Severe Plastic Deformation through High-Pressure Torsion for Preparation of Hydrogen Storage Materials -A Review

Abstract

Severe plastic deformation (SPD) processes are excellent processing methods that can refine grains, introduce crystal defects and improve mechanical properties and functionality. Among them, high-pressure torsion (HPT) was used for the preparation of hydrogen storage materials in recent years owing to the fact that HPT can process powder materials and introduce greater plastic deformation. HPT technology is not only an effective method to prepare bulk samples from powders, but also a brilliant route to enhance the kinetics, activation, air resistivity and long-term stability of hydrogen storage materials. In addition, new alloys, intermetallics, composites, and hydrogen storage materials even in the immiscible Mg-based alloy systems can be successfully synthesized through the HPT process. In this review, different SPD methods and HPT technology applied for different hydrogen storage materials are carefully reviewed.

1. Introduction

With the depletion of traditional fossil energy and increasingly serious environmental problems around the world, many countries have gradually shifted their attention to clean and pollution-free hydrogen energy and formulated corresponding development plans to promote the development of the hydrogen energy industry. Hydrogen is the lightest gas, flammable and explosive with a low density (0.0899 kg/m³) under standard conditions.¹⁾ How to store hydrogen safely and efficiently is a worldwide problem in the development of the hydrogen energy industry. Hydrogen is generally stored in three ways: high-pressure gas, low-temperature liquid and solid-state hydrogen storage materials.^2,3) Solid hydrogen storage does not have a high risk of safety and hydrogen embrittlement like high-pressure hydrogen storage, or high energy consumption and hydrogen volatilization issues like in liquid hydrogen storage.²⁾ There are many kinds of solid-state hydrogen storage materials such as zeolites, carbon materials, metal-organic frameworks (MOFs), metal hydrides, complex hydrides and so on.^4–7) Among them, metal hydrides are one kind of the most promising materials for commercial application and many solid-state hydrogen storage system prototypes have been developed based on different metallic alloys such as TiFe,^8,9) AB₂ alloys,^10,11) AB₅ alloys,¹²⁾ TiCrVMo,¹³⁾ VTiCrFe^14,15) and Mg-based alloys,^16–18) etc. Nevertheless, metallic alloys are still suffering from several tough issues such as difficult activation like in TiFe,¹⁹⁾ low reaction kinetics, poor resistance to air contaminants and high reaction temperature like in Mg-based alloys^20,21) and low hydrogen content like in AB₅ alloys.²²⁾

Severe plastic deformation (SPD) processes are recently developed processes, which are used to fabricate ultrafine or nanocrystalline materials. SPD processes are defined as material forming processes through ultra-large plastic strain (usually larger than 6) without obvious shape change. SPD processes include high-pressure torsion (HPT), equal-channel angular pressing (ECAP), accumulative roll-bonding (ARB), etc. SPD processes are mainly used to prepare high-strength metallic structural materials by the principle of the Hall–Petch relationship,²³⁾ which is expressed as eq. (1):   

\begin{equation} \sigma = \sigma_{0} + kd^{-0.5} \end{equation} (1)

where σ is the yield stress, k is a measure of the local stress needed to initiate plastic flow at a grain boundary, σ₀ is the resistance to dislocation motion in the grain interior and d is the grain size.²⁴⁾ The refined grains thus contribute to ultra-high strength for metallic materials.

In recent years, the SPD processes are also employed to prepare metallic functional materials, such as hydrogen storage materials, in order to improve their hydrogen storage performances.²⁵⁾ HPT is one of the most frequently adopted SPD methods to optimize both the kinetic and thermodynamic properties of hydrogen storage alloys. In this review paper, the influence of several main SPD methods, particularly HPT processing, on different hydrogen storage alloy systems are introduced in detail, so that researchers can better understand the importance of SPD and HPT in the preparation of hydrogen storage materials.

