Hot Corrosion Behavior of Ti-48Al and Ti-48Al-2Cr Intermetallic Alloys Produced by Electric Current Activated Sintering
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INTRODUCTION
THE technological importance of the intermetallic compounds of the Ti-Al binary system and the alloys developed based on these compounds is increasing rapidly.[1] In the Ti-Al binary system, alloys containing 40 to 50 pct Al are called c-TiAl–based alloys.[2] c-TiAl–based alloys have low density, high strength in a wide range of temperatures, high stiffness, good creep and fatigue resistance, rather good corrosion, and oxidation behavior. Therefore, these alloys are highly promising materials for high-temperature structural materials used in automotive, aerospace, and gas turbine industry applications.[3–6] Furthermore, TiAl-based alloys are considered to have replaced some Ni-based superalloys used in gas turbine engines.[7] However, the fact that TiAl-based alloys have limited ductility and toughness at low temperatures is a significant disadvantage.[8] Recent studies have focused on the production of ultrafine-grain TiAl-based alloys to improve the ductility of these alloys.[9] Since TiAl-based alloys have poor oxidation resistance above 800 C, the application areas of these alloys are limited. When TiAl-based alloys are exposed to oxidation by air, nonprotective TiO2 and
Y. GARIP and O. OZDEMIR are with the Technology Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, Sakarya University, 54187 Sakarya, Turkey. Contact email: [email protected] Manuscript submitted November 23, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Al2O3 mixed oxides are formed on the surface of the alloys. In fact, alumina layers show a passivating behavior and then they grow much slower than those made of titanium oxides. According to the research, it is necessary to form pure Al2O3 on the surfaces of these alloys in order to improve the oxidation resistance of TiAl-based alloys.[10] The oxidation resistance of these alloys is increased by modifying the alloy composition. In particular, alloying elements Mo, Nb, and W contribute significantly to the oxidation resistance of TiAl-based alloys, but room-temperature ductility decreases. It is known that alloying elements, such as Cr, Mn, and V, damage the oxidation resistance of TiAl-based alloys while increasing their room-temperature ductility.[11,12] It is expected that TiAl-based alloys used in the aviation industry will not only be resistant to oxidation but also resistant to hot corrosion.[13] Hot corrosion is traditionally characterized by an incubation stage with a relatively low reaction rate followed by a propagation stage in which the material rapidly degrades.[14] As a result, the load carrying capacity of the component decreases and catastrophic failure occurs.[15] Few studies have been conducted on the hot corrosion of TiAl-based alloys in salt-containing environments.[7] The electric current activated sintering (ECAS) was based on passing an electric current with an additional applied pressure, a green compact, or a loose powder during the heating process. ECAS has many advantages over conventional methods. Since this method h
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