Hydride Approach in Blended Elemental Powder Metallurgy of Beta Titanium Alloys

The physical bases of hydrogenated tanium powders applica on in blended elemental powder metallurgy (BEPM) of tanium alloys were earlier developed. Hydrogen as temporary alloying addi on for tanium strongly affects diffusion processes upon transforma on of powder blends into alloys ensuring produc on of α+β and metastable β tanium alloys which mechanical proper es meet standard requirements. At the same me, synthesis of metastable β alloys is complicated by a big amount of alloying elements which diffusion redistribu on upon sintering has a strong impact on microstructure evolu on. In present study BEPM hydride approach was expanded for produc on of biocompa ble low modulus Ti-Zr-Nb and Ti-Zr-Nb-Ta alloys having BCC structure which are a�rac ve materials for medical applica on. The alloys of prescribed composi ons were produced using various star ng powders, including TiH2, ZrH2, hydrogenated niobium, tantalum and Ti-Nb master alloys. Peculiari es of volume changes of mul component powder blends on dehydrogena on were inves gated. The specific volume changes of powder components during dehydrogena on affect densifica on kine c of powder blends and microstructure of as-sintered alloys.

big amount of alloying elements which diffusion redistribu on upon sintering has a strong impact on microstructure evolu on.
In present study BEPM hydride approach was expanded for produc on of biocompa ble low modulus Ti-Zr-Nb and Ti-Zr-Nb-Ta alloys having BCC structure which are a�rac ve materials for medical applica on. The alloys of prescribed composi ons were produced using various star ng powders, including TiH 2 , ZrH 2 , hydrogenated niobium, tantalum and Ti-Nb master alloys. Peculiari es of volume changes of mul component powder blends on dehydrogena on were inves gated. The specific volume changes of powder components during dehydrogena on affect densifica on kine c of powder blends and microstructure of as-sintered alloys.
Blended elemental powder metallurgy (BEPM) has a great poten al for manufacturing of tanium components [1] owing to significant cost reduc on as compared to conven onal cast/wrought manufacturing approach. The use of hydrogenated tanium ( tanium hydride TiH 2 ) powder instead of tanium metal powder blended with alloying par cles provides an unique opportunity to achieve mechanical proper es sufficient for many prac cal applica ons of BEPM produced tanium parts [2][3]. In this approach (further referred as TiH 2 BEPM) hydrogen serves as a temporary alloying element for tanium affec ng its microstructure and, therefore, mechanical proper es. TiH 2 ®Ti+2H phase transforma on takes place upon vacuum hea ng of hydrogenated tanium. The phase transforma on increases density of crystal la ce defects, ac vates solid state diffusion, thus resul ng in faster homogeniza on and densifica on of powder blends. Atomic hydrogen released from each TiH 2 par cle reduces surface TiO 2 layer on them. Cleaning of the par cle surface makes them open for interpar cle diffusion. Open porosity provides the pathway for H 2 O vapor to go out. As a result, oxygen content in the compact is being decreased [4]. Hydrogen also takes away some other impuri es from the par cle surface, such as chlorine and carbon. The former is especially important since the chlorine rich powder in solid state PM processing always result in the poor duc lity and corresponding decrease in the proper es [5]. If properly done, the TiH 2 BEPM that includes dehydrogena on, sintering and chemical homogeniza on of powder blends (Fig 1) results in produc on of nearly dense (up to 99%) chemically and microstructurally homogeneous α+β and near-β tanium products with admissible impurity content and mechanical proper es which met requirements and specifica ons for corresponding cast/wrought materials [2][3][6][7]. However, homogeniza on and densifica on of powder blends and hence, microstructure of alloy products are strongly affected by their composi on and type of alloying powders (AP -elemental metals or master alloys) used in the blends to a�ain prescribed chemical composi on. Forma on of low-porous and fine-grained as-sintered microstructures has proved to be especially challenging for near-β composi ons with high content of alloying elements, such as high-strength Ti-10V-2Fe-3Al, Ti-5Al-5V-5Mo-3Cr [2][3] and Ti-1Al-8V-5Fe alloys. It has been found that although TiH 2 BEPM is advantageous compared to conven onal Ti PM, its specific features have to be taken into account at manufacturing tanium composi ons with higher content of alloying elements.

