Experimental study of the feasibility of producing materials based on the metastable phase Ti₂Fe through explosive compaction and heat treatment

Introduction

The development of hydrogen energy imposes ever-increasing demands on hydrogen storage systems [1]. One of the most promising and safe methods for hydrogen storage is the use of hydride-forming intermetallics [2]. For example, the volumetric density of hydrogen in hydrides based on intermetallics ranges from approximately 60 to 100 kg/m³, whereas in the gaseous state, even at a pressure of 400 bar, it is only about 20 kg/m³ [3]. Such high values of hydrogen capacity enable the creation of anode materials for metal hydride batteries with a discharge capacity reaching 400 mA·h/g [1].

In addition to the currently most widespread intermetallic LaNi₅, one of the most attractive materials for hydrogen storage is the intermetallic TiFe, which is characterized by low cost and the ability to operate at low pressures and ambient temperatures [1].

Currently, many researchers [4–11] are focused on studying the possibility of solving the “activation” problem of this material [12–15]. The issue is that TiFe is very sensitive to air and forms a passivating layer that prevents hydrogen absorption. The classical activation method involves heat treatment, which consists of cyclic exposure to elevated and room temperatures at high hydrogen pressure [16]. However, other methods are now considered more promising, such as mechanical treatment (ball milling, cold rolling, high-pressure torsion) [8; 17–20], the use of increased (compared to stoichiometric) Ti content [5; 9], and the addition of alloying elements such as Mn, Cr, Zr, Y, etc. [6; 10; 21–23].

When using an excess of Ti and alloying, it was found [5; 6; 9; 10; 21–23] that in addition to TiFe, a solid solution based on β-Ti (referred to in some works as the BCC phase or Ti₄Fe phase) and the intermetallic Ti₂Fe appear in the structure of the material, which act as a kind of “gate” for hydrogen [6–8]. Several studies [4; 5; 9–11] also note a general increase in the hydrogen capacity of materials during primary hydriding, indicating hydrogen absorption not only by the main TiFe phase but also by the secondary β-Ti and Ti₂Fe phases. Statistical analysis of data obtained by various authors for three-phase materials based on TiFe, which do not require activation and contain excess amounts of Ti and/or Mn and Zr, showed that their hydrogen capacity during primary hydriding follows the law of additivity and can be described by the equation [24; 25]

H = 0.01537[TiFe] + 0.03213[Ti₂Fe] + 0.03847[Ti₄Fe],

where H is the hydrogen capacity of the material, wt. %; [TiFe], [Ti₂Fe], and [Ti₄Fe] – are the contents of the respective phases, wt. %.

As follows from the given equation, the hydrogen capacity of the TiFe phase in the multiphase TiFe–Ti₂Fe–Ti₄Fe material is about 1.54 wt. % H (which is quite close to the experimental value of 1.70–1.85 wt. % H for the TiFeH hydride given in [10]), the capacity of the BCC solid solution Ti₄Fe is 3.85 wt. % H (slightly exceeding the known capacity of 3.7 wt. % H for the Ti₄FeH₈ hydride [9]), and the capacity of Ti₂Fe is about 3.21 wt. % H (also slightly higher than the theoretical estimate of 3.09 wt. % H for the Ti₂FeH_4.75 hydride given in [9]). It is interesting to note that in the presence of secondary β-Ti and Ti₂Fe phases, the main TiFe phase apparently does not form the TiFeH₂ hydride with the maximum possible capacity of 1.8–1.98 wt. % H [10], which is more than compensated by the contribution of β-Ti and Ti₂Fe.

Another important fact from the statistical analysis [24; 25] and consistent with the conclusions of [9] is that increasing the content of β-Ti in Ti and Fe-based materials reduces their reversible hydrogen capacity, indicating that the saturation of this phase with hydrogen is irreversible. The ability of the intermetallic Ti₂Fe to release stored hydrogen depends on the content of alloying elements: in their absence or with a small amount of Mn, hydrogen saturation is reversible [25], but with Zr alloying, ensuring maximum Ti₂Fe content in the material’s structure, it is irreversible [9; 25]. This leads to the conclusion that the most promising way to improve Ti–Fe system materials, capable of doubling their reversible hydrogen capacity, is to increase the Ti₂Fe content in their structure without using alloying. However, known attempts to solve this problem, based on melting the components followed by annealing [16; 26; 27], have failed [5].

This work explores the possibility of using explosive compaction of iron and titanium powder mixtures for this purpose. The starting point for choosing this method was the well-known fact of frequent intermetallic formation during explosion welding of steels and titanium alloys [28; 29], as well as the fact that during explosive compaction of powders and explosion welding, the area of intense plastic deformation on the surface of the joined components forms similarly [30; 31]. Using powder significantly increases the surface area in the material volume and reduces the effective diffusion paths of elements during interfacial interactions.

Materials and methods

To produce the materials, titanium powder grade PTM-1 and PZhV powder with particle sizes up to 200 µm from a commercial supplier in as-received condition were used. The Ti powder particles had a spongy shape, while the Fe particles were rounded with a pronounced polycrystalline structure (Fig. 1, a). The component content in the powder mixture was 36 wt. % Fe and 64 wt. % Ti, which is nearly stoichiometric for Ti₂Fe (see Fig. 1, b) and ensures equal volumetric content of Fe and Ti in the mixture.

