Effect of Cu addition on consolidating Ti 5 Si 3 by the elemental powder-metallurgical method
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TRODUCTION
INTERMETALLIC compounds based on Ti-Si have high melting points and low densities, suitable for structural materials in space and aircraft applications. Among them, Ti5Si3 has an attractive combination of melting point and density,[1,2,3] 2130 7C and 4.32 g cm23, respectively, which stimulated significant efforts to synthesize the compound to a high integrity. The compound, however, is not amenable to fabrication into a defect-free condition, one of the limitations being its highly anisotropic thermal expansion characteristics.[4] Murata et al.[5] and Liu et al.[6] investigated Ti5Si3 by a vacuum-melting technique, whereas Frommeyer et al. employed a hot isostatic pressing (HIP). An important factor considered in these works was the sintering aid. Elements such as V, Nb, Cr, Zr, and Al were tried as a possible sintering aid. Successful verification of their effect, however, has not been well documented. Present work aimed to achieve a methodology for obtaining a fully dense Ti5Si3 compact by elemental powder metallurgy (EPM), an economical route to make intermetallics.[7–10] Sintering additives were evaluated theoretically according to the ‘‘figure of merit’’ approach[11,12,13] and the prediction was verified experimentally. The original figure of merit calculation proposed by Miodownik[13] uses the heats of solution, surface energies, and the energy of formation for vacancies in the activators. The ‘‘new’’ figure of merit approach employed in the present work was followed according to Madan and German,[12] which was based on regular solution model. A chemical composition selected therefrom underwent an integrating process that consisted of reactive sintering and pseudo-hot isostatic pressing (PHIP).
K.J. PARK, formerly Graduate Student, Department of Metallurgical Engineering, Inha University, is now a Research Engineer at Dong-Bu Steel Co., Inchon, Korea, 404-250. J.K. HONG, formerly Graduate Student, Department of Metallurgical Engineering, Inha University, is now a Manager at Fine Semitech Co., LTD, Suwon, Korea, 442-380. S.K. HWANG, Professor, is with the Department of Metallurgical Engineering, Inha University, Inchon, Korea 402-751. Manuscript submitted September 11, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
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EXPERIMENTAL PROCEDURE
Experiments were conducted in two steps. The first step was a preliminary investigation to determine a right sintering additive from the analysis of the relationship between the density and the theoretically calculated efficiency of candidate elements such as Al, Cr, Fe, Ni, and Cu. From this study, Cu was chosen as the proper sintering aid for Ti5Si3. The second step consisted of an in-depth experiment of improving the integrity of Ti5Si3-Cu compounds utilizing a PHIP apparatus. The experimental setup for PHIP was constructed according to the design ideas of Ferguson.[14] The term ‘‘pseudoisostatic’’ is used because the external pressure applied to samples was not truly isostatic because of the different pressure-transmitting medium used in the present wor
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