2. SPD Methods and Mechanisms

2.1 High-pressure torsion (HPT)

Bridgman developed a concept in 1935,²⁶⁾ when materials are subjected to static axial compressive stress, much larger torsion deformation could be produced than that without axial compressive stress. At the same time, materials could suffer higher torque.²⁷⁾ He also proposed that although there was no suitable method to introduce a torsion test for large deformation, the same effect (large torsion deformation) would be produced by introducing torsion and compression tests at the same time.²⁷⁾ These reports have laid a theoretical foundation for the HPT process. Figure 1 shows the schematic diagram of the HPT process.²⁸⁾ A circular sample is placed between two rigid molds (anvils), and the mold exerts large compressive stress on the sample. Under the action of static pressure, the shear stress is applied to the sample through the surface friction generated by the rotation of the lower die, which causes the sample to twist. In the process of HPT deformation, many materials can show much greater plasticity than single torsion deformation.

Fig. 1

Schematic illustration of HPT processing.²⁸⁾

The HPT process was later used to strengthen metallic structural materials by refining grains such as aluminum alloys,^29–33) copper alloys,^34–38) titanium alloys,^39–43) etc. In recent years, this technology has expanded to the preparation of functional materials, including hydrogen storage materials, thermoelectric materials, shape memory alloys, etc. Different from other SPD processes, HPT can process alloy powders into bulk materials. HPT refines the grains and introduces a lot of defects during the deformation process. In some alloys, the super-large cold deformation leads to the transformation of crystals into amorphous, such as the NiTi alloys.^44–53) As a result, the functionality of materials can be changed. On many occasions, the functionality of materials is greatly improved.

The grain size of alloys prepared by the HPT method can reach the submicrometer or nanometer levels, but there are still some disadvantages of the HPT method. The sample obtained by the HPT method is on a small scale, which limits its application. The amount of plastic deformation gradually increases from the center to the edge of the sample prepared by HPT, which makes the size of nanocrystalline grains different, that is, the size of the nanocrystalline grains from the center to the edge is uneven (Fig. 2).

Fig. 2

Schematics of disc sample in HPT processing. The strain on HPT samples is closely related to the distance from the center of the sample. The strain is calculated as γ = r · φ/h, where r is the distance from the center of the sample, φ is the torsion angle and h is the thickness of the sample. The strains at spots A, B and C are 0, r · φ/h and R · φ/h, respectively.

2.2 Equal-channel angular pressing (ECAP)

The ECAP method was first developed by Segal to impose large simple shear in metallic materials.⁵⁴⁾ The schematic of the ECAP process is shown in Fig. 3. Under the extrusion force, the sample is deformed in a mold having two channels with the same section size and shape and intersecting at a certain angle. The ECAP process is an effective method to obtain ultrafine-grained or nanocrystalline materials, which are larger than that obtained by HPT. It is a technology that can homogenize and refine ingots. It is considered one of the most promising technologies for further application to refine the grain size of conventional materials to the submicron or even nanometer scales. The alloys prepared by the ECAP process experience nearly pure shear deformation. To increase the amount of cold deformation, the samples can pass through the channel several times. Compared with HPT, ECAP can process large-size samples; however, a high-temperature working environment is required in some cases, leading to difficulties to prepare nanocrystalline alloys. On the basis of traditional ECAP technology, several new ECAP processes have been developed in recent years, such as rotary-die ECAP,⁵⁵⁾ multi-pass ECAP,⁵⁶⁾ ECAP–conform,⁵⁷⁾ ECAP with rotating tooling⁵⁸⁾ and parallel tubular channel angular pressing (PTCAP).⁵⁹⁾ These ECAP processes widen the application areas of the ECAP process.⁶⁰⁾ The schematic of the ECAP process is shown in Fig. 3.⁶¹⁾

Fig. 3

A schematic of the ECAP process.⁶¹⁾

2.3 Accumulative roll-bonding (ARB)

ARB is an SPD process that can significantly refine the grain size of metallic materials.^62–68) The schematic of the ARB process is shown in Fig. 4. First, a sheet is rolled to reduce its thickness by 50%. The rolled sheet is cut into two sheets and the surfaces of two metallic sheets with the same shape are degreased and rolled to make them welded. Then, the same process of laminating, rolling and welding is repeated to refine the structure of the metallic material. The ARB process can greatly improve the mechanical properties of metallic materials.

Fig. 4

A schematic of the ARB process.

