Liquid-Solid Phase Equilibria in Metal-Rich Nb-Ti-Hf-Si Alloys
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Basic and Applied Research: Section I
Liquid-Solid Phase Equilibria in Metal-Rich Nb-Ti-Hf-Si Alloys Y. Yang, B. P. Bewlay and Y. A. Chang
(Submitted October 12, 2006) A thermodynamic description of the Nb-Ti-Hf-Si system is extrapolated from descriptions of the constituent Nb-Ti-Si, Nb-Hf-Si, Hf-Ti-Si, and Nb-Hf-Ti systems using the CALPHAD technique. From this thermodynamic description, the liquidus projection at the metal-rich region of the NbTi-Hf-Si system with the Si concentration up to 40% is calculated. The calculated liquidus surface at this region consists of six primary solidification regions: (Nb), Hf2Si, (Hf,Ti)5Si3, (Nb,Ti)3Si, aNb5Si3, and bNb5Si3. Three five-phase equilibria involving these phases are identified on the liquidus surface by this calculation. They are L þ (Nb) þ aNb5 Si3 ! Hf 2 Si þ (Nb,Ti)3 Si at 1814 C, L þ Hf 2 Si þ aNb5 Si3 ! ðHf,Ti)5 Si3 þ ðNb,TiÞ3 Si at 1739 C, L þ Hf 2 Si ! ðNb) þ (Hf,Ti)5 Si3 þ (Nb,Ti)3 Si at 1400 C. To validate the calculated liquidus surface, a total of 10 alloys was directionally solidified. The microstructure of these 10 as-cast alloys was examined by using scanning electron microscopy (back scatter electron (BSE) imaging) and energy dispersive spectrometry (EDS). The solidification simulation of these 10 alloys was then performed by using the Scheil model that is integrated in the multicomponent phase diagram calculation software Pandat. The observed phases presented in the as-cast microstructure of these 10 alloys are in good agreement with those predicted from the Scheil simulation.
Keywords
CALPHAD, liquidus surface, multicomponent, phase diagram, phase equilibria, quaternary
1. Introduction Directionally solidified in situ composites based on niobium and niobium-based silicides are presently being developed for high-temperature structural applications. There has been extensive work on composites generated from binary Nb-Si alloys,[1-5] as well as those with additions such as Ti, Hf, Cr, and Al. Hf and Ti are important alloying additions because they can improve oxidation resistance and strength.[1,2,5,6]. However, there is little previous knowledge of phase equilibria in the Nb-Ti-Hf-Si quaternary system. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by The Alloy Phase Committee of the joint EMPMD/SMD of the Minerals, Metals, and Materials Society (TMS), held in San Antonio, Texas, March 12-16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany. Y. Yang, CompuTherm LLC, 437 S. Yellowstone Dr., Madison, WI 53719; B. P. Bewlay, General Electric Company, GE Global Research Center, 1 Research Circle, Niskayuna, NY 12309; Y. A. Chang, Department of Materials Science and Engineering,
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