Influence of the Diffusion Saturation with Oxygen on the Durability and Long-Term Static Strength of Titanium Alloys
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INFLUENCE OF THE DIFFUSION SATURATION WITH OXYGEN ON THE DURABILITY AND LONG-TERM STATIC STRENGTH OF TITANIUM ALLOYS V. M. Fedirko,1, 2 O. H. Luk’yanenko,1 and V. S. Trush1
UDC 669.017.163: 669.295
We study the influence of gradient hardening of the surface layer of the metal with an interstitial impurity (oxygen) on the durability of VT1-0, VT5, PT-7M, and OT4-1 titanium alloys under the conditions of pure and rotating bending, cyclic tension, and long-term static loading (up to 1000 h). Keywords: titanium alloys, subsurface layer, solid-solution hardening, fatigue, fatigue strength.
Titanium and alloys based on titanium are structural materials with high physicomechanical characteristics, such as specific strength, corrosion resistance in aggressive media, etc. The implementation of new technologies and the application of traditional methods for the production and treatment (thermal and thermochemical treatment, welding, etc.) of products made of titanium alloys is inevitably connected with the problem of gassaturation of the surface layers of the metal (formation of solid solutions of interstitial impurities) caused the high chemical affinity of titanium alloys to interstitial elements (O, N, C). This phenomenon affects the serviceability of the products [1–3]. There are opposite points of view on the solid-solution hardening of the subsurface layers of titanium alloys. One of these ideas is to use the phenomenon of surface hardening to enhance the wear resistance, improve the corrosion resistance of the metal in solutions of oxygen-containing acids, etc. [4–6]. The second commonly accepted opinion is that the solid-solution hardening of the subsurface layer embrittles the metal, decreases its ability to deformation, facilitates the origination of fatigue cracks, and decreases the fatigue resistance [7, 8]. At the same time, the experimental data [9–11] on the favorable influence of gas-saturated layers of certain thickness and structure on the mechanical properties of titanium alloys are less known. Thus, as early as in 1972 [12], it was discovered that the fatigue resistance of titanium alloys can be improved by annealing in air with subsequent thermal treatment in argon at temperatures of 530–600°С. This promotes the dissolution of the preliminarily formed oxide layer and the formation of a thicker diffusive layer [13], which enhances the durability of products made of titanium alloys. Hence, in order to increase the fatigue life and strength of titanium alloys, it is necessary to form a diffusive layer without phase film. Some researchers solve this problem by removing a part of the gas-saturated layer by mechanical treatment or chemical etching. Thus, in particular, in some works [9–11], it was shown that a partial removal of the gas-saturated layer by the chemical etching increases the durability of the metal under a repeated static tension of VT6 and VT14 sheet alloys. Thus, it is possible to enhance the durability of titanium alloys by the solid-solution hardening of the subsurface layer without phase f
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