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An investigation of hydrogen storage in amagnesium-based alloy processed byequal-channel angular pressing

Alberto Moreira Jorge Jr.a,*, Egor Prokofiev a, Gisele Ferreira de Lima a,Edgar Rauch b, Muriel Veron b, Walter Jose Botta a, Megumi Kawasaki c,d,Terence G. Langdon d,e

aDepartamento de Engenharia de Materiais, Universidade Federal de Sao Carlos, Via Washington Luiz, km 235,

Sao Carlos 13565-905, SP, Brazilb SiMap Laboratory CNRS, CNRS UMR 5266 INPG e UJF, Grenoble, BP75 38402 St-Martin d’Heres, FrancecDivision of Materials Science and Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong,

Seongdong-gu, Seoul 133-791, South KoreadDepartment of Aerospace & Mechanical Engineering and Materials Science and Materials Science,

University of Southern California, Los Angeles, CA 90089-1453, USAeMaterials Research Group, Faculty of Engineering and the Environment, University of Southampton,

Southampton SO17 1BJ, UK

a r t i c l e i n f o

Article history:

Received 21 January 2013

Received in revised form

28 March 2013

Accepted 29 March 2013

Available online 24 May 2013

Keywords:

Equal-channel angular pressing

Hydrogen storage

Magnesium alloys

Severe plastic deformation

Ultrafine grains

* Corresponding author. Tel.: þ55 16 3351947E-mail addresses: moreira@ufscar.br, mo

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.03.1

a b s t r a c t

Equal-Channel Angular Pressing (ECAP) can be successfully used to process Mg and Mg-

based hydrides to produce bulk samples with enhanced hydrogen sorption properties.

The primary advantages associated with ECAP processing are the shorter processing time,

lower cost and the production of safer and more air-resistant bulk material by comparison

with powders produced by high-energy ball milling. ECAP can produce special features for

hydrogen absorption such as preferential textures, an increased density of defects and

submicrometer grain sizes. In this research, ECAP was used to process a commercial AZ31

extruded alloy in order to evaluate its use as a hydrogen storage material. The ECAP was

conducted under conditions of temperature and number of passes in order to avoid grain

growth. Additional experiments were conducted on commercial coarse-grained magne-

sium to evaluate the effect of sample thickness on the sorption properties. The ECAP

sample was evaluated in two different orientations and it is shown that better hydrogen

properties are related to a refined microstructure allied to the (0001) texture.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction attractive for hydrogen storage in the solid state because these

Hydrogen storage is currently attracting considerable atten-

tion with special emphasis on the fabrication of appropriate

storage materials [1,2]. Magnesium alloys are especially

8; fax: þ55 16 33615404.reira@dema.ufscar.br (A.M2013, Hydrogen Energy P58

alloys are light-weight and they can absorb up tow7.6 wt.% of

hydrogen in the form of reversiblemagnesium hydride (MgH2)

[1,3]. In addition, together with the abundance and low cost of

magnesium, these alloys represent an outstanding potential

. Jorge Jr.).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 3 0 6e8 3 1 2 8307

for commercial applications and they are more effective and

safe as hydrogen storage media in the solid state than in the

pressurized or liquefied conditions. Nevertheless, the kinetics

of the reaction is very slow even at elevated temperatures and

this effectively places a limit on their practical utility. The ki-

netics is reduced primarily because of two factors. First, the

diffusion rate of hydrogen is very low within the magnesium

hydride [4,5]. Second, there are oxide layers on the surfaces

whichpreventordelay thepenetrationofhydrogen [6,7]. There

is also evidence that the presence of porosity may create easy

paths for hydrogen penetration in bulk samples [8,9].

