Formation and mechanism of nanocrystalline AZ91 powders during HDDR processing

Accepted Manuscript Formation and mechanism of nanocrystalline AZ91 powders during HDDR processing

Yafen Liu, Jianfeng Fan, Hua Zhang, Qiang Zhang, Jing Gao, Hongbiao Dong, Bingshe Xu PII: DOI: Reference:

S1044-5803(16)30975-5 doi: 10.1016/j.matchar.2017.01.028 MTL 8540

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

17 November 2016 18 January 2017 20 January 2017

Please cite this article as: Yafen Liu, Jianfeng Fan, Hua Zhang, Qiang Zhang, Jing Gao, Hongbiao Dong, Bingshe Xu , Formation and mechanism of nanocrystalline AZ91 powders during HDDR processing. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Mtl(2017), doi: 10.1016/ j.matchar.2017.01.028

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Formation and mechanism of nanocrystalline AZ91 powders during HDDR processing Yafen Liu, Jianfeng Fan*, Hua Zhang, Qiang Zhang, Jing Gao, Hongbiao Dong*, Bingshe Xu (1 Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology; Taiyuan 030024; P.R. China; 2 3 4

Shanxi Key Laboratory of Advanced Magnesium-based Materials, Taiyuan 030024; P.R. China;

Shanxi Research Center of Advanced Materials Science and Technology; Taiyuan 030024; P.R. China;

College of Materials Science and Engineering, Taiyuan University of Technology; Taiyuan 030024; P.R. China)

PT

Abstract

RI

Grain sizes of AZ91 alloy powders were markedly refined to about 15 nm from 100-160 μm by an optimized

SC

hydrogenation-disproportionation-desorption-recombination (HDDR) process. The effect of temperature, hydrogen

NU

pressure and processing time on phase and microstructure evolution of AZ91 alloy powders during HDDR process was investigated systematically by X-ray diffraction, optical microscopy, scanning electron microscopy and

MA

transmission electron microscopy, respectively. The optimal HDDR process for preparing nanocrystalline Mg alloy powders is hydriding at temperature of 350℃ under 4MPa hydrogen pressure for 12 hours and dehydriding at 350℃

PT E

D

for 3 hours in vacuum. A modified unreacted core model was introduced to describe the mechanism of grain refinement of during HDDR process.

Introduction

AC

1.

CE

Keywords: Nanocrystalline materials, Phase transformation, Magnesium alloy, HDDR, Powder technology

Due to the low density, high specific strength and reasonable cost, et al, magnesium alloys have attracted much interest in automotive and aerospace applications [1,2]. However, the poor mechanical properties have restricted their high-end application in these fields [3]. So it has been a hotspot to improve the properties of magnesium alloys in the past years. At room temperatures, the yield strength generally follows the Hall–Petch relationship:

*

Corresponding author：Jianfeng Fan, E-mail: [email protected]; Tel.: +86 351 6014852 Hongbiao Dong, E-mail: [email protected]. 1

ACCEPTED MANUSCRIPT

Where

is the yield stress,

is the lattice friction stress,

is a constant of yielding and d is the grain size. Thus,

the strength of the material increases when the grain size is reduced, and the larger the effectively the metal is strengthened. At room temperature, the

value is, the more

value of pure magnesium is 280 MPam1/2, which is

PT

quite higher than that of the pure aluminum (68 MPam1/2). So, compared with traditional reinforcing methods, such as

RI

solution strengthening, strain hardening and dispersion strengthening, grain refinement is considered to be one of the

SC

most efficient ways to improve both strength and plasticity of magnesium alloys. However, it is difficult to refine the grains of Mg alloys to nanometer size with the present grain refining techniques although a grain size of 0.5-1 μm can

NU

be acquired by severe plastic deforming (SPD) [4-7]. Furthermore, people have been wondering if the Hall-Petch

MA

relationship works for the nano-sized Magnesium alloys. To the best of our knowledge, the maximum compression yield strength for Mg alloy was 550 MPa with bimodal microstructure in Mg-10Al alloy processed via cryomilling,

D

spark plasma sintering and extrusion[8] while the maximum compression yield strength for Mg base composite was

PT E

710 ± 35 MPa in Mg2Zn matrix composite with about 14 vol% nano-SiC particles[9]. Moreover, H.J. Choi et al[10] thought when grain size was further reduced down to 100 nm the compression yield strength decreased with

AC

important and urgent.

