Materials Science, Vol. 38, No. 6, 2002 KINETICS OF CARBONIZATION OF TITANIUM ALLOYS IN A CARBON-OXYGEN-CONTAINING MEDIUM I. Pohrelyuk, O. Yas’kiv, and S. Mynyk UDC 669.295.620.196 We investigate the kinetics of saturation of surface layers of VT1-0 and OT4 titanium alloys in the process of carbonization from graphite in an atmosphere of argon at temperatures of 900 and 1000°C for 1 – 15 h. We revealed the formation of a deep hardened zone for long-holding times. The redistribution of oxygen in the case of dissociation of a surface oxide film promotes the formation of complex TiC x O y -type compounds based on titanium carbide, which addi- tionally harden surface layers. Thermochemical treatment, in particular carbonization, is an efficient method for increasing the durability of products made of titanium alloys [1 – 3]. The rate of formation and growth of surface carbide layers, their phase composition, and the structure are determined by the activity of the saturating medium, the temperature, and the time of treatment [4]. Using the results of investigation related to carbonization, one can estimate the intensity of interaction of a metal with the environment and trace the evolution of phase transformations in its surface layers. Saturation of titanium alloys from a powder substance (graphite) in argon [4] is one of the efficient methods for their carbonization. Using this fact, we investigate the kinetics of carbonization of titanium alloys from this medium. Methodical Aspects We investigate specimens 15 × 10 × 1 mm in size made of commercial titanium VT1-0 (commercially pure titanium) and OT4 (Ti – 3.5, Al – 1.5 Mn) alloys. The specimens were polished (R a = 0.4 μm ) and subjected to the thermochemical treatment at temperatures of 900 and 1000° C for an isothermal holding of 1 – 15 h. The specimens were saturated from graphite in a static atmosphere of argon (100 kPa) under a partial oxygen pressure < 0.2 Pa. We determine the phase composition of surface layers performing X-ray phase analysis with a DRON-3.0 diffractograph in a monochromatic CuK α -irradiation focused according to the Bragg – Bretano scheme. The voltage across the anode of the X-ray tube is equal to 30 kV and the current through it equals 20 μA. The results are processed with a software of the diffractograph. To identify the obtained diffraction spectrum of the specimen presented in the form of an array of the values of d i and T i , we compare them with the data of the JCPDS–ASTM card file [5]. We estimate hardening on the basis of microhardness (a PMT-3 device). We identify a hardened layer with a zone whose hardness is greater than the hardness of the core by 0.2 GPa [6]. Results and Discussion As a measure of saturation, we take the increase in the mass of specimens per unit area Δ m / S. Curves of variation in the increase in mass of specimens of VT1-0 and OT4 titanium alloys with time (Fig. 1) can be satisfactorily described by the power dependence [7, 8] Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materi- aliv, Vol. 38, No. 6, pp. 58 – 62, November – December, 2002. Original article submitted November 19, 2001. 1068–820X/02/3806–0833 $27.00 © 2002 Plenum Publishing Corporation 833