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

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 3 2021 contains fractions of small and large particles. Most of the particles of the fine fraction are up to 7.3 µm in size, its amount is 37.2 % of the total, and agglomerated particles are up to 55 µm in size [22]. X-ray analysis was used to determine the structure and phase composition of the samples. XRD patterns were obtained using a DRON-type diffractometer (Russia) with CuK a radiation with an exposure at each recording point to provide statistical accuracy of not less than 0.5 %. The ICDD PDF-2 powder database was used to identify phase composition. Crystal lattice parameters were determined using the rtp32 program for XRD calculations [23] from interplanar distances for all reflections in the angular range of 10°–100°. The relative phase content was determined from integral intensities of all identified phases in the angle range of 35°–50°; the most intense reflections of all phases are located in this range. The sum of all lines is 100 %. The size of the coherently diffracting domain / coherent scattering region (CDD/CSR) was calculated from the first line of X-ray spectra using the Scherrer equation [24]; for calculation, FWHM was determined for each phase. For powder, all reflections characteristic of the phases are difficult to determine due to significant broadening of X-ray diffraction lines; therefore, the Williamson-Hall method cannot be used. Nevertheless, it should be noted that the calculated CDD/CSR sizes are slightly underestimated since the contributions of the CDD/CSR size and microdistortion to broadening are not separated in this case. The diffraction profiles were approximated using the Lorentz function. Results and discussion Fig. 1 shows SEM images of powders and particle size distribution after mechanical treatment (a, b) and after mechanical treatment of the powder hydrogenated for 90 minutes (c, d), and 180 minutes (e, f). As can be seen, after mechanical treatment (Fig. 1, a), the average particle size decreased by 2 % and is 10.9 µm, while the size dispersion increased from 7.5 to 10.9 µm. In the powder, fractions of small and large particles can be distinguished (Fig. 1,b). The size of fine particles with a smooth surface is up to 3.6 μm, its number is 33.3 % of the total number, and agglomerated particles are up to 83.1 μm in size. After mechani - cal treatment with preliminary hydrogenation of the powder for 90 minutes (Fig. 1, c), the average particle size decreased by 13 % and is 9.7 µm, the size dispersion is 9.6 µm. Fine particles with a smooth surface are up to 3.6 µm in size, and its number increased to 37.6 % of the total number; agglomerated particles are up to 80.6 µm in size (Fig. 1,d). After mechanical treatment with preliminary hydrogenation of the powder for 180 minutes (Fig. 1, e), the average particle size decreased by 40 % and is 6.7 µm, the size dispersion is 7.7 µm. Fine particles with a smooth surface are up to 2.6 µm in size, its number is 41.2 % of the total, and the size of agglomerated particles decreased to 62.9 µm (Fig. 1,f). During mechanical treatment and pre-hydrogenation, the particle shape does not change and is close to spherical apparently due to the effect of high-energy grinding in the planetary mill. Fig. 2 shows XRD patterns of the powders in the initial state, after mechanical treatment (sample No. 1), after mechanical treatment with pre-hydrogenation for 90 minutes (sample No. 2) and 180 minutes (sample No. 3). As can be seen, all XRD diffraction patterns contain diffraction reflections of the TiNi austenite phase, traces of the TiNi, Ti 2 Ni, and Ni 3 Ti martensitic phase. The XRD diffraction pattern obtained after mechanical treatment (MT) shows no change in phase composition (sample No. 1), while FWHM increased insignificantly, not more than by 19 %. The XRD diffraction patterns obtained for powders after MT with pre-hydrogenation (samples 2 and 3) show two intense peaks at angles 32.1° and 45.8°, which belong to the cubic phase of Ti 2 NiH x hydride with reflection indices (400) and (531). The relative content of the Ti 2 Ni phase increases from 36±5 % (powder in the initial state) to 42±5 %, and that of TiNi (austenite + martensite) decreases from 62±5 % (powder in the initial state) to 54±5 % during mechanical treatment of the powder hydrogenated for 180 minutes. After mechanical treatment of the initial powder and the powder hydrogenated for 90 minutes, the relative content of the latter virtually does not change with respect to the initial powder, all powders contain a small amount of the Ni 3 Ti phase, not more than 5 %. After hydrogenation, the angular positions of the lines of different phases are shifted (Fig. 2); therefore, the parameters of the crystal lattices of the phases are calculated. The calculation results are