable in these applications because of their poor oxidation resistance and creep-rupture strength at the usual operating temperatures of these reactors (~ 650 C). Carbide strengthening is not acceptable because

A. MEHARWAL, M. KUMAR, S.K. KARAK, J. DUTTA MAJUMDAR and I. MANNA are with the Metallurgical & Materials Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur, W.B. 721302, India. Contact e-mail: [email protected] Manuscript submitted July 5, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

of the possibility of selective leaching of carbon in sodium environment. Ferritic alloys with ultrafine/nanometric oxide dispersion are projected as suitable materials for liquid sodium cooled nuclear power plants up to a temperature of 650 C to 670 C as they possess adequate void swelling and creep resistance and very low carbon concentration to suppress formation of TiC.[2–6] Mechanically alloyed ferritic steels with yttria (Y2O3) dispersion were found to register higher creep resistance than conventional ferritic or martensitic steel of identical base composition.[5–8] Furthermore, presence of yttria dispersion at grain boundaries is considered an effective strategy to reduce grain boundary diffusion and prevent internal oxidation of the bulk.[9,10] In the past, four ferritic steels with nominal compositions of 83.0 Fe-13.5Cr-2.0Al-0.5Ti (alloy A), 79.0Fe-17.5Cr-2.0Al0.5Ti (alloy B), 75.0Fe-21.5Cr-2.0Al-0.5Ti (alloy C), and 71.0Fe-25.5Cr-2.0Al-0.5Ti (alloy D) alloys (all in wt pct) were synthesized by us using solid state processing route comprising mechanical alloying and sintering (at 1000 C) by four competitive methods of hot isostatic pressing, high pressure sintering, hydrostatic extrusion and pulse plasma sintering, respectively.[11–15] These alloys recorded very attractive range of bulk mechanical strength under compression, indentation hardness, Young’s modulus and wear resistance under optimum processing condition, which were superior to conventional ferritic steels. Among these alloys, alloy D with the highest Cr content (25.5 wt pct Cr) manifested the best combination of mechanical properties in terms of

compressive strength, hardness and elastic modulus.[1