Many studies have been conducted in attempts to solve

theseproblemsand in recent years therehasbeena remarkable

improvement in thedesorptionkineticsassociatedwith theuse

of oxides and catalysts especially in nanocrystalline magne-

sium [10e13]. High-energy ballmilling (HEBM) techniques have

been applied successfully for the preparation of Mg-based

nanocomposites which provide fast H-sorption kinetics at

300 �C or even at lower temperatures [1,14e18]. However, there

are several disadvantages in processing by powder metallurgy

including the occurrence of surface contamination, the

expended time, the potential fire risk and health concerns.

These various shortcomings may be overcome by using se-

vere plastic deformation (SPD) processing techniques which

provide the capability of converting conventional coarse-

grained metals into ultrafine-grained or nanocrystalline mate-

rials under a high hydrostatic pressure and at relatively low

deformation temperatures [19e21]. ProcessingbySPDproduces

multiple defects in the crystalline lattice such as vacancies and

dislocations and this has a positive effect on the diffusion ki-

netics. Nevertheless, the presence of porosity, which is an

important factor in improving the diffusion kinetics, is essen-

tially non-existent after processing by SPD techniques. In

practice, investigations suggest there is improveddiffusionand

H2 storage capacity in Mg alloys after processing by SPD due to

thepresenceof excess vacancieswhichdramatically accelerate

the diffusion process and allow the entrapment of up to six

hydrogen atoms per vacancy [22e26].

Processing by an SPD technique such as equal-channel

angular pressing (ECAP) [20] has been widely used for intro-

ducing ultrafine grain sizes into light metal alloys, especially

magnesium-based alloys where it is possible to attain

remarkable superplastic ductilities [27e34]. In practice, as

already observed for ZK60 alloy processed by ECAP, defect

structure [35] and refinement of the microstructure [35,36] are

also important in improving theH-kinetics sorption properties

and structural stability during cycles of absorption/desorption

andECAPmayproduce textureswhich improve theH-sorption

properties [37,38]. There is also evidence that hydrogen ab-

sorption is improved in magnesium processed by the alterna-

tive SPD technique of high-pressure torsion (HPT) [39].

In an investigation of the structural and hydrogen storage

properties in nanostructured thin films of Mg deposited on Si

(001) substrates, X-ray diffraction showed that the conversion

of Mg to MgH2 follows a martensitic-like orientation rela-

tionship with Mg (002) // MgH2 (110) and Mg ½120� // MgH2 [111]

[40]. Experiments combining ECAP, cold rolling (CR) and HEBM

to process commercial extruded AZ31 alloy showed the

deformability of the alloy during ECAP processingwas suitable

for temperatures above 150 �C. However, a [002] texture

favorable for the absorption of hydrogen was obtained only

with a combination of ECAP and subsequent cold rolling [41].

These results suggest that the presence of excess vacancies

and the reduced grain size produced by ECAP are not sufficient

to produce an improvement in the absorption properties and

instead other aspects, such as the density of nucleation sites

for the hydride formation, must also be considered. An early

study of the ECAP processing of Mg showed that the basal

planes become aligned with the theoretical shearing plane

and it follows that different directions within the processed

samples may have different properties relating to the texture

[42]. Based on these results, the objective of this investigation

was to use ECAP processing on a commercialmagnesium alloy

to determine the processing route that produces the best

texture for hydrogen absorption and to analyze the differ-

ences in the hydrogenation behavior for samples cut in

different orientations.

2. Experimental materials and procedures

The experiments were conducted using a commercial AZ31

magnesiumalloy, supplied byTimmincoCo. (Aurora,CO) in the

form of extruded rod having diameter of 10 mm. The chemical

composition of this alloy (inwt.%) was 2.50 Al, 0.37Mn and 0.92

Zn with the balance as Mg. Inspection showed the initial grain

size in the as-received extruded condition wasw14.5 mm.

