The Microstructure of the Surface Layer of the Titanium Alloy Modified by Vacuum Ion Nitriding in the Pulse Mode
Authors:   National Academy of sciences of Ukraine G.S. Pisarenko institute for problems of strength, Kalinichenko Vitalii , Rutkovskіy Anatoly ,   National Academy of sciences of Ukraine G.S. Pisarenko institute for problems of strength, Markovych Sergiy ,   Central Ukrainian National Technical University
Published: December 2023

The mechanical strength and durability of structural elements is ensured by the correct selection of the materials from which they are made and the special functional properties of the surface layer, which are provided by applying coatings or by changing the structural state of the surface layers of the material by means of their modification. The use of titanium alloys provides unique properties, including high specific strength, corrosion resistance and a fairly high melting point. However, the use of these materials is limited by their low hardness, extremely low wear resistance, high tendency to sticking, high coefficient of friction in pairs with almost all materials, high chemical activity during heating above 500ᵒС and sensitivity to stress concentrators. To reduce the disadvantages of titanium alloys, various technological processes of strengthening are used by modifying the surface layers: by implantation of ions of various metals; surface treatment with concentrated energy flows; plasma flows; surface plastic deformation; ultrasonic treatment; laser processing; diffusion saturation and chemical-thermal treatment. Of all the modification methods, such a method of HTO as nitriding has become the most widespread, however, it involves long-term high-temperature heating, which causes fragility of the obtained nitrided layers, leads to a change in the metal structure and a decrease in strength characteristics. Task statement: conduct metallographic studies of the microstructure of modified surfaces with determination of the structure and depth of saturation of the diffusion layer with nitrogen Microsands for metallographic studies were produced on the "Alpha & Beta" installation (manufacturer of TM BUEHLER) according to the standard method (preventing tempering and overheating) with subsequent detection of the microstructure by chemical etching in a herbarium: НNO3 : HF : H2O (7 ml. : 2 ml. : 50 ml.). Metalohrafichni doslidzhennya zrazkiv provodyly v pozdovzhnʹomu peretyni po tovshchyni na optychnomu invertovanomu mikroskopi «AXIOVERT 40 MAT» z fiksatsiyeyu mikrostruktur tsyfrovoyu fotokameroyu AXIOCAM 305 v prohrami «ZEN core v3.1» Yak pokazaly doslidzhennya, tovshchyna nitrydnoho sharu pry vakuumnomu ionnomu azotuvanni v impulʹsnomu rezhymi tytanovoho splavu VT1-0 skladaye ne bilʹshe 1520 mkm, pid yakym znakhodytʹsya zona vnutrishnʹoho azotuvannya menshoyi tverdosti, yaka postupovo zmenshuyetʹsya po eksponentsiyniy zalezhnosti do tverdosti osnovy. Metallographic studies of the samples were carried out in a longitudinal cross-section by thickness on an optical inverted microscope "AXIOVERT 40 MAT" with fixation of microstructures by a digital camera АXIOCAM 305 in the program "ZEN core v3.1". As the studies showed, the thickness of the nitride layer during vacuum ion nitriding in the pulse mode of titanium alloy VT1-0 is no more than 1520 μm, under which there is a zone of internal nitriding of lower hardness, which gradually decreases exponentially depending on the hardness of the base. The structure of the base material does not change during low-temperature nitriding. A nitrided layer is formed on the surface with a nitride zone of varying thickness, which depends on the mode of nitriding and is weakly etched by the herbarium. The microstructure of the samples is an equiaxed polyhedral structure of the α-phase with a liquation band in the axial zone. Microhardness was determined under a load of 50, 100, and 200 g. The holding time under the load was 10 seconds. Microhardness was measured on the sandpapers in the radial direction of the segment with a step of 0.05 mm. The load depended on the dispersion and thickness of the reinforced layer. Conclusions. Nitriding (up to 900ᵒС) in the glow discharge of titanium VT1-0 and VT1-00 allows to significantly increase the microhardness due to the properties of nitrogen and its compounds with titanium, while preserving the original mechanical properties of the base material. A change in the parameters of the nitriding process (temperature, pressure, composition of the saturating medium and nitriding time) causes a change in the physical and mechanical characteristics, structure, thickness, phase and chemical composition of the surface nitriding layer: microhardness of the surface up to 10500 MPa; the thickness of the nitrided layer is up to 500 microns; the thickness of the nitride layer is up to 25 microns; phase shift difference TiN, Ti2N, Ti(N); different gradient of hardness in depth, which allows to optimize the power of the surface of the ball in specific conditions of operation.