Nanocrystalline and bimodal Fe-Cr alloys: synthesis, mechanical properties and oxidation resistance
thesisposted on 28.02.2017 by Brahmadesham Venkataraman, Mahesh
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Nanocrystalline (nc) materials, a single or multi-phase polycrystalline solids with a grain size of <100 nm have been of considerable research interest in the past two decades. However, investigation of corrosion/oxidation resistance of these materials has attracted limited attention. Moreover, the ductility and thermal grain size stability of nanocrystalline materials has not been adequately addressed in the literature. The aim of the current study is to develop a nano-structured alloy with good oxidation resistance and at the same time address the issues of grain size stabilization and poor ductility. In the process, the study attempts to understand the effect of nanocrystalline and bimodal grain size distributions on the high temperature oxidation resistance of Fe-Cr alloys and validates the hypothesis good oxidation resistance and mechanical properties in Fe-Cr alloys can be simultaneously achieved, even at much lower bulk Cr concentration as compared to conventional stainless steel, if the microstructure is composed of a homogeneous dispersion of nanocrystalline and microcrystalline grains. The alloys that have been studied here include nanocrystalline Fe-alloys (with 5 wt% Ni, 2 wt% Zr and varying Cr contents between 2 and 10 wt%) and bimodal Fe-Cr-Ni-Zr (with 7 and 10 wt% Cr). The mechanical properties and oxidation behaviour have been compared with microcrystalline Fe-Cr-Ni alloys with 10 and 20 wt% Cr. Pellets of nanocrystalline and microcrystalline Fe-alloys were prepared by mechanical alloying followed by hot-compaction and sintering. A new 2-stage hot compaction technique has been designed, based on a systematic study of grain growth in nanocrystalline Fe-Cr. The addition of Zr has been shown to improve the high temperature processibility of Fe-based alloys. A uniform distribution of microcrystalline grains in a nanocrystalline matrix (viz. bimodal alloys) is shown to improve the ductility of nc alloys by 80-100%. Oxidation resistance of nanocrystalline, bimodal and microcrystalline alloys was compared at 550 C. Chromium content in the inner oxide scale was found to be comparable with that of mc 20% alloy even when the bulk Cr concentration in the nanocrystalline/bimdal Fe-alloys was just 7% which suggests that nanocrystalline grain size distribution can be exploited to develop highly oxidation resistant alloys with much lower amounts of expensive alloying element (Cr). The critical amount of Cr for development of a protective chromia film was experimentally determined to be ~7%. This value is consistent with the theoretically predicted value based on Wagner's analysis. A mechanistic understanding of the high temperature oxidation in nanocrystalline alloys has been presented. This work also investigates the role of active element Zr on the high temperature oxidation resistance of nanocrystalline Fe-Cr-Ni. Based on the elemental composition along the depth of the oxide layer (obtained using SIMS), a transient mechanism for oxidation in bimodal alloys has also been proposed.