Billets were cut from the rod with lengths of w60 mm for

processing by ECAP and these billets were processed using a

hydraulic press of 150-tons capacity operating at a pressing

speed of w7 mm s�1. All processing by ECAP was performed

using a solid die having an internal channel angle of F ¼ 110�

and an angle at the outer arc of curvature of the two parts of

the channel of J ¼ 20�. These angles produce a strain of w0.8

on each separate passage through the die [43] and repetitive

pressings were performed using route A in which the billet is

pressed through the die without any rotation between each

pass [44].

In this investigation, the billets were pressed under condi-

tions in terms of the numbers of passes and processing tem-

peratures such that the samples were able to achieve high

numbers of ECAP passes with little or no grain growth. Specif-

ically, AZ31 alloy was pressed for two passes at 473 K and a

subsequent two passes at 443 K to give a total of 4 passes and a

strain of w3.2. Each processing temperature was maintained

stable so that itwaswithin an error range of�2�. For changes inthe processing temperature after every two passes, the pro-

cessing was conducted after the ECAP die achieved a constant

temperature within �2�. The samples were kept at room tem-

peratureuntil thedieachievedastableprocessing temperature.

Following ECAP, the processed samples were cut in two

directions: (i) perpendicular to the pressing direction to give

the cross-sectional plane and (ii) parallel to the pressing di-

rection and perpendicular to the upper surface of the billet to

give the longitudinal plane. For both sections, pieces were

prepared having mean thicknesses of about 0.3 mm.

To evaluate the effect of thickness on the hydrogen ab-

sorption, some additional experiments were conducted using

commercial coarse-grained magnesium supplied by Baofull

Trading Co. (Liuzhou, China) in the form of an ingot having an

Fig. 1 e XRD patterns of AZ31 alloy processed by ECAP: (a)

for cross-section and (b) for longitudinal section,

respectively.

Fig. 2 e Bright field TEM image taken at a longitudinal

section in the processed AZ31.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 3 0 6e8 3 1 28308

initial grain size of w27 mm. Specimens were cut from the

ingot with square size of 10 mm and then thinned to thick-

nesses of 500 mm and 100 mm. The chemical composition of

this commercial magnesium (in wt.%) was 0.005 Fe, 0.03 Si,

0.002 Ni, 0.02 Cu, 0.05 Al, 0.005 Cl, 0.06 Mn and 0.2 impurities

with the balance as Mg.

The hydrogenation and kineticmeasurements of hydrogen

absorption were carried out using a Sieverts apparatus with

the samples hydrogenated at 573 K under a hydrogen pressure

of 2.0 MPa. The hydrogenation of commercial Mg was carried

out in the same Sieverts apparatus following the same pro-

cedure as above.

The desorption analysis of commercial Mg was performed

in a Netzsch Simultaneous Thermal Analyzer (STA) 449

Jupiter calorimeter which takes simultaneous differential

scanning calorimetric (DSC) and thermogravimetric (TG)

measurements and quadrupole mass spectrometer (QMS)

Aeolos equipment. Hydrogen desorption temperatures were

measured during continuous heating in DSC, using purified

and dried argon gas in an overflow regime. The buoyancy

effect was considered due to the use of argon as a carrier gas

and the necessary background treatment was performed as

usual.

The phases were identified by X-ray diffraction (XRD) using

monochromatic Cu-Ka radiation with an angular pass of

0.032� in a Rigaku DMAX diffractometer equipped with a C-

monochromator (LCE-DEMa-UFSCar-Brazil). The microstruc-

ture and orientation-phase mapping were characterized by

transmission electron microscopy (TEM) using a TEM JEOL

Fig. 3 e Equivalent bright field image obtained by TEM for

the AZ31 sample at a longitudinal section. High-angle

boundaries are represented by blue lines (misorientation

angles of over 15�) and low-angle grain boundaries are

represented by red lines (misorientation angles of below

15�). (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of

this article.)

Fig. 4 e Grain size distribution for the longitudinal section

of the AZ31 alloy.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 3 0 6e8 3 1 2 8309

3010 facility equipped with an orientation-phase mapping

precession unit NanoMEGAS (model ASTAR) and with a Digi-

star P1000 unit (SIMAP e CNRS e France). Orthotropic sample

symmetry was used in the texture calculations to reduce the

number of coefficients in the harmonic series expansion.

