*3.1. Cryogenic Milling*

The SEM micrograph of gas atomized Ti-15Mo powder is shown in Figure 3a. The initial gas-atomized powder particles are ball-shaped with the size from several μm up to 30 μm. Powder particles after cryomilling are shown in Figure 3b. Powder particles were not refined by cryomilling due to high ductility of titanium and its alloys even at low temperatures [22]. Powder particles were, however, plastically deformed and changed their shape from balls to disks. The size of disk-shaped powder particles is up to around 50 μm.

The contamination by oxygen (shown in Table 1) increased during milling from 0.20 wt. % to 0.78 wt. % despite milling in inert atmosphere of liquid argon. It was previously shown that handling powders in a glove box does not reduce the contamination significantly and therefore it is assumed that contamination origins from the milling [16]. Contamination during milling can be caused both by addition of the process control agent—the stearic acid—and by contamination from the used liquid argon, which may contain some trace amount of oxygen. It has been calculated that adding of 1 wt. % of stearic acid can cause contamination by oxygen by 0.11 wt. %. On the other hand, almost 100 L of 99.999% purity argon was used to mill 180 g of powder. The total content of oxygen in the liquid argon (Ar) corresponds to about 0.5 wt. % of oxygen in the milled powder. Some additional oxygen may be also absorbed.

**Figure 3.** Scanning electron microscopy (SEM) micrographs of (**a**) initial powder and (**b**) milled powder.

**Table 1.** Contamination of initial and milled powder by oxygen.

