Liquid titanium solute diffusion measured by pulsed ion-beam melting
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INTRODUCTION
TITANIUM alloys are important commercial materials in the aerospace industry because of their high strengthto-weight ratio. The manufacture of Ti components often involves melting to obtain the desired alloy composition and casting. In understanding the process of alloy solidification and solute partitioning, the liquid diffusivity is a critical parameter. In addition to the absence of experimental measurements of Ti solute liquid diffusivities in the literature,[1.2] there is also a general deficit in knowledge of the thermophysical properties of the liquid phase, including latent heat, specific heat, and thermal conductivity. This article reports on direct experimental determination of liquid diffusivities of some common alloying elements in Ti. Liquid diffusivity is a particularly challenging property to measure accurately. Two potentially serious problems associated with liquid diffusivity measurements are convective contamination and container wall interactions.[1] The elevated melting point (Tm) and high reactivity of Ti exacerbate both of these problems. Convective contamination generally occurs when temperature gradients in the liquid create instabilities leading to the formation of convective cells. High temperatures and extended times increase the likelihood of cell formation. Although it is exceedingly difficult to completely eliminate convective contamination in terrestrial diffusion measurements, these effects can be minimized by using fine capillaries that limit the formation of convection cells. Although container wall interactions have been ruled out in some diffusion experiments with low Tm materials,[3] there is a general concern that fine capillaries introduce additional problems due to wall interactions—a particularly troublesome concern when working with reactive materials at high temperatures. Although microgravity diffusion measurements can effectively eliminate buoyancy convective currents, surface tension-driven flows (Marangoni) may remain. Liquid diffusivity measurements following pulsed melting P.G. SANDERS, Researcher, is with Ford Research Laboratory, Ford Motor Company, Dearborn, MI 48121. M.O. THOMPSON, Professor, is with the Materials Science and Engineering Department, Cornell University, Ithaca, NY 14853. T.J. RENK, Researcher, is with the Beam Applications and Initiatives, Sandia National Laboratories, Albuquerque, NM 87185. M.J. AZIZ, Professor, is with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. Manuscript submitted October 7, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
can minimize many of these difficulties. The thin film geometry and short melt duration permit the avoidance of convection currents. The planar geometry permits accurate measurement of the submicron diffusion distances resulting from the short melt duration using techniques such as Rutherford backscattering spectrometry (RBS) or secondary ion mass spectrometry. While molten, the liquid is surrounded by a solid of the same composition, so container
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