Fig. 5 e Pole figures for the processed A

3. Results and discussion

Fig. 1 shows representative XRD patterns after processing by

ECAP for the AZ31 alloy taken on the cross-sectional plane (a)

and the longitudinal plane (b). These patterns reveal only the

presence of a-Mg but, when one compares the theoretical and

observed relative intensities, it is apparent that the a-Mg

phase has preferred orientations in different sections. In

Fig. 1(a) the phase a-Mg has preferred orientations along the

pyramidal ð1011Þ plane and prismatic ð1010Þ and ð1120Þ planesthat are activated at high temperatures and become more

pronounced in the ð1011Þ plane. In Fig. 1(b) the basal (0001)

orientation is strongly pronounced, and grows at the expense

of all other random orientations. The (0001) orientation is the

main slip plane of a-Mg and, as observed earlier [45], is the

best orientation for hydrogen absorption.

Fig. 2 shows bright field TEM image taken at the longitu-

dinal section of the processed AZ31 alloy. In this image one

can see clear evidence for a bimodal distribution of grains.

Measurements gave average grain/subgrain sizes of w1.0 mm.

It is also evident from Fig. 2 that there are large numbers of

dislocations within the grains.

Orientation mapping was performed using TEM in the

same regions of the alloy as shown in Fig. 2 and themapping is

shown in Fig. 3. This mapping is similar to using electron

Z31 sample (longitudinal section).

Fig. 6 e Hydrogen absorption at 573 K under 2 MPa of H2 for

the processed AZ31: comparison is made between the

cross-section and the longitudinal section.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 3 0 6e8 3 1 28310

backscatter diffraction (EBSD) in scanning electron micro-

scopy (SEM) except that it has a higher resolution so that it can

be used with nanometric sizes and it provides a capability for

performing the analysis in heavily deformed samples as after

processing by ECAP.

For comparison purposes, Fig. 3 shows the equivalent

bright field image obtained by regular TEM for the longitudinal

section of the AZ31 alloy. This image shows a non-

homogeneous distribution of grains/subgrains. It should be

noted that, although this image was taken in the same region

as in Fig. 2, the two sets of images are not identical because

Fig. 3 was taken under conditions where orientation infor-

mation can rapidly differentiate between boundaries and sub-

boundaries. The misorientation angles of grain boundaries

were indexed and Fig. 3 shows high-angle boundaries (blue)

with misorientation angles equal to or greater than 15� and

low-angle grain boundaries (red) havingmisorientation angles

less than 15�. The high-angle boundaries are associated with

new grains and the low-angle boundaries denote subgrains.

By indexing these colored boundaries, it can be seen that the

sample contain w72% of high-angle boundaries. This con-

firms the observations from Fig. 2 regarding the amount of

deformation present in the samples.

Fig. 7 e (a) Optical micrograph and (b) the XRD p

The grain size distribution is shown in Fig. 4 for the pro-

cessed sample in the longitudinal section. The measured

average grain size was w960 nm. This value is similar to that

obtained from analysis of the images in Fig. 2.

Fig. 5 shows the pole figures for the processed sample in

the longitudinal section of the AZ31 alloy. These pole figures

reveal a tendency to have a strong texture (8 times the random

value) in the preferential direction of (0001) for H-sorption.

This is the same trend observed in the X-ray diffraction

spectra in Fig. 1(b).

