Solute diffusion in liquid nickel measured by pulsed ion beam melting
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Solute Diffusion in Liquid Nickel Measured by Pulsed Ion Beam Melting J.P. LEONARD, T.J. RENK, M.O. THOMPSON, and M.J. AZIZ Measurements of liquid-phase diffusion coefficients for dilute tungsten and molybdenum in molten nickel were made using a pulsed ion-beam melting technique. A high-intensity beam of nitrogen ions is focused on the surface of a nickel substrate that was implanted with known concentration profiles of W and Mo. Melting of the surface to a depth of 1 m allows broadening of the implant profiles while molten. Solute concentration-depth profiles were determined before and after melting using Rutherford backscattering spectrometry. Using a series of numerical simulations to estimate the melt history and diffusional broadening for mean liquid temperatures in the range 1755 to 2022 K, an effective diffusion coefficient is determined in each case by comparison to the measured depth profiles. This is found to be (2.4 0.2) 105 cm2/s for W and (1.6 0.4) 105 cm2/s for Mo, with an additional systematic uncertainty of 0.5 105 due to instrumental and surface effects.
I. INTRODUCTION
NICKEL alloys have important high-temperature structural applications. Nickel is currently found in over 90 pct of all superalloys used in gas turbine blades, furnace equipment, and power plant components. Nickel is favored because of its high melting temperature, oxidation resistance, and favorable mechanical properties at high temperatures.[1] A wide array of alloy formulations and processing techniques have been developed to meet critical property requirements such as high strength, corrosion resistance, creep, and fatigue resistance. Key to these properties are engineered microstructures, including grain size control and texturing in directionally solidified (or single crystal) components, grain boundary composition control, and the formation and tailoring of secondary phase precipitates and oxide dispersion-strengthened structures. Process modeling of this complex behavior necessarily requires accurate values of liquid-phase thermophysical properties. In particular, the diffusion coefficient of alloying elements in molten nickel is crucial, yet, as in most liquid metals systems, is lacking.[1] This article reports on measurement of the diffusion coefficients of Mo and W in nickel, which are commonly added for solid solution strengthening. These elements reside in the primary austenitic phase, often in concentrations as high as 10 pct.[2] They are also known to react to form carbides of the form MC and upon decomposition M6C and M23C6, often associated with grain boundaries. Measurement of diffusion in molten metals is difficult, particularly in refractory and reactive systems.[3] Under ambient terrestrial conditions, these measurements are limited by convective instabilities in the melt, often obscuring diffuJ.P. LEONARD, formerly Postdoctoral Researcher, Division of Engineering and Applied Sciences, Harvard University, is Assistant Professor, Materials Science and Engineering, U
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