Hydrogen behavior in ultrafine-grained palladium processed by high-pressure torsion
Introduction
It is important to investigate the hydrogen behavior in metals and alloys for hydrogen production, storage and utilization. Hydrogen may be trapped by lattice defects such as dislocations and vacancies and grain boundaries. However, there is controversy as to hydrogen interactions with grain boundaries as raised by Oudriss et al. [1], [2]. Some studies reported that hydrogen diffusivity is enhanced by short-circuit diffusion along grain boundaries [3], [4], [5], [6], [7], whereas an opposite trend was reported in other studies that the grain boundaries act as trapping sites for hydrogen, thus decreasing the hydrogen diffusivity [8], [9], [10].
Solving this controversy has been restricted because of the difficulty of preparing nanograin-structured metals and alloys having abundant grain boundaries in a bulk form of samples. Conventional rolling and drawing processes may refine the grain size to the order of a few micrometers but the grain size is still insufficient to investigate the effect of grain boundary because the volume fraction of the grain boundary is very few in this range of the grain size [11].
Since it was reported that grain refinement to the nanoscale leads to marked improvement of mechanical properties including various physical properties, production of nanograined materials has gained great interest. Several methods are available for the production of nanograins, such as gas deposition [12], electrical deposition [13] and severe plastic deformation (SPD) process [14]. Although gas deposition and electrical deposition have been used for the grain refinement and the investigation of grain boundary effect on hydrogen behavior [1], [2], [3], [4], [5], [15], [16], [17], SPD process has hardly been used.
The SPD process includes Equal-Channel Angular Pressing (ECAP) [18], Accumulative Roll-Bonding (ARB) [19] and High-Pressure Torsion (HPT) [20]. Theoretically, these processes can introduce strain infinitely because sample shape does not change with processing and the processing can be repeated. Therefore, ultrafine grains with sizes of hundreds nanometer can be obtained even pure metal. Especially, the HPT process can produce smaller grains than the other SPD processes [21].
In HPT processing, a disk sample is placed between upper and lower anvils and the lower anvil is rotated with respect to the upper anvil under a high pressure. Large strain is then introduced and grain refinement to the submicrometer and/or nanometer range is achieved by the HPT processing. Application of high pressure during the HPT operation prevents cracks and voids from formation. In addition, no impurity is introduced into the sample during the HPT processing unlike the grain refinement using the processes of gas deposition and electrical deposition. Therefore, it is anticipated that evaluation becomes more reliable for the interaction of hydrogen with grain boundaries. In this study, HPT process is applied to palladium which has active interaction with hydrogen. Thus, hydrogen behavior in ultrafine-grained palladium is examined using gas permeation test in comparison with annealed palladium which has less dislocations and grain boundaries.
Section snippets
Experimental procedures
High purity palladium (99.9%) in the forms of sheet with 0.5 mm thickness and of rod with 10 mm diameter was used in the present investigation. The Pd sheet was cut to disks with 8.9 mm diameter and 0.5 mm thickness and annealed at 1073 K for 10.8 ks, giving a grain size of ∼80 μm (hereafter referred to Annealed sample). The Pd rod was cut to disks with 2.6 mm thickness and pressed with HPT anvils under a pressure of 1.5 GPa–20 mm diameter disks having the same size as the anvil hole. The
Results
Fig. 3 plots the Vickers microhardness against the distance from the disk center including the hardness level of the annealed disk. The hardness significantly increases when compared to the annealed sample and gradually saturates to a constant level with increasing the distance from the disk center. Grain refinement by the HPT process was achieved as shown in Fig. 4: (a) an OM microstructure with an average grain size of ∼80 μm before the HPT processing and (b) a bright-field image and (c) a
Dislocations and grain boundaries
In present work, an increase of hydrogen flux by HPT processing was confirmed using the hydrogen permeation tests. It is considered that this is due to the effect of grain boundaries on hydrogen behavior in the material.
When the grain size becomes finer by HPT processing, dislocations are also introduced significantly. There would be a possibility that dislocations increase the hydrogen flux. Sakamoto et al. conducted an electrochemical permeation test using an annealed and a cold-rolled Pd
Summary and conclusions
- 1.
Pure palladium was processed by HPT to achieve grain refinement to ∼330 nm and enhancement of Vickers microhardness to ∼200 Hv which is four times higher than the annealed state.
- 2.