ARB process not only has low equipment investment costs, simple process and high yield, but also can refine the structure and improve the strength of materials without limiting the size of strain during sheet rolling. This process is considered to be the most promising continuous method for industrial production of large size and high-performance fine-grained alloy plates. This technology has attractive application prospects, but there are still some disadvantages when preparing alloy plates by this method, such as cracking of thick plates during processing, easy oxidation during heating, and difficulty in preparing plates from brittle materials. At present, the ARB process is mainly used to make high-performance thin sheets.

2.4 Others

Besides the above processes, SPD also includes bundle drawing process die,^69–73) cyclic extrusion compression (CEC),⁷⁴⁾ repetitive corrugation and straightening (RCS),⁷⁵⁾ simple shear extrusion (SSE),⁷⁶⁾ etc. The bundle-drawing process is to wrap multiple metal wires in the outsourcing material, and then carry out continuous drawing through the multi-stage drawing die. Heat treatment and other processes can be set as intermediate processes. Any parameter variations in the process of drawing and heat treatment change the properties of the fiber, thus affecting the fiber quality. The CEC process can be used for grain refinement of magnesium, aluminum and some other alloys. A similar process is cyclic expansion-extrusion (CEE),⁷⁷⁾ the CEE process use extrusion instead of compression in CEC process. CEE is considered as a substitute for CEC, its advantage is processing samples for the desired number of passes with no need to remove them from the die until the whole number of passes are accomplished.⁷⁷⁾ In the RCS process, without changing the shape of the section of the samples, the samples undergo repeated wrinkling and straightening to obtain large plastic deformation.⁷⁵⁾ In the SSE process, the extruded sample passes through a specially shaped straight passage, which causes severe deformation and can keep the section of the sample unchanged.⁷⁶⁾

In general, SPD are excellent processing methods that can refine grains and enhance mechanical properties and functionality. The basic principle of grain refinement by the SPD process is to accumulate a large number of defects such as dislocations or twins. The combination of the SPD process and low-temperature annealing can make the metallic materials recrystallized, and then form nanocrystals or ultrafine grains in the metallic materials. In addition, due to the large strain introduced by the SPD process, the defect density in the metallic materials is close to the saturation limit. Moreover, the element distribution in the metallic alloys are also different from the conventional alloys, especially in terms of segregation and the generation and disappearance of the second phase.^78–81) The main reason for this phenomenon is that a large number of defects and the presence of non-equilibrium grain boundaries provide abundant nucleation positions for the segregation and second phase.

However, not all SPD processes are suitable for hydrogen storage materials. The HPT process has been extensively used to prepare hydrogen storage materials recently^25,82) due to the ability to process powders and introduce greater plastic deformation. Here, this paper focuses on the application of the HPT process in the preparation of hydrogen storage materials.

3. HPT to Improve the Performance of Hydrogen Storage Materials

The HPT process provides an opportunity to produce many ultrafine-grained structures which are essentially beneficial for hydrogen diffusion and absorption. Up to now, a series of alloys have been optimized for hydrogen storage performance by means of HPT.

3.1 TiFe alloys

TiFe alloy is a typical hydrogen storage material which can nominally absorb 1.9 wt.% H; however, it usually suffers from severe oxidation with deactivation.⁸³⁾ In 2013, Edalati et al.⁸⁴⁾ employed HPT to activate TiFe alloy and investigated its activation mechanism. Their experimental results showed that nanostructured TiFe became active by HPT and was not deactivated for a long time (hundreds of days) even when exposed to the air. The experiments indicated the formation of grain boundaries, cracks and surface segregation and the generation of Fe-rich islands after HPT processing. The Fe-rich islands acted as catalysts and cracks and nanograin boundaries acted as pathways to transport hydrogen through the oxide layer (Fig. 5). Their results confirmed that HPT is effective not only to improve the hydrogenation rate but also to activate the TiFe alloy at ambient conditions. Another study⁸⁵⁾ also reported TiFe alloy processed by HPT could reversibly store 1.7 wt.% hydrogen without activation at room temperature. The hydrogen uptake pressure decreased from 2 MPa to 0.7 MPa after four cycles. Hydrogen storage performances of TiFe with different treatment routes (annealing, groove rolling and HPT) were also compared by Edalati et al.⁸⁶⁾ Experimental results indicated samples activated by groove rolling and HPT processing both kept activated in the air for a long time while the annealing sample did not absorb hydrogen.

