Properties Evaluation and Studying Production Mechanism of Nanocrystalline NiAl Intermetallic Compound by Mechanical All
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INTRODUCTION
NANOCRYSTALLINE materials, defined as polycrystalline solids with crystallite sizes of usually less than 100 nm, can exhibit unique properties due to large fraction of grain boundaries they possess.[1] Intermetallic compounds such as nickel aluminides, iron aluminides, and titanium aluminides are a new brand of advanced materials with outstanding physical and mechanical properties suitable for high-temperature applications.[2–4] Owing to their excellent physical and mechanical properties used for producing homogeneous materials with controlled structure and uniform properties, these compounds are of interest for researchers and engineers.[5] The NiAl intermetallic compound offers a combination of attractive properties such as low density (5.86 g/cm3), high melting point 1911 K (1638 °C), high thermal conductivity, good temperature stability, high-temperature oxidation, corrosion resistance, attractive modulus, and metal-like properties. These properties cause NiAl intermetallic compound to be considered for many structural applications such as edges of the rotor in gas turbine engine blades and aerospace industries.[4,6–8] However, low toughness at low temperature and low creep strength at elevated temperatures limit its industrial applications. Hence, numerous attempts have been made in recent years to overcome this problem. The common methods employed for improving room-temperature ductility of nickel aluminide intermetallics include grain
refining, improving slip systems, and alloying.[9–12] Mechanical alloying (MA) can provide all the above-mentioned properties simultaneously; therefore, this technique has been extensively applied to synthesize nickel aluminide intermetallic. Many researchers have reported manufacturing nanocrystalline NiAl intermetallic combinations by MA of initial material powders.[13,14] Ivanov et al.[15] were the first to synthesize nanostructured NiAl intermetallic compound powder by MA. Mashreghi et al.[16] investigated the formation of NiAl intermetallic compound by MA after 30 hours. Kubaski et al.[17] examined the effect of milling variables on the synthesis of NiAl intermetallic compound by MA and reported ignition time as a function of the initial particle size of Ni, process control agent, and ball-to-powder ratio. Ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion during high-energy ball milling. The powder particles were flat and flake shaped in the early stage of milling, but change to a spherical shape with the crystallite size of about 30 nm after milling, and the synthesis mechanism was a gradual exothermic reaction.[18] In this work, the formation mechanism of Ni50Al50 intermetallic compound was studied with a planetary ball mill and the effect of different milling times on structure and hardness of Ni50Al50 intermetallic manufactured by MA method was investigated.
II. ALI KHAJESARVI, Research Student, and GOLAMHOSSEIN AKBARI, Professor, are with the Department of Materials Science and Engineering, Shahid Bahonar University, Ke
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