Improvement in mechanical properties of dental cast Ti-6Al-7Nb by thermochemical processing
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TION
TITANIUM alloys are used for biomaterials, which substitute failed hard tissue.[1] Ti-6Al-4V extra low interstitial (ELI) is the most widely used titanium alloy for biomedical applications among various titanium alloys. Ti-6Al-4V ELI, which is an ␣ / titanium alloy, has a good balance of strength and toughness. It has also a good balance of workability, heat treatability, and weldability.[2] Ti-6Al-4V ELI is the first titanium alloy registered as a biomaterial in the ASTM standard F 136.[3] However, V in Ti-6Al-4V has been reported to be toxic for the human body.[4] V-free ␣ / titanium alloys, Ti-6Al-7Nb and Ti-5Al-2.5Fe, therefore, have been developed. Ti-6Al-7Nb has been recently standardized as a biomaterial in the ASTM standard F 1295.[5] Ti-6Al-7Nb has been developed in Europe in the 1980s for biomedical applications and its mechanical properties, corrosion resistance, and biocompatibility have been investigated.[6,7] This alloy has been basically developed for use in the surgical field, for example, for implant instruments such as artificial hip joints and artificial elbow joints. Recently, this alloy has been attempted to be used in the dental field, for example, not only for dental implants but also for denture bases, crowns, clasps, etc., which are produced mainly by the dental precision casting method. Precision casting is also an attractive method especially for producing titanium alloy parts, because it is a near-net-shape production method advantageous to reducing the very high machining cost of titanium alloy parts. Cast titanium (Ti) alloy parts are generally inferior to wrought or forged ones in elongation and fatigue strength because of their Widmansta¨tten-like coarse microstructures. Cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo achieve greater 0.2 pct proof stress, fatigue strength, and creep resistance than those of both as-cast alloys through heat treatments.[8] TOSHIKAZU AKAHORI, Research Associate, and MITSUO NIINOMI, Professor, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan. AKIHIRO SUZUKI, Engineer, is with the R&D Laboratory, Daido Steel Co. Ltd., Nagoya 457-8545, Japan. Manuscript submitted May 1, 2001.
METALLURGICAL AND MATERIALS TRANSACTIONS A
By keeping these alloys at a relatively higher temperature in the ␣ ⫹  phase field, the volume fraction of ␣ phase decreases, while the width of  lamellae increases with decreasing width ␣ lamellae. According to the proper subsequent cooling rate, the widened  lamellae are hardened by the precipitation of very fine ␣ in different orientation variants. Cast Ti-6Al-4V conducted with thermochemical processing shows an excellent balance of strength and ductility where the tensile strength is around 1100 MPa and the elongation is around 14 pct, because the coarse Widmansta¨tten ␣ phase is modified to be super fine ␣ phase.[9] Therefore, the microstructural refining without conducting deformation processing, that is, without changing the shape of products, is possible to impro
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