Crystallographic Orientation Dependence of Corrosion Behavior of a Single Crystal Nickel-Based Alloy
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TRODUCTION
NICKEL-BASED alloys have been widely applied as hot section components in aerospace gas turbine engines due to their excellent mechanical properties and corrosion resistance at high temperatures.[1–5] In order to further enhance the high-temperature performance of nickel-based alloys, directional solidification casting is used to produce single crystal (SX) alloys. Previous investigations demonstrate that the properties of SX materials are generally known to vary significantly with crystallographic orientation. Several studies have focused on understanding high-temperature performance of SX nickel-based alloys under various service conditions.[6–12] The SX nickel-based alloys are known to exhibit higher creep properties along the [001] crystallographic direction compared to the [011] or [111] directions.[11] There is an anisotropic variation in yielding strength and deformation for SX nickel-based alloys with different orientations.[12] However, aside from high-temperature properties, corrosion behavior under ambient conditions with different corrosion media is fundamental and equally worthwhile to investigate for practical application as well, as turbine vanes in aircraft engines may suffer from
L.N. ZHANG and O.A. OJO are with the Department of Mechanical Engineering, University of Manitoba, Room E2-327, 75A Chancellors Circle, Winnipeg, MB R3T 5V6, Canada. Contact e-mail: [email protected] Manuscript submitted June 6, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
room-temperature corrosion during downtime.[13] Resistance to a corrosive environment at room temperature is another important property which can influence durability and service life of the alloy.[14–17] Nevertheless, very limited research has been conducted on corrosion behavior of SX nickel-based alloys. It is known that alloying elements, such as nickel, chromium, and titanium are beneficial for nickel-based alloys to form very thin oxidized films on their surfaces in various aggressive environments.[18,19] This type of layer which is known as passive film, can significantly reduce the electrochemical reaction rates on the surface, and prominently improve corrosion resistance of the alloy. Previous results demonstrated that crystallographic orientation can control dissolution, passivation, and corrosion behavior of materials.[19–28] Although, a study has found that fabrication of nickel-based alloys in SX form can significantly enhance their resistance to corrosion compared to polycrystalline alloys,[18] the dependence of crystallographic orientation on passivation and corrosion behavior of the alloy has not yet been thoroughly investigated. The objective of this research is to study crystallographic orientation dependence of corrosion behavior of a SX nickel-based alloy, IN 738. Electron backscatter diffraction (EBSD) was applied to determine crystallographic orientation of SX specimens. The corrosion behavior of SX nickel-based specimens with different crystallographic orientations was investigated in various corrosion environments by em
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