Modeling of Alpha-Case Formation and Its Effects on the Mechanical Properties of Titanium Alloy Castings
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TRODUCTION
TITANIUM (Ti) castings generally contain a surface layer of a grains, commonly referred to as an alpha-case (a-case), which is enriched with oxygen[1–6] and, in certain cases, some metallic elements released from oxides in the investment material by molten Ti.[6–11] The a-case is formed as the result of reaction between liquid Ti metal and the mold wall that leads to first the formation of TiO2 and then a solid solution of oxygen into titanium at the casting surface in contact with the mold. As the molten metal solidifies, oxygen diffuses further below the surface at a high temperature, producing an a-case layer whose thickness depends on the cooling rate, casting geometry, and the amount of oxygen at the mold/casting interface. Alpha-case thickness in the range of 50 to 500 lm has been reported for Ti castings.[4] The a-case, which is hard and brittle because of its high oxygen content, is usually removed from the castings by chemical milling. Alternatively, a-case formation in Ti castings can be prevented by coating the mold face with an oxide, such as yttria (Y2O3) that is more stable than titania (TiO2) so that a minimal reaction occurs between the mold wall and Ti as the liquid metal is poured into the mold.[11] While the mold face-coating method has been shown to be effective in preventing the formation of an a-case on Ti castings, the KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238, USA. Contact e-mail: kchan@ swri.edu MARIE KOIKE, Assistant Professor, BENJAMIN W. JOHNSON, Dental Student, and TORU OKABE, Regents Professor and Chairman, are with the Baylor College of Dentistry, Texas A&M University System Health Science Center, Dallas, TX 75246, USA. Manuscript submitted April 17, 2007. Article published online December 4, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
process is more involved and increases the cost of Ti castings. As a result, the face-coating method becomes unattractive when a low-cost casting process is desired. There have been several studies of a-case formation in Ti castings for dental applications.[10–12] Most of these studies involved characterization of the microhardness in the a-case as a function of distance below the surface in order to establish the depth of the a-case. Using wedge-shaped casting specimens, Kikuchi et al.[12] showed that the a-case thickness increases with increasing wedge thickness for both CP-Ti and Ti-6Al-7Nb. Furthermore, the a-case in the CP-Ti is larger than that in Ti-6Al-7Nb at identical wedge thickness values. Because of the small casting size, a-case can be a significant fraction of the thickness or cross-sectional area of dental castings. Tensile testing of Ti castings revealed that microcracks formed readily in the a-case[13] and the growth of these microcracks caused ultimate fracture and determined the tensile ductility of Ti castings. In general, a large a-case can lower the fatigue strength and tensile ductility[14] because of larger microcracks in the a-case. On the other hand, a hardened
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