Laser-Surface Alloying of Nimonic 80 with Silicon and Aluminum and its Oxidation Behavior
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NTRODUCTION
NI-BASED superalloys (for example, Nimonic 80) are widely used materials for turbine blades and housing in jet engines and gas turbines because of a unique combination of low thermal expansion coefficient, high temperature strength and toughness, and resistance to oxidation and fatigue.[1,2] However, the high-temperature oxidation resistance of Nimonic alloys needs to be enhanced for increasing the engine thrust or speed.[3] Several attempts have been made to improve the high-temperature oxidation resistance of Ni-based superalloys. However, most of the literature concerns the application of thermal barrier coating in enhancing the performance of the alloy in a hightemperature oxidation environment.[4,5] Schlager et al.[6] developed NiCrAlY and NiCrW alloy cladding by laser surface cladding technique on Nimonic 80A substrate and discussed an improvement in high-temperature corrosion resistance compared with the substrate. Furthermore, the high-temperature corrosion resistance increased with increase in chromium content.[6] Du et al.[7] studied the effect of TiN coating (developed by physical vapor deposition technique) on the high-temperature sulfidation of Inconel 600 and Nimonic PE 11 alloy against sulfidation attack. It was observed that TiN acts as a barrier layer and, JYOTSNA DUTTA MAJUMDAR, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India. INDRANIL MANNA, Director and Professor, is with the Department of Metallurgical and Materials Engineering, Central Glass and Ceramic Research Institute, Kolkata, West Bengal 700 032, India, and also with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur. Manuscript submitted October 17, 2011. Article published online May 23, 2012 3786—VOLUME 43A, OCTOBER 2012
hence, reduces the counterionic transport. However, the presence of a sharp interface and degradation of coating in thermal cycling are the problems associated with ceramic barrier coating. Si and Al are known to form a protective oxide scale and improve the high-temperature oxidation resistance of most metallic alloys.[8] But Si/Al rich alloys that provide superior oxidation resistance are often brittle and unsuitable for intricate fabrication routines. Because oxidation is mainly a surface-dependent degradation, surface engineering may offer an appropriate solution to combat degradation of such alloys in oxidizing environment by surface alloying with Si and Al without adversely affecting the bulk mechanical properties. In this regard, conventional surface engineering techniques like pack cementation and calorizing are usually time/material/ energy consuming, less precise, and lacking in scope of automation.[9] Furthermore, the thermodynamic constraint of poor solid solubility and kinetic constraint of slow rate of solid-state diffusion further restrict the process to only a limited number of systems.[10] In contrast, laser-surface alloying (LSA), which involves transien
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