Fig. 5

(a) Pressure-Composition-Temperature (PCT) curves of the first hydrogenation cycles of annealed TiFe and TiFe processed by compression under 6 GPa (N = 0) and HPT for N = 1/4–10 turns at 25°C. (b) Schematic illustration of activation mechanism of TiFe hydrogen storage alloys by HPT.⁸⁴⁾

Edalati et al.⁸⁷⁾ severely deformed TiFe_1−xMn_x intermetallics (x = 0, 0.15 and 0.3) by HPT to improve their activation and air resistivity for hydrogenation. The HPT-processed samples absorbed hydrogen fast at room temperature while the as-cast ingots did not. The formation of lattice defects and amorphous regions, acting as channels for hydrogen diffusion, played an important role in the improvement of hydrogen storage performance.

HPT was also employed as a synthesis approach to prepare nanostructured TiFe alloy in which Ti and Fe micro-powders were used as starting materials.¹⁹⁾ The as synthesized TiFe could absorb hydrogen at room temperature with a reasonable kinetics after activation at 400°C for 2 h. Experimental results indicated that a combination of ingot casting and HPT is more effective than a single HPT to overcome the activation problem of the synthesized TiFe.

3.2 Mg-based alloys

Mg-based alloys have a large hydrogen storage capacity (up to 7.6 wt.% in MgH₂) and good absorption/desorption reversibility, and they are becoming more and more important as solid hydrogen storage materials.⁸⁸⁾ Nevertheless, most Mg-based alloys suffer from sluggish kinetics and high dehydrogenation temperatures due to the high activation energy E_a = 143.0–160.6 kJ/mol⁸⁹⁾ and decomposition enthalpy of MgH₂ (75 kJ mol⁻¹ H₂).⁹⁰⁾ The HPT process is a good method to introduce SPD in different Mg-based hydrogen storage materials. The introduction of SPD can improve hydrogen storage performance such as increasing the hydrogenation kinetics, facilitating the activation, decreasing the hydrogenation pressure in the first cycle, improving the air resistance and enhancing recycling stability.⁹¹⁾ Recent studies showed that HPT processing is an effective method for the optimization of hydrogen storage performance of various Mg-based alloys. The main achievements are carefully described below for Mg (or MgH₂), Mg–Ni alloys and other Mg-based alloys.

3.2.1 Mg (or MgH₂)

Leiva et al.²⁰⁾ used 5 GPa of HPT to investigate the microstructural development process and H-sorption properties of Mg, MgH₂ and MgH₂–Fe mixtures. Some interesting phenomena were observed, such as significant grain refinement of MgH₂ to 10∼20 nm size, the strong growth of (001) texture in β-MgH₂ hydride and the formation of metastable γ-MgH₂ phase (Fig. 6). HPT treatment significantly improved the hydrogen desorption and absorption behaviors of MgH₂ + 5%Fe, and such properties could be further improved with the simultaneous use of both HPT and high-energy ball-milling (HEBM) processes.

Fig. 6

(a) XRD patterns of MgH₂ disks processed by HPT under 5 GPa with 1–10 turns and (b) photograph of the disk from commercial MgH₂ powder after 5 turns of HPT.²⁰⁾

In 2011, Edalati et al.⁹²⁾ investigated the hydrogen storage capacity of pure Mg using HPT treatment. They found that the properties of hardness, tensile strength, total elongation to failure and hydrogen absorption were all improved. Besides those, grain refinement was proved to play a greater role in hydrogen storage capacity improvement than dislocation density. Hydrogen absorption was not improved by only 0.25 revolutions of HPT, but it was significantly improved by 10 revolutions, and a total hydrogen absorption of 6.9 wt.% was obtained (Fig. 7).

Fig. 7

Amount of hydrogen absorption vs. exposure time under a H₂ pressure of 3 MPa for annealed and HPT-processed Mg.⁹²⁾

Grosdidier et al.⁹³⁾ used HPT to consolidate ultrafine Mg powder particles prepared by arc plasma evaporation. They found HPT processing could further reduce the grain size and the fragmentation of the oxide layer could prevent grain growth in the Mg–MgO composite. Consolidation by HPT could improve hydrogen storage kinetics and thermodynamics which was mainly attributed to the structure refinement of Mg and dispersion of fine MgO. In addition, they found the consolidation of a passivated Mg + 2 wt% Ni powder led to a Mg–MgO–Ni composite with a further improvement of hydrogen kinetics and absorption at 100°C.

