OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 3 2023 Introduction There are two intermetallic compounds on the iron-titanium equilibrium diagram: Fe2Ti and FeTi (fig. 1). Iron monotitanide FeTi is the most studied among the intermetallic compounds of the iron-titanium system. This compound is known as a perspective hydrogen storage material. FeTi is inferior to the most promising magnesium alloys and alloys of rare-earth metals in terms of hydrogen capacity, ability to pass into an active state with respect to hydrogen sorption, kinetic characteristics of sorption-desorption and cyclic stability [1]. However, due to the low cost of raw materials, attempts continue to obtain a material with improved sorption characteristics. In this case, iron and titanium powders are used as raw materials, and mechanical activation (MA) of synthesis is used as a method for obtaining the material during longterm processing of the powder mixture in planetary mills. Intensive studies of the behavior of iron-titanium powder mixtures during mechanical activation began in the early 2000s. It was found out in [2] that during long-term (up to 92 hours) processing of mixtures in a SpexMixer/MillModel 8000 magnetic vibratory mill, full amorphization of titanium and iron is completed. The formation of intermetallic compounds was not observed in this case. Extensive studies of the sorption properties of materials obtained by mechanical activation of iron and titanium powder mixtures were studied by A.V. Zadorozhny and colleagues [3–8]. For mechanical activation, an AGO-2S planetary mill was used. The processing was carried out in an argon media at a rotation speed of 840 rpm. The phase composition of the mechanical synthesis’ products depended on the dispersity of titanium and iron powders. The formation of intermetallic compounds was not observed when using coarse powders, i.e. 280 and 450 µm, and processing time of 120 minutes [3]. When 5–10 µm fine titanium powder was used, the product obtained by processing of an equiatomic mixture for 30 minutes at the same intensity, i.e. 840 rpm, consisted of singlephase FeTi [8]. This result is in conflict with the results of [2], in which no intermetallic compound was formed even after 92 hours of processing in a SpexMixer/MillModel 8000 mill. The authors of [8] explain the reason for this mismatch by the low intensity of MA in the mill used in [2]. Pressed FeTi single-phase powder samples with a nanocrystalline structure during thermal cycling in a hydrogen medium retained its shape without cracking due to the formation of bridges that hold adjacent particles together [4, 7]. In order to improve the sorption properties of FeTi obtained by intensive MA, various powder additives were added into the mixture of titanium and iron: 20 at. % Al or 6 at. % Cr [6], 1 at. % S or 2 at. % Mg [5]. It was shown, that these additives improve the sorption characteristics – simplification of the procedure for activating hydrogen uptake and a decrease in the pressure of the plateau area. In a the brief follow-up review [9], hydrogen storage alloys based on magnesium and rare earth metals such as FeTi have improved sorption characteristics in the nanocrystalline state. Along with mechanical activation, to obtain nanocrystalline intermetallic compounds of the iron-titanium system, attempts are made to use other methods, in particular, severe plastic deformation of powder mixtures in a Bridgman anvil [10]. To create a nanocrystalline structure, it looks perspective to intensively grind FeTi powder, previously obtained by casting or using powder technologies. The most technologically simple way to obtain intermetallic compounds is the synthesis in iron-titanium powder mixtures of binary intermetallic compounds [11, 12]. This synthesis can be realized either directly in the process of mechanical activation [12] or during the subsequent initiation of the reaction in mechanically activated mixtures [13, 14]. In [15], FeTi and Fe2Ti compounds were obtained from powder mixtures of the respective compositions. Since the mixtures were not subjected to preliminary mechanical activation, it becomes possible to initiate Fig. 1. Phase diagram of the Fe-Ti system
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