TiH 2 BEPM processing of metastable β alloys
The base TiH 2 par cles and AP par cles form two subsystems with numerous interfaces between par cles of different subsystems. Dehydrogena on results in a significant shrinkage of TiH 2 par cles, only slightly compensated by thermal expansion, while AP are subjected to thermal expansion only. Such mismatch in volume changes makes TiH 2 /AP interfaces weak elements of the powder compacts further leading to their damaged integrity. Forma on of gaps and voids at such interfaces adds to the porosity and retards densifica on. Another issue is a delayed homogeniza on since corresponding contact areas become smaller and which is generally slow in such alloys because of low diffusion mobility of many β stabilizers (Mo, Nb, Ta) in tanium. Higher temperatures and/or longer mes of sintering cannot be a solu on for an op mized processing since this leads to coarsening of grain structure.  The above issues of TiH 2 BEPM processing of metastable β alloys were partly overcome by proper selec on of alloying powders, their sizing and modifica on of sintering parameters [2][3].
However, for some composi ons such methods were not sufficient to achieve desirable microstructures and proper es.
This study was aimed at analysis of poten al pathways for further improvement of TiH 2 BEPM manufacturing of heavily alloyed tanium alloys. The main strategy was to minimize a difference in volume mismatch on TiH 2 /AP interfaces in order to minimize the reasons of gap/void forma on. In principle, there are two possibili es for that. Since dehydrogena on of TiH 2 par cles is a dominant process, the first possibility is based on the assump on that such shrinkage should be compensated by expansion of the AP in the same temperature range. It would be possible if AP par cles absorb the part of hydrogen released from the TiH 2 par cles and expand due to their hydrogena on.
More promising is a second possibility based on a selec on of AP which par cles behave similar to TiH 2 par cles upon compac on and hea ng. It is known [2] that compac on mechanism of the TiH 2 par cles includes their crushing and forming specific locks between crushed pieces, strong enough to keep them together upon handling of the green parts and following hea ng. This ensures compact integrity despite of shrinkage of each individual par cle. So, if the AP par cles would exhibit the mechanical proper es similar to that of the TiH 2 par cles, namely bri�leness and low strength, they would be involved in the forma on of compact similar to the compact consisted of the TiH 2 par cles only. To sa sfy the above requirements to the AP it is proposed to use their hydrogenated modifica ons.
Ingot of 18Ti-6Al-47V-29Fe (mass.%) master alloy was melted and crushed to powder. Part of the AP was compacted with TiH 2 powder. It was important that material of such composi on could undergo hydrogena on/dehydrogena on cycle [8] upon hea ng. So we could believe that AP hydrogena on would be in favor of compact integrity. Another part was preliminary hydrogenated and then compacted together with TiH 2 powder as a blend of two hydride powders.
Shrinkage curves for both such compacts are shown on Fig. 3. One can see that they are quite similar. Moreover, overall linear shrinkage was in excess of 10%, the same as that typically observed upon sintering of the pure TiH 2 powder. This was expected for the blend that included preliminary hydrogenated AP (curve 1) but similar shrinkage behavior of the blend with non hydrogenated AP was even above expecta ons (curve 2) clearly indica ng that AP par cles can be temporarily hydrogenated by hydrogen evolved from TiH 2 on hea ng. Absence of gaps around AP a�er dehydrogena on was completed (Fig. 4) gives an addi onal confirma on for that. It is important that for both compacts densifica on was no ceably be�er than that earlier observed for Ti-1023 alloy [2][3], in which visible gaps between AP and dehydrogenated tanium matrix formed on hea ng (See Fig. 2).  Although both above approaches have proved to be successful in TiH 2 BEPM processing of the Ti-185 alloy with regard to its final density and chemical homogeneity, this was achieved at lower temperature and/or shorter me of sintering in the approach based on using preliminary hydrogenated AP, indica ng on an important advantage of using the hydrogenated PAs. Therefore just this approach was used in TiH 2 BEPM processing of low modulus tanium alloys.