Panda et al.⁹⁴⁾ studied the effect of two different Mg powder precursors (atomized micro-sized and condensed ultrafine powder particles) on the hydrogen storage performance of HPT-processed Mg. The results indicated that the HPT product obtained from the condensed ultrafine powder showed faster absorption kinetics than that from atomized powder. However, the HPT product obtained from atomized powder absorbed more hydrogen and presented faster desorption kinetics. It indicated that the nature of the starting powder precursor had a great effect on hydrogen storage behavior.

Révész et al.⁹⁵⁾ investigated the correlation between the microstructural parameters and hydrogen storage using nanocrystalline Mg processed by HPT. With the increasing of HPT-turns to N = 10, the dislocation defect density reached the largest value of ρ = 8 × 10¹⁵ m⁻² and the hydrogen storage capacity reached the maximum of 3.5 wt.%. In their experiments, the as-cast Mg disks were HPT processed in air which inevitably results in partial oxidation of Mg. Besides that, no other activation process were conducted. These might be the main reasons leading to the maximum hydrogen absorption capacity of only 3.5 wt.%, a value much lower than that of pure Mg (7.6 wt.%).

More recently, Edalati et al.⁹⁶⁾ used HPT to process MgH₂ with α tetragonal structure and found that α-MgH₂ fully transformed to nanocrystalline γ-MgH₂ with orthorhombic structure by increasing strain. Importantly, the dehydrogenation temperature of nanocrystalline high-pressure γ-MgH₂ became 80°C lower than that of low-pressure α-MgH₂. The pressure-temperature phase diagram of MgH₂ is shown in Fig. 8.

Fig. 8

Pressure-temperature phase diagram of MgH₂.⁹⁶⁾

3.2.2 Mg–Ni alloys

Révész et al.⁹⁷⁾ studied the effect of HPT on the microstructure and hydrogen absorption kinetics of ball-milled Mg₇₀Ni₃₀. Because of the new possible hydrogen absorption sites at the grain boundaries and lattice defects produced by heavy shear deformation of HPT, the maximum hydrogen absorption capacity was increased by 30–50% by comparison with the only ball-milled samples. Nevertheless, HPT slightly decreased the initial hydrogen absorption rate.

The HPT technology was also applied for the synthesis of hydrogen storage alloys with new microstructures. Hongo et al.⁹⁸⁾ investigated the hydrogen storage of Mg₂Ni intermetallics treated by three different routes: annealing, HPT and HPT+annealing. It was found both HPT and HPT+annealing samples were activated by formation of nanograins and stacking faults, and exhibited 3.3 wt.% hydrogen absorption at 150°C. Both HPT and HPT+annealing treatments enhanced the kinetics of hydrogen absorption. The experimental results indicated the same significance of grain boundaries, stacking faults and fine cracks as pathways to transport H from the surface to the interior of Mg₂Ni (Fig. 9).

Fig. 9

(a) Schematic illustration of the role of stacking faults for transportation of hydrogen from surface to bulk and (b) mechanism for easy activation and fast hydrogenation of Mg₂Ni by HPT and post-HPT annealing.⁹⁸⁾

Edalati et al.⁹⁹⁾ designed a Mg-based alloy Mg₄NiPd with a BCC-based CsCl-type structure and a low hydrogen binding energy using first-principles calculations. The designated material was successfully synthesized by HPT with 1500 turns and exhibited 0.7 wt.% reversible hydrogen storage properties at room temperature with good phase stability up to 167°C (Fig. 10). This discovery described a feasible route to design and synthesize novel alloys for hydrogen storage via the binding energy engineering.

Fig. 10

Mg₄NiPd with BCC structure designed by binding-energy engineering and synthesized by 1500 turns of HPT exhibits room-temperature reversible hydrogen storage and keeps stable after 5 absorption/desorption cycles.⁹⁹⁾ (a) Hydrogen PCT results at 32°C under 10 MPa of H₂, (b) XRD pattern after PCT measurements.