Explosive compaction was performed by placing the initial powder mixture on the surface of a steel substrate and loading it with a flat, normally incident detonation wave through an intermediate layer separating the detonation products from the powder (Fig. 2). The calculation of the physical compression parameters implemented in the experiments was performed using the (P, U)-diagram method [33]. The calculation results are presented in the Table.

The phase composition, structure, and chemical composition of the phases of the obtained samples were investigated by X-ray phase analysis using a D8 Advance X-ray diffractometer (Bruker Optik GmbH, Germany) and scanning electron microscopy using a Versa 3D microscope (FEI, Czech Republic) with an integrated Apollo-X energy-dispersive X-ray microanalysis system (EDAX, USA).

Experimental results

The study of the materials obtained through explosive compaction showed that using this method, which ensures a shock-wave compression pressure of P = 11.5 GPa and a heating temperature up to 777 °C, the powder mixture is compacted into an almost monolithic material (Fig. 3, a). In this process, the deformation of the particles occurs due to uniform plastic flow, resulting in noticeable flattening in the direction of shock compression and spreading in the transverse direction. No traces of changes in the initial phase composition were detected (Fig. 3, c). Even in the immediate vicinity of the interfacial boundaries, the chemical composition of the phases remained practically unchanged (Fig. 3, b).

Under more severe shock-wave compression conditions (Р = 12.5 GPa and t = 831 °С) the deformation character of the component particles of the powder mixture changes fundamentally. Plastic deformation localizes in their surface layers and exhibits a pronounced jet character with the formation of specific “vortices,” as described in detail in [33; 35].

At the interfacial surfaces, solid layers up to 20 µm thick of an intermetallic compound (Fig. 4, b) are formed, which, based on its chemical composition (Fig. 4, c) and crystalline structure (Fig. 4, d), can be identified as the metastable phase Ti₂Fe.

Detailed examination of the microstructure in the interaction zone of the original powder mixture components indicates its chemical heterogeneity, expressed as periodic (with a period of 1.5–2.0 µm) composition oscillations of the intermetallic compound (Fig. 4, d) The deviation from the average stoichiometric ratio of components reaches up to 7 %.

Results and discussion

The data obtained from the conducted studies indicate that the mechanism of plastic deformation of titanium and iron particles during explosive compaction significantly influences the process of forming the metastable phase Ti₂Fe. Several hypotheses can be suggested about the nature of this influence. The most plausible hypothesis is that the intermetallic compound forms when a liquid phase is generated in the material during shock-wave treatment. This assertion is primarily supported by the experience of explosive welding of titanium alloys and steels [28; 29; 36].

Given that the contact melting temperature in the Ti–Fe system is low, at 1085 °C according to the phase diagram (see Fig. 1), achieving this temperature at particle boundaries during explosive compaction under conditions ensuring an average calculated heating temperature of 831 °C, combined with jet flows of metal and extreme temperature field heterogeneity, is quite likely. The heterogeneity of the formed intermetallic layer in this case could be a result of the simultaneous growth of its grains from a considerable number of crystallization centers within the liquid phase under rapid cooling due to heat dissipation into the “cold” areas of the structure and the metal substrate.

To test the proposed hypothesis, compacts obtained under loading conditions ensuring uniform plastic flow of the material particles without intermetallic phases were heated to 1100 °C in a vacuum with a holding time of 1 h, followed by cooling in an argon flow. As a result, the initial components of the compacts fully reacted with each other, forming a structure consisting of the intermetallic compounds TiFe and Ti₂Fe (Fig. 5, a, b).

The associated impurity elements – carbon, oxygen, and nitrogen – were bound in titanium oxycarbonitrides (Fig. 5, c). Through coagulation and coalescence in the liquid phase, these formed relatively large inclusions that were uniformly distributed throughout the material (Fig. 5, a, b). The incorporation of part of the Ti into these inclusions appears to be the main reason why the TiFe intermetallic phase formed in addition to the Ti₂Fe phase in the material’s structure. Addressing this issue requires increasing the Ti content (above stoichiometric) in the initial powder mixture.

Another factor likely contributing to the presence of the TiFe intermetallic phase in the structure of compacts obtained by heat treatment is the significant increase in the duration of the interfacial interaction process when transitioning from purely explosive treatment to a combination of explosive and heat treatments. As a result, the likelihood of forming the stable TiFe phase and dissolving the metastable Ti₂Fe phase increased.

Conclusions

1. Under conditions of explosive compaction of iron and titanium powder mixtures that ensure uniform deformation of the initial particles without jet flows, the powder mixture compacts to an almost non-porous state while maintaining the original phase composition.

2. Using compaction regimes with localized plastic deformation and jet flows of the material particles in the original powder mixture forms a chemically heterogeneous metastable intermetallic Ti₂Fe as continuous layers between iron and titanium particles.

3. An effective method for producing materials with a high content of the metastable intermetallic Ti₂Fe is to combine explosive compaction of Fe and Ti powder mixtures with subsequent heat treatment in the intercritical temperature range (reactive sintering in the presence of a liquid phase).