Specific kinetic features of nitriding of titanium caused by phase-structural transformations
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SPECIFIC KINETIC FEATURES OF NITRIDING OF TITANIUM CAUSED BY PHASE-STRUCTURAL TRANSFORMATIONS Ya. S. Matychak
UDC 621.7856 539.219.3
We model analytically the high-temperature ( T = 1000°С) nitriding of titanium. The role of nitrogen as an α-stabilizer in the formation of a three-layer diffusion zone is analyzed. The problem of nitrogen diffusion in this inhomogeneous medium is solved with regard for the motion of interfaces. We calculate constants of the parabolic growth of layers, which enables us to predict the distribution of nitrogen in the diffusion zone. The evolution (after holding for 1 and 5 h) of the microstructure of a diffusion layer caused by structural-phase transformation during hydrogen diffusion is detected. Keywords: diffusion, modeling, kinetics, titanium, evolution of microstructure.
Diffusion in solid bodies is often accompanied by interrelated and interdependent structural and phase transformations that can be stimulated by diffusion. Furthermore, the structural transformations change the conditions of its development. At present, it is difficult to describe these processes analytically. Of special interest for these investigations, from both theoretical and experimental points of view, is titanium, which undergoes a polymorphic transformation (transformation of the hcp lattice into the bcc lattice) at Tα⇔β = 882°С. To impart functional properties to products of titanium alloys, one uses chemicothermal treatment, in particular, nitriding [1]. In contrast to many alloying elements (V and Mo), which are β -stabilizers, nitrogen, like other interstitial elements (oxygen and carbon), is an α -stabilizer, which stimulates structural transformations during nitriding. To develop methods for the prediction of surface hardening of titanium alloys after nitriding, it is necessary to obtain detailed (experimental or theoretical) information on the kinetics of interaction processes with regard for the influence of treatment parameters and the state of the material. The kinetics of nitriding of titanium and (α + β) -titanium alloys at temperatures T < Tα⇔β was analytically and experimentally investigated in [2, 3]. The specific features of the morphology of the surface layers of titanium alloys of different structural classes nitrided at temperatures above the temperature of polymorphic transformation were studied in [4]. The evolution of their microstructure during the α ⇔ β phase transformation as a result of the migration of β stabilizers was described in [5]. However, the role of nitrogen itself as an α -stabilizer in these transformations was not taken into account. In [6], the interrelation between nitrogen diffusion and structural transformations during high-temperature nitriding, specifically, from a rarefied atmosphere when the conditions for the formation of a continuous nitride layer are absent for a long time, was also studied. In what follows, we develop a physicomathematical model of the diffusion saturation of titanium from an environment with high nitrogen potential at temperatures T > Tα
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