Révész et al.¹⁰⁰⁾ employed HPT to study the hydrogen storage performance of melt-spun amorphous Mg₆₅Ni₂₀Cu₅Y₁₀ alloy in 2012. A deformation dependent microstructure originated from HPT was verified by XRD and SEM. High-pressure DSC indicated that hydrogen uptake temperature (37∼147°C) in the fully amorphous alloy (N = 5 of HPT) was significantly lower than that of the crystallized state (207∼277°C). Due to the formation of Mg₂Ni crystals, the hydrogen absorption amount increased largely after heavy shear deformation of HPT from 0.25 to 0.65 wt.%.

3.2.3 Other Mg-based alloys

Mg is immiscible with Zr, V and Cr, it is hard to produce Mg–Zr or Mg–V–Cr alloys via melting method. Nevertheless, the abovementioned Mg-based alloys can be prepared by HPT processing and present a certain hydrogen storage capacity.

Grill et al.¹⁰¹⁾ studied the hydrogen storage performance of Mg alloy ZK60 (Mg–5Zn–0.8Zr) using HPT. After the HPT treatment, both kinetics and storage capacity were stable for more than 200 cycles which was much better than that of pure Mg. Edalati et al.¹⁰²⁾ dissolved immiscible Mg and Zr in each other via HPT method and observed the formation of several new metastable phases: nanostructured HCP, nano-twinned FCC, BCC or ordered BCC-based phases. These supersaturated Mg–Zr phases reversibly stored hydrogen at room temperature and were stable up to 500°C. About 1 wt.% of hydrogen could be absorbed at 9 MPa in one-third of 1 minute and all absorbed hydrogen could be desorbed in the air.

Fujiwara et al.¹⁰³⁾ successfully synthesized the first Mg–V–Cr BCC alloys from MgH₂, V and Cr powders by HPT under 3 GPa. The as-synthesized MgV₂Cr and MgVCr alloys could absorb ∼0.9 wt.% hydrogen at room temperature. MgVCr alloy exhibited the best homogeneity and stability during thermal treatment or hydrogenation process.

Floriano et al.¹⁰⁴⁾ synthesized MgVCr via HEBM followed by HPT treatment and investigated its hydrogen storage performance. MgVCr kept a very refined nanostructure with the presence of a BCC solid-solution phase. It exhibited fast kinetics, and good phase stability but a low reversible hydrogenation capacity (0.9 wt.% at 350°C).

Mg–5 wt%Ni–2 wt%Nb₂O₅ alloy with ultrafine grains was prepared by Osorio-García et al.¹⁰⁵⁾ using HPT processing. It exhibited faster kinetics and higher storage capacities (5.5 wt.%) in comparison with the initial milled powder mixture. More importantly, it kept fast activation even when stored in air for several months.

3.3 High-entropy alloys

High-entropy alloys are potential hydrogen storage materials containing at least five principal elements which have received wide investigation in recent years.¹⁰⁶⁾ For instance, TiZrCrMnFeNi was reported to reversibly absorb 1.7 wt.% hydrogen at room temperature with fast kinetics and without activation treatment.¹⁰⁷⁾ Usually, high-entropy alloys are prepared by arc melting,^108,109) HEBM¹¹⁰⁾ or laser-engineered net shaping.^111,112) More and more works using the HPT technology to process high-entropy alloys appeared in recent years as described below.

de Marco et al.¹⁰⁴⁾ synthesized MgVTiCrFe via HEBM followed by HPT processing and studied its hydrogen storage behavior. MgVTiCrFe kept bimodal crystalline and amorphous phases and exhibited a low hydrogen absorption (0.3 wt.% at 350°C in Fig. 11) and partial decomposition during hydrogenation cycling. This was the first successful attempt to synthesize high entropy alloys with reversible hydrogen storage performance.

Fig. 11

PCT isotherms at 350°C for high-entropy alloy MgVTiCrFe, synthesized by ball milling followed by 200 HPT turns for hydrogen storage.¹⁰⁴⁾

3.4 Others

There are some other studies on the interaction of hydrogen with HPT-processed materials which can be of importance for understanding the effect of SPD on hydrogen storage materials. Some of these studies are reviewed here.

