Structure and composition of titanium spark erosion powder obtained in liquid nitrogen
Chem. Met. Alloys 4 (2011) 126-142
Gennady MONASTYRSKY, Patrick OCHIN, Guillaume Y. WANG, Victor KOLOMYTSEV, Yuri KOVAL, Vitaliy TINKOV, Anatoliy SHCHERBA, Sergey ZAHARCHENKO
Titanium powders were obtained by the spark-erosion method in liquid nitrogen. As-processed powder particles with typical sizes between 1 and 50 microns have spherical shape. The surface of the Ti powder particles has irregular structure. Many of the micron-sized particles contain irregular holes inside. The inner surface of the holes has well arranged bubble-like structure. The chemical analysis, made by the Kjeldahl method, showed that the powder contains 11.24±0.06 wt.% of nitrogen, while the more reliable XRD method gave 12.4±0.2 wt.%. Backscattering scanning electron microscopy images of cross sections of powder particles, as well as EDX analyses, showed a cellular structure of the particles with nitrogen-rich areas of several microns, separated by relatively thin boundaries of α-Ti(N). The XRD study confirmed that the powder contains about 85 wt.% of the δ-TiNx nitride (osbornite, Fm-3m), 10 wt.% of α-Ti(N) (P63/mmc) and minor quantities of α- and b-Ti (no more than 4 % in total). The TEM investigation showed that particles with sizes between 10 and 100 nm are mainly δ-TiNx nitride, although the quantity of oxygen in the nanoparticles is high. The proposed model for solidification of Ti powder particles considers multiple formation of nuclei of the solid phase on the surface of liquid droplets. It makes it possible to estimate, on the one hand the upper and lower temperatures of the molten droplets that were reached during the spark erosion, and on the other hand the cooling rate, and eventually to specify the mechanism of pore formation inside the powder particles.
Typical XRD pattern of a Ti-N powder with grain size less than 32 microns. The inset shows an enlargement of the pattern between 40o and 48o.
Powder / Spark-erosion method / Ti-N / Titanium nitride