Kinetic measurements are presented in Fig. 6 where the

hydrogen absorption is plotted against time. This plot com-

pares the first absorption curves between the cross-section

and longitudinal sections for the AZ31 alloy after processing

by ECAP. Inspection shows that much faster hydrogenation

kinetic is observed for the sample with a preferential texture

as in the longitudinal section where a maximum hydrogen

content of approximately 4.0 wt.% was reached after 15 h. For

the cross-section a maximum content of w2 wt.% was

reached after 32 h. These different hydrogen absorptions are

due mainly to the significant differences in texture in the two

types of sections and consequently to the preferred texture

that is available in specific directions. Nevertheless, if one

compares the obtained result in the least favorable direction

of the AZ31 alloy with that obtained by Skripnyuk et al. [35] for

ZK60 alloy, it is interesting to note that AZ31 can absorb more

hydrogen than for the same condition (without preferred

orientation) in the ZK60 alloy. This difference can be associ-

ated with the difference in grain sizes as the authors have also

reported a bimodal microstructure with average sizes of the

larger grains of 27 mm and of the smaller grains of 4 mm in the

same temperature of this work. This demonstrates, thereby,

the advantage of the smaller grain size in the AZ31 alloy and

the advantage of processing materials to a more deformed

state.

It is also important to note that, although there is the

presence of a preferred texture, the ability for hydrogen ab-

sorption is substantially lower than the theoretical capacity of

7.6%. This behavior may be related to the large thickness of

the sample since the excess vacancies produced by ECAP

processing, combined with the absence of any porosity, may

not be sufficient to create an easy path for hydrogen

attern for a commercial magnesium alloy.

Fig. 8 e Thermogravimetric properties of the commercial

magnesium alloy with different thicknesses of (a) 500 mm

and (b) 100 mm.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 3 0 6e8 3 1 2 8311

penetration [8,9]. In order to more fully examine this possi-

bility, a commercial coarse-grainedmagnesiumwith an initial

grain size of w27 mm was prepared having two different

thicknesses of w500 and w100 mm. Fig. 7 shows (a) an optical

micrograph and (b) the XRD pattern for this material where

there is no preferential orientation and the x-ray spectrum

follows the theoretical peak sequence of intensity. Fig. 8(a)

and (b) shows the thermogravimetric desorption properties of

the hydrogenated commercial magnesium samples with

thicknesses of w500 and w100 mm, respectively. From these

plots, it is evident that the thicker samples desorbed practi-

cally no hydrogen (w0.07 wt.%) whereas the thinner samples

desorbed w2.54 wt.%.

From these results it is apparent that different hydrogen

absorption results may be achieved primarily due to the dif-

ferences in texture but also to the grain size distribution and

thickness of the sample in the magnesium alloy after pro-

cessing by ECAP. It is concluded that promising new applica-

tions related to hydrogen storage may be achieved by using

ECAP processing to fabricate Mg and Mg-based alloys with

exceptionally small grain sizes.

4. Summary and conclusions

1. A commercial magnesium AZ31 extruded alloy was pro-

cessed by ECAP under conditions of temperature and

numbers of passes in order to avoid grain growth. Following

ECAP, the grain size was measured as w1.0 mm.

2. To analyze the effect of texture, the sample was cut in the

longitudinal and cross-sectional directions. A preferential

(0001) texture fiber was found in the longitudinal section of

the AZ31 alloy.

3. There was an influence of texture on the capacity and ki-

netics. The longitudinal sectional reached a maximum

hydrogen capacity of w4 wt.% in 15 h and the cross

sectional reached a maximum content of w2 wt.% after

32 h. Even the highest capacity is low and correlates with

the thickness of the samples. Additional testing on com-

mercial Mg samples of different thicknesses (500 and

100 mm) confirmed this effect.

4. It is proposed that promising new applications related to

hydrogen storage may be investigated by using ECAP to

fabricate Mg and Mg-based alloys with exceptionally small

grain sizes.

Acknowledgments

This work was supported in part by award FAPESP# 2011/

51245-8 under a cooperation agreement with the University of

Southampton, in part by the National Science Foundation of

the United States under Grant No. DMR-1160966 and in part by

the European Research Council under ERC Grant Agreement

No. 267464-SPDMETALS (MK and TGL).

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