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

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 3 2021 Fig. 2. XRD patterns of powders: in the initial state, sample No. 1 – after mechanical treatment, samples No. 2,3 – after mechanical treatment with pre-hydrogenation for 90 and 180 minutes, respectively summarized in Table 1. After mechanical treatment of the initial powder and hydrogenated powder, the lattice parameter of the martensite phase is difficult to determine due to significant broadening of the X-ray diffraction lines; therefore, this value is indicated by a “question mark” in the table. As can be seen from the table, the parameters of the crystal lattices of all phases detected in the powder in the initial state are in good agreement with the literature data [25–28]. After mechanical treatment, the lattice parameters of TiNi (austenite), Ti 2 Ni, and Ni 3 Ti phases do not change within the error limits. After MT of the powder hydrogenated for 180 minutes, the lattice parameter of the Ni 3 Ti phase decreases from 0.8316 nm (powder in the initial state) to 0.8241 nm, and the lattice parameter of the Ti 2 Ni phase increases from 1.1333 nm (powder in the initial state) to 1.1457 nm. This means that during mechanical treatment of the hydrogenated powder, the lattice parameter of only Ti 2 Ni phase changes significantly, and its value is close to the lattice parameter of hydride with Ti 2 NiH 0.5 stoichiometry, which equals 1.1500 nm (ICDD PDF2 270346) [29, 30]. Table 2 shows data on microstructural parameters of phases. It also presents an estimate of the disloca- tion density (ρ) calculated in accordance with [31] from the first line of the X-ray spectra under the assump - tion that its width depends on the CDD/CSR size only; therefore, the density is overestimated. As can be seen from the table, FWHM of the TiNi (austenite) phase does not change within the error during mechani - cal treatment of the initial and hydrogenated powder. Whereas, FWHM of the Ni 3 Ti phase increases from 0.084° (powder in the initial state) to 0.260° (powder hydrogenated for 90 minutes) and then decreases to 0.143° (powder hydrogenated for 180 minutes). The CDD/CSR size calculated for the TiNi and Ti 2 Ni phases does not change within the error range and is 29±5 nm and 11±5 nm for all powders, while for the Ni 3 Ti phase after MT it decreases from 108±5 nm to 49±5 nm, after MT of the hydrogenated powders it decreases to 35±5 nm at 90 minutes, increases to 63±5 nm (180 minutes). An absolute increase in the defect density in the Ni 3 Ti phase exceeds that in TiNi and Ti 2 Ni phases, however, the value of the dislocation density for the Ti 2 Ni phase is 6 fold greater than that for the TiNi phase and 67 fold greater than that for the Ni 3 Ti phase in the initial state. Apparently, high defect density in the Ti 2 Ni phase in the initial state intensifies interaction of hydrogen with this phase [20], which also intensifies crushing during mechanical treatment due to formation of brittle hydride.