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

Abstract

Introduction. Industrial nickel-titanium alloy PN55T45 closed to the equiatomic composition is widely used for the manufacture of products by powder metallurgy. To achieve high physical and mechanical properties of the material obtained by this method, it is necessary to use fine powders, which can be obtained by implementing high-intensity grinding in a planetary ball mill. However, during such treatment, contamination, powder oxidation and particle aggregation, etc. are possible. To solve this problem, preliminary hydrogenation is proposed for subsequent grinding in a planetary ball mill. The aim of the work is to study the effect of hydrogen on the grinding of titanium nickelide powder. Materials and methods. The morphology and average particle size of the powders were studied by scanning electron microscopy. The structure and phase composition of the powders were investigated by the methods of X-ray structural and X-ray phase analysis. The data of X-ray structural analysis were used to estimate the dislocation density. Results and discussions. It is shown that the use of pre-hydrogenation for 180 minutes before machining allows reducing the average particle size by about a half. After mechanical treatment of the powder, the parameters of the crystal lattices of the TiNi (austenite), Ti2Ni and Ni3Ti phases do not change within the error range. After mechanical treatment of the powder with preliminary hydrogenation, the crystal lattice parameter of only the Ti2Ni phase changes significantly, in particular, at 180 minutes of hydrogenation, the lattice parameter increases to 1.1457 ± 5×10-4 nm, which corresponds to the stoichiometry of the Ti2NiH0.5 hydride with a lattice parameter of 1.1500 nm. The highest dislocation density estimated by X-ray diffraction analysis is contained in the Ti2Ni (511) phase than in the TiNi (austenite) (110) and Ni3Ti (202) phases. Thus, preliminary hydrogenation can be an effective method of powder grinding due to the formation of brittle hydride and suppression of the aggregation of fine particles during high-intensity mechanical treatment.