Characterization of Solidification Microstructures in Vacuum Arc Remelted Nickel Alloy 718
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CAST-AND-WROUGHT nickel alloy 718 is the most used superalloy in the aerospace industry[1–3] because of its useful combination of mechanical properties[4–11] and manufacturability.[9,11,12] If processed appropriately, this material can exhibit good creep strength, oxidation resistance, and microstructural stability up to 650 C.[4,5,8,10] These mechanical properties are derived from a uniform distribution of coherent c¢¢ (Ni3Nb) intermetallic precipitates distributed throughout the c-nickel lattice.[4,5,13,14] Formation of the c¢¢ phase is promoted by niobium in alloy 718; the nominal composition of alloy 718 is provided in Table I.[5] Mechanical properties depend heavily on the microstructure produced by liquid metal processing (LMP), i.e., casting. Defects formed during LMP often cannot be remediated by subsequent processing steps such as homogenization and mechanical deformation.[15–21] Some defects can cause non-uniform
THOMAS A. IVANOFF is with Materials Mechanics & Tribology, Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM, 87185-0889. Contact e-mail: [email protected] TREVOR J. WATT is with Avid R&D LLC, Jollyville Rd., 202376, Austin, TX, 78712. ERIC M. TALEFF is with the Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712. Manuscript submitted September 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
precipitation of c¢¢ during artificial aging, which creates regions with poor mechanical strength.[22–24] For critical applications in gas-turbine engines, these weak regions may produce catastrophic failures, which cannot be tolerated. Consequently, inferior ingots must be melted again, which is costly and time consuming. Typical LMP of alloy 718 often includes vacuum-induction melting (VIM) followed by electro-slag remelting (ESR) and/or vacuum arc remelting (VAR).[25–29] VAR, the process shown schematically in Figure 1, is often the last step in LMP. VAR improves ingot quality by removing undesirable oxides and volatile gases and by refining solidification microstructures.[25] VAR uses a direct current to melt an electrode, typically an ingot produced by VIM or ESR, which then solidifies in a water-cooled crucible. Solidification begins on the chilled stool at the bottom of the crucible. As molten metal from the electrode falls into the crucible, a molten pool of metal, called the melt pool, is formed. The size and geometry of the ingot, the electrode current, and the melt rate all influence the melt pool profile and the local solidification rate of the material. The melt pool profile is the average shape of the interface between the melt pool (liquidus isotherm) and the solidified material (solidus isotherm). Local solidification time is typically defined by the time required to reach the solidus temperature from the liquidus temperature.[30] The melt pool profile and local solidification time are important to control because they influence the likelihood of
The Specified Elemental Composition Limits of Nickel Alloy 718 Are Provided in Weight Percent[5]
Table I
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