**1. Introduction**

Metastable β-alloys constitute a specific group of Ti alloys in which none of the α-phase, α'-phase or α"-phase form after quenching from a temperature above the temperature of β-transus (774 ◦C for Ti-15Mo [1]). β-Ti alloys are widely used in the aircraft industry due to their high specific strength [2]. Utilization of these alloys in biomedicine is also expected [3]. Currently, Ti-6Al-4V is a commonly used alloy for biomedical implants manufacturing, despite vanadium being considered as a toxic element [4]. Therefore, Ti-15Mo, which contains only biocompatible elements, is a perspective biocompatible alternative. Furthermore, β-Ti alloys in β-solution treated condition exhibit lower modulus of elasticity, which is closer to that of a human bone [5].

Mechanical properties of metastable β-Ti alloys can be further improved by thermomechanical treatment. Both microstructure refinement and precipitation of α-phase increase the strength of the alloy. α-phase particles precipitate typically at temperatures in the range of 500–750 ◦C. During this phase transformation, so-called element partitioning occurs. β-stabilizing elements (molybdenum (Mo), niobium (Nb), vanadium (V), etc.) diffuse from the growing α-phase particles to the surrounding β-matrix [6,7]. In addition to α and β-phase, metastable ω-phase can also occur in some metastable β-alloys including Ti-15Mo alloy. During quenching, so-called ωath (athermal) forms by a displacive transformation. When annealing at temperatures in range 250–450 ◦C, ωiso (isothermal) forms by a diffusional transformation [8]. ω-phase also strengthens the material.

Powder metallurgy is an alternative and often more suitable way of producing titanium alloys [9,10]. Due to the possibility of near-net-shape processing, the material waste and costs associated with the

material processing are reduced. Spark plasma sintering (SPS) was used as a compaction method in this study. During SPS, the powder is compacted by pulse electric current and the powder particles are joined together by the Joule heat. The sintering therefore occurs primarily at the point of contact of powder particles [11]. Therefore, lower temperatures and shorter times are sufficient for compaction in comparison to other compaction methods. Utilization of SPS for compaction helps therefore to preserve the fine microstructure and restrict the grain growth [11,12].

Elemental powders, master alloys or pre-alloyed powder can be used for sintering. Sintering of elemental powders or master alloys must provide alloying (homogenization) of the material and therefore high sintering temperatures (1200–1700 ◦C) must be used for processing of metastable β-Ti alloys due to high melting points of β-stabilizing elements and their low diffusivity [2,13]. On the other hand, successful compaction of pre-alloyed powder can be achieved at temperatures comparable to the temperature of β-transus of an alloy [14].

Besides the parameters of sintering, the final bulk material is also affected by the shape, the size and the microstructure of powder particles [14]. Mechanical milling is commonly used to fragment powder particles [15]. In this study, intensive ball milling at cryogenic temperatures was used. Cryogenic temperatures are often used for milling of organic materials to ensure their brittleness [16]. In metallic materials, cryogenic temperatures help to suppress dynamic recovery and recrystallization and to achieve very refined microstructure [17].

Cryogenic milling employing the Szegvari type attritor has been previously applied to commercially pure titanium [18–21]. It was found that titanium remains ductile even at cryogenic temperatures (unless contaminated by nitrogen [21]) and powder particles are not significantly refined [22]. On the other hand, powder particles are significantly deformed by repetitive plastic deformation [15,22,23]. In our previous study we proved that deformation by ball milling resembles the multi-directional forging and causes grain refinement [24]. In this respect, mechanical milling can be regarded as a method of severe plastic deformation (SPD). Various metastable β-Ti alloys have been prepared by SPD methods and improvement of strength was reported [25,26]. Recently the Ti-15Mo alloy was prepared by high pressure torsion (HPT), which caused the grain refinement and an increase of dislocation density [27,28]. On the other hand, HPT samples are very small and thermal stability of ultra-fine grained Ti alloys prepared by SPD methods is limited [29,30]. Similarly to bulk SPD methods, grain size after cryogenic milling can be reduced to tens of nanometers as shown for commercially pure Ti (CP Ti) in [22]. However, the grain size in CP Ti significantly increases after sintering due to recrystallization [24].

Ti-15Mo pre-alloyed powder was milled in liquid argon slurry. Note that liquid nitrogen cannot be used, because N atoms diffuse into the powder during milling causing embrittlement [21,31]. In order to prevent cold welding of the powder to the milling tank, shaft and balls, a process control agent such as stearic acid must be added to it [15,22].

Milled powder was subsequently sintered at temperatures in the range of 750–850 ◦C for 3 min. For comparison, initial gas atomized powder was sintered using the same parameters. Due to the fact that titanium is a strong gatherer of nitrogen and oxygen, some contamination is unavoidable during powder metallurgy processing [22,32]. Therefore, contamination by oxygen, nitrogen (and possibly by carbon and hydrogen) must be always monitored in order to assess strengthening mechanisms appropriately.
