Resistance of Nanostructured Fe-Cr Alloys to Oxidative Degradation: Role of Zr and Cr Contents
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TRODUCTION
NANOCRYSTALLINE materials are polycrystals, structurally characterized by a nanometric (1 to 100 nm) grain size and a large density of interfaces (typically 1018/cm3 for grain sizes around 5 nm). In such materials the volume fraction of the grain boundaries is considerable as compared to the crystalline component resulting in remarkably different optical, mechanical, electrical, and electrochemical properties. The current study investigates the high temperature oxidation resistance of Fe-Cr alloys with varying Cr concentrations and grain sizes in the nanometer range. Various synthesis techniques for nanocrystalline alloys have been extensively studied over the past two decades. The most notable techniques for synthesis of bulk nanocrystalline alloys include pulsed electro-deposition[1] and severe plastic deformation (SPD).[2–4] Among the SPD techniques, mechanical alloying/ball milling is highly
B.V. MAHESH, formerly Ph.D. Student with the Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia, is now Research Fellow with the Solar Thermal Group, Australian National University, Canberra, Australia. R.K. SINGH RAMAN, Full Professor, is with the Department of Mechanical and Aerospace Engineering, Monash University, and also with the Department of Chemical Engineering, Monash University, Clayton, Australia. Contact e-mail: [email protected] C.C. KOCH, Full Professor, is with the Department of Material Science and Engineering, North Carolina State University (NCSU), Raleigh, NC. Manuscript submitted February 4, 2014. Article published online 31 January 2015 1814—VOLUME 46A, APRIL 2015
versatile and can produce artifact-free nanocrystalline powders. However, the compaction of Fe-Cr nanocrystalline powders is a non-trivial task because of the high hardness and restrictions on plastic deformation due to a BCC lattice structure. In an attempt to circumvent these difficulties, processing at excessively high temperatures would lead to considerable grain growth and loss of nanocrystallinity. Groza[5] has reviewed some of the commonly used compaction techniques such as in situ consolidation,[4] vacuum hot pressing,[6,7] hot rolling,[8] hot isostatic pressing,[7,9] explosive compaction,[7,10] sparkplasma sintering,[11] hot extrusion[12,13] etc. The synthesis technique used in the current work involves grain size stabilization with Zr addition and follows the hot compaction route described elsewhere.[14,15] The unique structure and area fraction of the grain boundaries, diffusion of impurities and alloying elements, and change in thermodynamic properties of the grain boundaries may lead to considerable difference in high temperature oxidation behavior of nanocrystalline alloys compared to their microcrystalline counterparts. Chromium, when present in sufficient amounts, forms a protective layer of chromia (Cr2O3) and improves oxidation resistance. However, even alloys with low Cr contents form chromia containing Fe-Cr oxides that can improve the oxidation resistance. The magnitude
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