Microstructural Evolution and Mechanical Properties Evaluation of IN-939 Bonds Made by Isothermal Solidification of a Li
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OWING to the extraordinary high-temperature mechanical properties, particularly tensile strength and creep resistance, and also superb hot corrosion resistance of nickel-based superalloys, they are widely used in manufacturing hot section components of modern aircraft and industrial power generation gas turbine engines which are designed to withstand harsh service conditions.[1–3] Continuous efforts to increase the performance and the overall efficiency of these apparatuses by operation at higher temperatures have urged scientists to develop superalloys featuring a better combination of corrosion/oxidation resistance and long-term mechanical strength.[4] IN-939 is one of these developed high chromium nickel-based superalloys which is extensively used in blades and vanes of land-based and marine gas turbines which operate in a temperature
FARZAM ARHAMI and SEYYED EHSAN MIRSALEHI are with the Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran 15875-4413, Iran. Contact e-mail: [email protected] Manuscript submitted March 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
range up to 850 C and at lower stresses as compared to aero engines.[5,6] Since blades and vanes of gas turbines are exposed to stringent operating conditions, these components are often deteriorated by a combination of various types of damages during service including surface erosion, hot corrosion, thermal fatigue cracking, oxidation, and sulfidation.[7] Furthermore, the expenses of manufacturing superalloy components are too high and the fabrication process is relatively time-consuming. Accordingly, some repairing and joining processes are economically more interesting in comparison to complete replacement of service failed parts.[8,9] Fusion welding, brazing, and diffusion welding processes are the most leading ones for repairing/joining superalloy components, which have been commonly applied in the industry.[10] IN-939 is a gamma prime (c0 ) precipitation strengthened superalloy which has been developed by modification of Waspaloy.[11] It is well known that precipitation hardened nickel-based superalloys, like Waspaloy, containing relatively high amounts of Al and Ti are generally considered to be difficult to weld due to their high susceptibility to heat-affected zone (HAZ) cracking.[12] Likewise, Inconel 939 suffers from the lack of weldability owing to a susceptibility to microfissuring particularly in the heat-affected zone (HAZ) during the welding cycle. This
is due to the high amounts of gamma prime (c0 ) formers such as Al and Ti in its chemical composition.[4,13,14] However, application of brazing as the repairing/joining technique for superalloy components has its inherent weaknesses. Melting point depressants (MPDs) (e.g., boron, silicon, and phosphorus) are added into the brazing filler alloys in order to enhance the fluidity and reduce the liquidus temperature of them. During cooling stage of the brazing process, positive segregation of MPDs leads to a formation of centerlin
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