Fracture characteristics of Ti-6Al-4V and Ti-5Al-2.5Fe with refined microstructure using hydrogen
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INTRODUCTION
M I C R O S T R U C T U R E has a dominating influence on the tensile properties, fatigue properties, and fracture toughness of c~ + /3 type titanium alloys. Control of microstructure by heat treatment or thermomechanical processing (TMP) I]] has been shown to be an effective way to improve the properties of cz + /3 type titanium alloys. Thermochemical Processing (TCP) [2] is a recently developed technique for improving mechanical properties by refining c~ + /3 microstructures through the use of hydrogen as a temporary alloying element. [3-11] Several TCP methods have been reported up to now. These include HVC (Hydrovac), pl /3Q-HD M. NIINOMI, Associate Professor, and T. KOBAYASHI, Professor, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan. B. GONG, formerly with the Graduate School, Toyohashi University of Technology, Toyohashi 44 1, Japan, is with the State Key Laboratory for Fatigue and Fracture, Institute of Metal Research, Academia Sinica, Shenyang 110015, China. Y. OHYABU, formerly with the Graduate School, Toyohashi University of Technology, Toyohashi 441, Japan is with Naoetsu Works, Sumitomo Metals Industries, Co., Ltd. Jyouetsu 942, Japan. O. TORIYAMA is with the Graduate School, Toyohashi University of Technology, Toyohashi 441, Japan. Manuscript submitted March 16, 1994.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(Hydrogenation/Dehydrogenation of Beta Quenched alloys), Is] CST (Constitutional Solution Treatment), 161and HTH (High Temperature Hydrogenation).171 These methods, in general, consist of a fl solution treatment before, during, or after hydrogenation, a possible aging treatment below the hydrogenated /3 transus, then dehydrogenation at a lower temperature. The TCP has been reported to increase the tensile strength and fatigue strength of ~ + /3 alloys and markedly decrease tensile ductility. t~~ There are no available reports on the fracture toughness of the TCP-treated titanium alloys. Studies of the mechanisms for refining microstructure during TCP are also limited, j"] One theory maintains that microstructural refinement is due to a eutectoid transformation during the aging step. t11} However, this explanation cannot account for the microstructure refinement during TCP treatments that do not have an aging step. In the present study, the hydrogenation behavior of Ti6AI-4V (Ti-6.4) with the starting microstructure of coarse equiaxed a and coarse Widmanst/itten a, respectively, is investigated. A new TCP method is proposed, in which the alloy is hydrogenated at a temperature below the/3 (H) transus. Changes in the microstructure and the constituent phases present during this new TCP are examined using optical microscopy, and X-ray and electron
VOLUME 26A, MAY 1995-- 1141
diffraction analyses. The tensile properties, fracture toughness, and fatigue strength of T i - 6 . 4 and Ti-5A12.5Fe (Ti-5.2.5), which is expected to be used widely for a implant material)~21 and various TCP treatments are t
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