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ODUCTION

OBTAINMENT of porous implants characterized by high-strength, elastic modulus being close to that of the replaced human bone tissue, and high osseointegration ability is an important issue which attracts the attention of many researchers.[1–13] Spherical titanium powders obtained by different melt atomization methods are often used for production of porous implants. The spherical powders are usually consolidated into porous materials by various methods such as electrodischarge sintering, selective laser melting, metal injection molding, and vacuum sintering.[8–13] Vacuum sintering is the simplest method which does not require any special equipment for obtaining porous materials, but this method has serious disadvantages such as high sintering temperatures (up to 1350 C) and long duration (up to 10 hours).[12–14] To enhance the efficiency of production and processing technologies for titanium alloys, thermohydrogen processing is extensively studied which includes hydrogenation of titanium alloys with various hydrogen concentrations, realization of required operations such as deformation, diffusion welding, cutting, etc., and then dehydrogenation.[15–24] The addition of hydrides, thermal decomposition of which promotes activation of sintering process, is used in powder metallurgy to produce various titanium alloys from initial powder components.[25–30] The role of hydrogen for atomized spherical powder sintering is poorly studied.

K.S. SENKEVICH is with the Department of Material Science and Heat Treatment, Moscow Aviation Institute, National Research University, Moscow 125993, Russia. Contact e-mail: [email protected] Manuscript submitted July 23, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

Spherical and nonspherical titanium powders (armstrong process powder, electrolytic powder, hydride–dehydride powder, and electrolytic powder) principally differ in sintering kinetics. Nonspherical powders are characterized by high surface energy conditioned by their well-developed surface.[31,32] They also may have a loose porous internal structure. Therefore, their sintering is accompanied by active diffusion processes as well as shrinkage of the powder already at lower temperatures (700 C and above).[31] Sintering of nonspherical titanium powders alloyed with 3 to 4 wt pct H occurs even more intensely and results in titanium hydride formation. The main factors contributing to the activation of diffusion processes during sintering of the nonspherical hydrogenated titanium powders are (1) thermal decomposition of titanium hydride forming a large number of crystal defects,[25,33–36] (2) reduction or loosening of oxide films by hydrogen,[35,37,38] (3) phase transformations associated with decomposition of titanium hydride and subsequent polymorphic transformation,[26,27,39] (4) sintering in the beta phase of titanium due to a decrease in the temperature of the polymorphic transformation (below 882 C) in hydrogenated titanium under conditions of abnormal self-diffusion in titanium near and above the b-transus temperat