BEPM produc on of β Ti-Zr-Nb and Ti-Zr-Nb-Ta alloys
Conven onal ingot/wrought processing of such alloys is hindered by high mel ng points of the components. Therefore their BEPM processing is of great interest. However, in turn, such processing is also challenging since mutual diffusivi es of most their components are very low. These alloys are based on Ti-Zr system with addi on of biocompa ble β stabilizers such as Nb and Ta [9]. As a first step, it was proposed to use Zr in the form of ZrH 2 hydride. Advantage of ZrH 2 over Zr powder in the sintering process [10] comes from its similarity with TiH 2 . Zirconium can be easily hydrogenated; bri�le and low-strength ZrH 2 phase is easily crushed to powder of desirable sizes; ZrH 2 ®Zr+2H phase transforma on upon vacuum hea ng provides sintering ac va on. As a result, TiH 2 +ZrH 2 compacts always are sintered to densi es higher than TiH 2 +Zr compacts (Fig. 5). However, addi ons of Nb in accordance with 39Ti-35Zr-26Nb and 19Ti-59Zr-22Nb prescribed alloy composi ons always lead to a significantly worse results. It was found that despite of low diffusion mobility of niobium, chemical homogeneity is possible to be achieved at high temperature sintering, but the final porosity remains high, typically in 6 to 9% range (Fig. 6). It is certainly a consequence of different behavior of the bri�le hydride TiH 2 +ZrH 2 part of the blend and duc le Nb par cles.
Hydrides are crushed on compac on and shrunk on hea ng leaving addi onal porosity around Nb par cles. Combined with Kirkendall`s type pores, the resul ng porosity on the interfaces between Nb and Ti-Zr matrix becomes high enough to be healed even at high temperatures. As it is seen from comparison of curves 1 and 3 (Fig. 7), addi on of Nb par cles to TiH 2 +ZrH 2 blend results in a delayed shrinkage of powder compacts.  Having in mind an aim to minimize the final porosity of 39Ti-35Zr-26Nb and 19Ti-59Zr-22Nb alloys, Nb powder was hydrogenated. However, that did not show any significant effect on the final result, since, as X-ray analysis showed, hydrogenated Nb had a two-phase Nb+NbH structure, not bri�le enough to be crushed upon compac on. Moreover, hydrogenated Nb was unstable in vacuum even at room temperature and therefore could not affect the microstructure evolu on upon hea ng.
A no ceable decrease in the porosity down to 2% was achieved only when hydrogenated AP (Ti-Nb)H x was used (Fig. 8). The shrinkage of such compact (Fig. 7, curve 2) looks similar to that of the TiH 2 +ZrH 2 without AP and much different from the shrinkage of the blend with Nb par cles.
Following the above results, for synthesis of Ti-35Nb-7Zr-5Ta alloy all components were used in their hydrogenated form, including hydrogenated tantalum inden fied as TaH phase. Its bri�leness allowed its easy milling to powders of desirable sizes and compac on together with the TiH 2 , ZrH 2 and (Ti-Nb)H X . Hydrogen release from TaH started at about 300 o C. Sintering of such blended compacts resulted in a forma on of low porous material. However, completed homogeneity was not achieved. Undissolved tantalum par cles were observed in as-sintered material because of the extremely slow diffusion mobility of Ta (5 x 10 -20 m 2 /s at 1250 o C [11], i.e. 7-8 orders of magnitude lower than diffusivity of other elements at noted temperature).
To achieve a fully homogeneous alloy, two stage sintering process was developed. A�er intermediate sintering, as-sintered product was hydrogenated again and milled to produce prealloyed hydrogenated powder which was then compacted and sintered again at 1250 o C, 4h. Such two stage sintering associated with double hydrogena on/dehydrogena on in order to ac vate diffusion resulted in a forma on of completely uniform single-phase b-BCC microstructure with grain size of about 100 mm and not more than 2% porosity (Fig. 9).
Mechanical proper es of the alloys manufactured with the modified TiH 2 BEPM approach are not analyzed in this paper. However, it is well known that three characteris cs are important to achieve balanced proper es of the PM processed tanium alloys. Those are high sintered density, not less than 98% of theore cal, admissible inters al content according to alloy specifica ons, and uniform microstructure in which beta grain size is limited by 100-150 mm. As it was shown above the modified TiH 2 BEPM approach which main feature consists in using not only TiH 2 but also all APs in their hydrogenated forms is capable to sa sfy these requirements. As an example, low modulus Ti-35Nb-7Zr-5Ta alloy processed with the above approach exhibit excellent strength/duc lity balance (Fig. 10). https://doi.org/10.1051/matecconf/202032103009 The 14 th World Conference on Titanium Fig. 9. Uniform Ti-35Nb-7Zr-5Ta alloy produced with sintering of hydrogenated prealloyed powder. Fig. 10. Stress/strain tensile tes ng curve of as-sintered Ti-35Nb-7Zr-5Ta alloy.

Conclusion
The TiH 2 BEPM of the heavily alloyed metastable β alloys is complicated because of numerous interfaces between TiH 2 and AP par cles. A no ceable shrinkage of the TiH 2 par cles upon dehydrogena on provokes a volume mismatch between them and AP par cles of different composi on thus affec ng integrity of compacted powder systems and resul ng in an excess as-sintered porosity.
Use of hydrogenated alloying powders allows minimiza on of the above disadvantage of TiH 2 BEPM. Low-porous, homogeneous β alloys were processed with this approach, including alloys for medical applica on.