Investigation of high-temperature oxidation behavior of silicon added 14Cr nanostructured ferritic alloys synthesized via mechanical alloying and spark plasma sintering

Abstract

In the present study, the effect of Si addition on the microstructure and subsequently, on the oxidation behavior of nanostructured oxide dispersion strengthened (ODS, 14YWT) ferritic steel was investigated. Two types of nanostructured ODS ferritic steel powders viz. Fe-14Cr-2W-0.3Ti-0.3Y2O3 (Si-free) and Fe-14Cr-2W-0.3Ti-1Si-0.3Y2O3 (Si-containing) were mechanically alloyed up to 50 h and consolidated via spark plasma sintering. The Si-containing sintered ODS steel contained single phase ferritic microstructure with finer grains and nanoparticles of Y2Ti2O7, SiO2, and Cr2TiO4; whereas, the Si-free one contained different phases such as ferrite, austenite, and martensite with nanoparticles of Cr2O3 and Y2Ti2O7. After performing oxidation at 850oC for 100 h, the weight gain in the Si-containing sample (1.5 mg/cm2) was ~ 18 times lower than that of the Si-free sample (27.5 mg/cm2). Analyzing the surface and cross section of the oxide layers via SEM-EDS, XRD, Rietveld refinement of XRD patterns and Raman spectroscopy, it was revealed that Si-free ODS steel consisted of outer Fe2O3 and inner FeCr2O4 layers which were porous and possessed whisker like morphology. However, the oxidized Si-containing ODS steel possessed thin, dense, adherent and strong protective outer layer of (Fe, Cr)2O3 and the inner layer consisted mixture of Cr2O3 and Fe2SiO4 oxides. Synergistic effect of the single crystal structure matrix and Si addition in the Si-containing ODS steel played a crucial role in forming a thin, dense and protective oxide layers during the oxidation process, resulting in improved oxidation resistance.