Microstructural study of transient liquid phase bonded cast INCONEL 738LC superalloy
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mprove the performance of aero-engine and power generation turbines, designers have been seeking alloys with improved high-temperature capabilities. Thus, over the past several decades, a number of nickel-, iron-nickel–, and cobalt-base superalloys, which generally can be used at temperatures above 540 °C, have been developed. Alloy INCONEL* 738 is one of the nickel-base superalloys, *INCONEL is a trademark of INCO Alloys International, Huntington, WV.
which was developed in this regard. It is designed to provide gas turbine engine components with good creep strength up to 982 °C combined with an ability to withstand long-time exposure to the hot corrosive environments encountered in aero and power generation turbines.[1] It derives its high-temperature strength primarily from the precipitation of L12-type ordered Ni3(Al,Ti) g9 intermetallic fcc phase in the g solid solution matrix, and by the presence of MC-type carbides at the grain boundaries. Despite the aforementioned excellent high-temperature properties of the alloy, high-temperature turbine components that are fabricated from this alloy suffer cracking and material loss due to exposure to extensive thermomechanical stresses as well as to the oxidizing/corrosive environment associated with the operating conditions in turbines. Consequently, in order to extend the total life of these damaged components at a cost that is less than their replacement cost, they are commonly repaired by welding. However, INCONEL 738, like most g9 precipitationhardened Ni-base superalloys that contain substantial amounts of Al and Ti, is extremely difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during welding and postweld heat treatments.[2,3,4] To solve this O.A. IDOWU, Graduate Student, O.A. OJO, Assistant Professor, and M.C. CHATURVEDI, Distinguished Professor and Canada Research Chair, are with the Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 5V6. Contact [email protected] Manuscript submitted August 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
problem, an alternative and very attractive repair technique was developed by Duvall et al.[5] and Hoppin and Berry.[6] This technique was named the ‘‘TLP bonding process’’ and has been widely used industrially over the past decades to repair degraded turbine components. During the TLP bonding process, a thin interlayer alloy, containing a melting point depressant (MPD), is sandwiched between the wellcleaned mating surfaces of the base material. Thereafter, the entire assembly is placed in a vacuum or an argon atmosphere and heated to the bonding temperature, which is above the liquidus temperature of the interlayer but below the solidus of the base material. This results in melting of the filler metal and formation of a liquid zone that subsequently fills the gap between the joint surfaces. While the parts are still held at the bonding temperature, rapid interdiffusion of alloying elements occurs between the liquid interlayer and
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