Hydrogen permeation tests were carried out under a hydrogen concentration gradient at a controlled temperature in the range of 100–500 °C. Hydrogen diffusion is almost the same between coarse-grained and ultrafine-grained states at temperatures above 200 °C but it is increased at temperatures below 200 °C. This suggests
Acknowledgments
One of the authors (HI) thanks the Japan Society for Promotion of Science (JSPS) for a postdoctoral scholarship for “Development of Hydrogen Storage Alloys Using Giant Straining Process”. This work was supported in part by WPI-I2CNER, in part by the Light Metals Educational Foundation of Japan, in part by a Grant-in-Aid for Scientific Research from the MEXT, Japan, in Innovative Areas “Bulk Nanostructured Metals” and in part by Kyushu University Interdisciplinary Programs in Education and
References (26)
- et al.
Grain size and grain-boundary effects on diffusion and trapping of hydrogen in pure nickel
Acta Mater
(2012) - et al.
The diffusion and trapping of hydrogen along the grain boundaries in polycrystalline nickel
Scr Mater
(2012) - et al.
Hydrogen in nanocrystalline palladium
J Alloys Compd
(1997) - et al.
An investigation of hydrogen diffusion in nanocrystalline Pd by neutron spectroscopy
J Alloys Compd
(1997) - et al.
SIMS study of hydrogen at the surface and grain boundaries of nickel bicrystals
Acta Metall
(1987) - et al.
Accelerated diffusion of hydrogen along grain boundaries in nickel
Acta Mater
(1996) - et al.
Atomistic study of hydrogen distribution and diffusion around a {112}<111> edge dislocation in alpha iron
Acta Mater
(2008) - et al.
Experimental studies of grain boundary diffusion of hydrogen in metals
Acta Metall Mater
(1991) - et al.
Relation between grain size and hydrogen diffusion coefficient in an industrial Pd–23% Ag alloy
Solid State Ion
(1999) Nanocrystalline materials
Prog Mater Sci
(1989)
On the contribution of triple junctions to the structure and properties of nanocrystalline materials
Scr Metall Mater
Hydrogen in amorphous and nanocrystalline metals
Mater Sci Eng
Intercrystalline hydrogen transport in nanocrystalline nickel
Scr Metall Mater
Cited by (9)
Hydrogen diffusivity and interaction with Fe<inf>20</inf>Mn<inf>20</inf>Ni<inf>20</inf>Co<inf>20</inf>Cr<inf>20</inf> and Fe<inf>22</inf>Mn<inf>40</inf>Ni<inf>30</inf>Co<inf>6</inf>Cr<inf>2</inf> high-entropy alloys
2020, Journal of Alloys and CompoundsCitation Excerpt :These results are in accordance with. Iwaoka et al. [18], suggesting that the decreasing in grain size can promote a short-circuit diffusion. Similar diffusion results showing the dependence of hydrogen diffusivity as a function of nickel grain sizes were reported by Oudriss et al. [19].
Mechanical property and hydrogen permeability of ultrafine-grained Pd–Ag alloy processed by high-pressure torsion
2017, International Journal of Hydrogen EnergyCitation Excerpt :Some studies show that grain boundaries in FCC metals act as fast diffusion path unlike dislocations [3–11]. Our previous study shows that both hardness and hydrogen permeation rate were increased in ultrafine-grained pure palladium (Pd) processed by high-pressure torsion (HPT) [12]. HPT processing can produce ultrafine-grained metallic materials by introducing intense shear-strain (For details, see Ref. [13]).
High-pressure torsion of palladium: Hydrogen-induced softening and plasticity in ultrafine grains and hydrogen-induced hardening and embrittlement in coarse grains
2014, Materials Science and Engineering: ACitation Excerpt :The average size of grains with misorientation angles higher than 15° is ~218 nm and the average size of subgrains with misorientation angles higher than 2° is ~175 nm. The average grain size of ~218 nm is well consistent with the earlier reports on Pd (350 nm [33], 210 nm [37], 220 nm [40] and 240 nm [47]) measured using transmission electron microscopy (TEM). It should be noted that the evolution of microstructures and lattice defects were investigated in most of these reports without exposing the samples to hydrogen or after the interstitial hydrogen was removed.
Severe Plastic Deformation through High-Pressure Torsion for Preparation of Hydrogen Storage Materials -A Review
2023, Materials Transactions