Measurement of the density and surface tension of Ni-based superalloys in the liquid and mushy states
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I. INTRODUCTION
NUMERICAL modeling of casting and solidification of metallic alloys is gaining increased importance in industrial processing. Successful modeling can lead to improvements of product quality, the optimization of the production processes, including the acceleration of introduction of new alloys into service, and to the design of new and improved casting techniques. Reliable thermophysical property data are required as input parameters in order to quantify the numerical modeling. The density and the surface tension are two critical parameters in modeling the fluid flow during casting and solidification, affecting both the dendrite morphology and the vulnerability of the solidifying alloys to the formation of defects that can limit the engineering performance. Ni-based superalloys contain heavy elements (e.g., W, Re, and Nb) to improve microstructural stability and mechanical performance. However, phenomena such as freckles, white spot, and the nucleation of spurious grains can result from convectional flows arising from differences in density () and its temperature dependence (d/dT). Surface tension () and its temperature dependence (d/dT) may affect the scale of microsegregation and the propensity for the formation of defects such as gas porosity as well as the evaluation of the convective contribution to heat transport. This study is in support of modeling to simulate the solidification of single-crystal superalloy components used in gasturbine engines for both aircraft propulsion and power generation. Ni-based superalloys contain elements (e.g. Al, ZUSHU LI, Postdoctoral Research Associate, and KENNETH C. MILLS and MALCOLM McLEAN, Professors, are with the Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom. Contact e-mail: [email protected] or [email protected] KUSUHIRO MUKAI, Professor Emeritus, is with the Department of Materials Science and Engineering, Kyushu Institute of Technology, Kitakyushu-shi, 804-8550 Japan. Manuscript submitted November 2, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
Ti, and Hf) that can be very reactive at high temperatures (with containers and atmosphere). This has made it difficult to measure thermophysical properties reliably; these parameters include densities in the liquid and “mushy” (i.e., liquidsolid coexistence or semisolid) states and the surface tensions of the liquid Ni-based superalloys. Sung et al.[1] have reported density values for five commercial Ni-based superalloys without giving any details of the experimental method used. Two recent reports describe the results of a Round Robin by a number of European laboratories (“Thermolab”) on the measurement of densities and surface tensions of liquid commercial superalloys.[2,3] The density of liquid CMSX-4 was determined using an optical measurement of the volume of an electromagnetically levitated specimen. Further measurements of the surface tension of liquid CMSX-4 have been obtained by the sessile drop and the pendant drop methods (at the liquidus temperature)
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