Hydrogen and its effect on the grinding of Ti-Ni powder

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 3 2021 Introduction It is known that the presence of hydrogen in a material has an ambiguous effect on its properties. In [1, 2], it is shown that hydrogen can reduce metal deformability, i.e. it makes it brittle [3] due to formation of brittle hydrides on the main slip and twin planes [4]. On the other hand, in [5], titanium alloy plasticity is found to increase due to the hydrogen plasticization effect, and in [6], it is shown that the presence of hydrogen in the lattice changes phase transformation temperatures. It is known that plastic materials are able to deformwithout breaking the continuity and crack formation [7]. One of these materials is titanium nickelide, which possesses a number of unique properties [8], and is widely applied in industry. Ti-Ni products are often produced by powder metallurgy techniques. This technology employs fine powders to produce materials with high physical and mechanical properties [9]. Fine powders can be prepared by different methods, such as physicochemical (reduction method, electrolytic method, carbonyl method) and mechanical (production of powder from metal melt, solid state grinding by ball, vortex, planetary ball mills and vibrating mills) production methods. One of grinding methods is high-intensity mechanical treatment of powder in a planetary ball mill. This method is relatively inexpensive, fast and easy-to-implement; however, milling can involve contamination, powder oxidation and particle aggregation, etc. [10]. It is known that metal hydrides are typically brittle substances [11, 12], which can prevent aggregation of fine particles during high-intensity grinding. Therefore, pre- hydrogenation can not only intensify powder grinding, but powder hydrogenation-dehydrogenation can also cause a self-grinding effect due to lattice expansion–contraction [13]. Moreover, sintering of hydrogenated powders according to [14-16] leads to hydrogen release, which helps remove impurities such as oxygen, carbon, and chlorine, whereas, after milling, impurities remain in heated powders. This is confirmed in [17, 18], which report a positive effect of hydrogen during sintering followed by hydrogen released. The aim of this study is to investigate the effect of hydrogen on Ti-Ni powder grinding. The main objectives of this study are as follows: 1) to investigate the morphology of Ti-Ni particles and its size by scanning electron microscopy before and after mechanical treatment of the initial and hydrogenated powder; 2) to study the change in phase composition and parameters of the fine crystal structure by X-ray structural and X-ray phase analyses after mechanical treatment of the initial and hydrogenated powder. Materials and methods The industrial powder of titanium nickelide of the PN55T45 grade produced by the Polema company was studied. The powder was milled in an AGO-2 planetary ball mill (Russia) in air at room temperature and atmospheric pressure. The steel ball diameter was 0.7 cm, the ball-to-powder ratio was 1:5, and the rotation speed of the planetary disk was 1.820 rpm, which provides 60g acceleration. The treatment time was 100 seconds. Hydrogenation before mechanical treatment was performed in a special cell by electrochemical method. The electrolyte was 20 % aqueous solution of sodium chloride (NaCl) and surfactant, dextrin (C 6 H 10 O 5 ) n , at a concentration of 1.5 g/L at room temperature [19]. The cathode current density was 55 mA/cm 2 , and the cell voltage was 4.0 V. A graphite plate was used as an anode, and powder, placed on a stainless-steel cup, was a cathode [20]. The hydrogenation time was 90 and 180 minutes. The morphology and average powder particle size were estimated from SEM images obtained with a scanning electron microscope (SEM) using a TESCAN VEGA3 SBH microscope. The particle shape was estimated by the method proposed by Hausner [21]. Two dimensions of the projection plane of each particle (maximum d max and minimum d min ) were measured to plot the particle size distributions; in total, about 650 particles were analyzed. The measurement data were used to calculate the average particle size, size dispersion and median diameter. The Ti-Ni powder in the initial state consists mainly of spherical particles with a smooth surface with an average size of 11.1 µm, and the size dispersion is 7.5 µm. The powder