Iwaoka et al.¹¹³⁾ investigated hydrogen absorption behavior in ultrafine-grained Pd processed by HPT. They found the hydrogen diffusion was similar between ultrafine-grained and coarse-grained states above 200°C but it was strengthened below 200°C in the ultrafine-grained state, indicating the facilitation of hydrogen diffusion by grain boundaries. More detailed experiments showed that grain boundaries acted as diffusion pass but not as the sites for the hydride formation.

Metal palladium (Pd) is able to absorb hydrogen to form PdH_0.6 solid solution with the FCC crystal structure,¹¹⁴⁾ as a result, it is usually used as a standard sample for hydrogen storage materials. Hongo et al.¹¹⁵⁾ used HPT to produce Pd with ultrafine-grained structure with an average grain size of 220 nm. By means of HPT processing, the hydride formation was facilitated. In HPT-processed samples, hydrogen-induced softening and plasticity were observed and the hydride phase was more stable than that of only annealed samples. Mito et al.¹¹⁶⁾ developed a magnetic measurement system to investigate the hydrogen desorption performance of HPT-processed PdH_0.64. Ti–V alloy was synthesized using HPT by Edalati et al.¹¹⁷⁾ for hydrogen storage. It absorbed around 4 wt.% of hydrogen at room temperature after an incubation period. HPT processing led to grain refinement to the nanometer level and ultrahigh density of edge dislocations (>10¹⁶ m⁻²) which was beneficial for easy activation.

Omranpour et al.¹¹⁸⁾ investigated the effect of a novel HPTE (High-Pressure Torsion Extrusion) technology on the hydrogenation of Nb and compared it with the conventional strategy of ECAP. The results revealed that HPTE was an effective technique for microstructure refinement, hardness increase and hydrogenation property enhancement. The hardness and hydrogenation kinetics of Nb samples processed by HPTE were both greatly improved and much better in comparison with ECAP (Fig. 12). Kitabayashi et al.²¹⁾ applied HPT processing to MgH₂–TiH₂ system for the synthesis of metastable hydrides and investigation of their phase transformations. The results showed that MgH₂ transformed to a high-pressure orthorhombic γ phase and TiH₂ did not transform at all. Application of 400 HPT turns to the immiscible MgH₂/TiH₂ composite resulted in atomic-scale mixing and formation of heterogeneous nanostructured FCC MgTiH₄ hydride with a lower dehydrogenation temperature than TiH₂. Taken altogether, HPT provides a new path for developing advanced hydrogen storage materials.¹¹⁹⁾

Fig. 12

Hardness testing (a) and first hydrogenation kinetics (b) of Nb samples processed by HPTE and ECAP.¹¹⁸⁾

4. Conclusion and Perspective

HPT, as one of the most popular processing methods of SPD, has been proven to be a powerful tool to improve hydrogen storage performance and fabricate new compounds for hydrogen storage.

1. (1)    HPT processing enhances the activation and air resistivity of TiFe alloys and improves the hydrogenation absorption rate in different Mg-based alloys without reducing their storage capacity. Such improvements in kinetics are due to the generation of induced defects (grain boundaries, dislocations and stacking faults) which can act as fast pathways for hydrogen transport.
2. (2)    HPT is not only an effective technology to improve hydrogen storage kinetics but also a novel method to synthesize advanced materials with favorable thermodynamics for hydrogen storage. HPT was used to successfully synthesize various alloys, intermetallics, composites, and new phases even from the immiscible Mg–V, Mg–Ti and Mg–Zr alloy systems by strain-induced mechanical alloying.
3. (3)    Though HPT is effective for the preparation and synthesis of hydrogen storage materials, it has a main drawback: it is still difficult to prepare samples with large sizes or volumes by HPT. In the future, SPD through the HPT process needs to be upscaled for commercial applications.
4. (4)    The potential of HPT in synthesizing various materials at the laboratory scale can scientifically contribute to the discovery of new hydrogen storage materials in the future. However, a combination of computational material design with the materials synthesis by HPT is required for optimized progress in this regard.

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (No. 52101249, 52171205, 52071157), Anhui Provincial Natural Science Foundation (No. 2108085QE191, 2108085ME144) and Youth Science and Technology Fund Project of China Machinery Industry Group Co., Ltd. (QNJJ-ZD-2022-01). K.E. thanks the MEXT, Japan, for Grants-in-Aid for Scientific Research (JP19H05176, JP21H00150, and JP22K18737).

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