On the evaluation of stability of rare earth oxides as face coats for investment casting of titanium
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INTRODUCTION
I N V E S T M E N T casting provides a high degree of dimensional tolerance (0.4 to 1.6 mm) and surface finish (0.8 to 3.8 ~m), which are far superior than those obtained in sand castings, tl,2,3] Investment castings also provide design flexibility to produce highly intricate shapes. The molds for investment casting are produced from refractory slurries containing finely divided materials for obtaining good surface fmishes. Refractories used in investment molds are relatively more stable and inert compared to those required for sand casting. A number of studies have been carried out on the reactivity of titanium with a variety of refractory materials including borides, carbides, sulfides, nitrides, and oxides. Borides of Cr, Ti, W, and Zr were found to severely contaminate the titanium melt. t4] Among the Cr, Mo, Ta, and Ti borides, TaB2 caused the least amount of contamination of molten titanium. ]5] In general, borides do not offer much promise as a mold face coat material and, thus, have not been actively pursued. Titanium carbide formation was observed in all castings using crucibles made of carbides of elements B, Mo, Si, Ta, Ti, W, and Zr. t41 It appears that the affinity of titanium for carbon triumphs even over the most stable carbides. NbC performed satisfactorily while in contact with molten titanium, probably because of a slower rate of carbon diffusion, trj Nitfides of refractory elements were also found to severely contaminate the melt. t4] As regards the performance of sulfides, computer calculations have shown significant advantages in the use of sulfides as mold coatings for titanium castings. Sulfides of calcium and magnesium were not satisfactory since appreciable pressure of metallic elements develops in equilibrium with molten titanium. The rare earth sulfides were found to perform better. Oxyfluorides have recently been investigated, and some have proven effective. Neodymium oxyfluoride t7] and lanthanum oxyfluoridd8] have been patented as the major
R.L. SAHA, Deputy Director, T.K. NANDY, Scientist, and R.D.K. MISRA, Scientist, are with the Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500 258, India. K.T. JACOB, Chairman, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted September 13, 1989. METALLURGICAL TRANSACTIONS B
constituents in a mold coating. However, oxysulfides or oxyfluorides need to be prepared carefully to ensure purity and correct stoichiometry. In search of suitable mold material, the oxide family of materials has received the most attention. The free energy diagram for oxides is useful for comparison of the relative stability of oxides, t9j The general reaction represented on the diagram is x 2 2 - M (s, 1, or g) + 02 (g) ~ - MxOy(s, 1, o r g) Y Y
[1]
Since only the standard free energy is plotted, the diagram does not provide a correct picture of the reactivity of oxides with liquid titanium. Solution effects need to be taken into consideration to obtain a more realis
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