High Temperature Resistant Intermetallic Nial-Based Alloys with Refractory Metals Cr, Mo, Re - Structures - Properties -
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HIGH TEMPERATURE RESISTANT INTERMETALLIC NiAl-BASED ALLOYS WITH REFRACTORY METALS Cr, Mo, Re - Structures - Properties - Applications G. Frommeyer and R. Rablbauer Department of Materials Technology, Max-Planck-Institut fuer Eisenforschung, Max-Planck-Str. 1, D-40237 Duesseldorf, Germany ABSTRACT The stoichiometric intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high-temperature mechanical properties, and excellent oxidation resistance. The main disadvantages of polycrystalline NiAl are the lack in plasticity and fracture toughness below the brittle-to-ductile-transition temperature of about 550°C. Insufficient hightemperature strength and creep resistance occur at temperatures above 800°C. Despite these facts NiAl-based alloys are still considered as promising structural materials for hightemperature applications. The refractory metals Cr, Mo, and Re with b.c.c. and h.c.p. lattice structures form with NiAl quasi-binary eutectic systems, showing high melting temperatures and thermally stable microstructures. Elasticity, solid solution hardening, fibre reinforcement, and creep properties were investigated in view of the constitutional defect structure and microstructural features. Especially the fibre reinforced NiAl matrix composites possess optimum high-temperature strength up to 1200 °C, and improved creep resistance as well.
INTRODUCTION Advanced design concepts for structural components of the new generations of energy conversion systems - stationary gas turbines of power plants, combustion engines and heating conductors - with improved thermal efficiency, reduced fuel consumption and exhaust gas emissions require the application of high temperature materials with appropriate physical and mechanical properties, and excellent oxidation resistance as well. The intermetallic compound NiAl exhibits some superior properties, such as high elastic stiffness (Eelast = 188 GPa) and thermal conductivity (λtherm = 92 W/m·K) [1], low density (ρ = 5.9 g/cm3) and excellent oxidation resistance up to 1300 °C [2]. The high melting point (T m = 1674 °C) [3] enables increasing operating temperatures of engine components in comparison with conventional nickel- or cobald-based superalloys. Newly developed NiAl base alloys with refractory metals are considered as promising structural materials for hightemperature applications in advanced energy conversion technologies [4-6]. NiAl is one of the most prominent Hume-Rothery compounds. The B2 superlattice structure and phase stability is basically determined by the valence electron concentration: e/a = 3:2. Deviations from the stoichiometry result in the formation of constitutional lattice defects. In Nirich off-stoichiometric NiAl antistructure atoms of Ni are present in the Al sublattice, whereas in Al-rich NiAl compounds vacancies are preferentially located in the Ni sublattice. However,
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atomic layer resolved atom probe field ion microscopy (APFIM) gave evidence of Ni and Al antistructure atoms to be present in very low c
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