CE

decreasing grain size (inverse Hall–Petch). Thus, developing new nanocrystallization approaches for Mg alloys is

The Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) processing has been used in NdFeB materials and led to significant refined grains of approximately 300 nm[11,12]. Recently, it is found that the HDDR process can also be applied to the Mg alloys. Through HDDR process, Takamura et al [13] acquired AZ31 powders with grain sizes of about 100 nm. Miyazawa et al [14] obtained AZ61, AZ91 and ZK60 Mg alloy with grain size of 100-300 nm. When ZK60 Mg alloys powders were hydrogenated by reaction milling in hydrogen at room temperature and then were dehydrogenated at 300℃ in vacuum, the average grain size was refined to about 25 nm [15]. However, 2

ACCEPTED MANUSCRIPT the HDDR-Mechanical Milling process needs more complicated equipment and can only process several-gram Mg powders every time, which cannot meet the requirement of industrial production. Moreover, it is quite surprising that although a lot of papers were reported on the thermodynamics and kinetics of hydrogen–Mg reaction [16-20], a clear understanding of the microstructure evolution of Mg alloys during the hydrogenation and dehydrogenation process has

PT

not yet been proposed. So in the present paper, the effect of the HDDR parameters on the phase and microstructure

RI

evolution of AZ91 alloy powders is investigated in detail, and an optimized HDDR technique is developed for mass

SC

production of AZ91 alloy powders with an average grain size of 15nm. In addition, the formation mechanism of nano-sized grains is interpreted by introducing a nucleation-growth model of hydrogenation and dehydrogenation

Material and Methods

MA

2.

NU

process.

The commercial as-cast AZ91 Mg alloy was used as the starting material. After removed oxide surface by using

D

SiC paper and cleaned by ultrasonic washing in acetone, Mg alloy ingot was then cut and ground into coarse powders

experiment is (2.000±0.005) g.

PT E

with a diameter from mesh 75 to mesh 65 in an Ar-filled grove box. Mass of Mg alloy powders used in every

CE

HDDR treatments were carried in a self-designed hydrogenation furnace. In our experiments, the hydrogenation

AC

temperatures varied from 250℃ to 450℃, and the hydrogen pressure differed from 2 to 4 MPa. The dehydrogenation process was performed at 300 and 350℃in vacuum (<5×10-3 Pa). All the samples treated at different HDDR parameters are shown in table 1.The evolution of phase and microstructure in the HDDR process was examined by powder XRD, OM and TEM, respectively.

3.

Results

3.1 Phase evolution Fig. 1 shows the XRD patterns of the original powders and the hydrogenated powders under 4 MPa hydrogen 3

ACCEPTED MANUSCRIPT pressure. The patterns of AZ91powders treated at 250℃for 12 hours are almost the same as those of the original powders, without formation of hydrides, which means that Mg alloy cannot be effectively activated to react with hydrogen at this condition. Distinct MgH2 peaks appeared in powders treated at 300℃for 12 hours while there still existed some Mg peaks; However, Mg17Al12was totally transformed into Mg3Al2 and MgH2 under this condition.

PT

Moreover, when the hydrogenation time was increased to 24 hours at 300℃, the alloy wasn't still hydrogenated

RI

completely. Continually increasing the hydrogenation temperature to 350℃, AZ91 powders were hydrogenated

SC

completely after treated for 12 hours and the Mg peaks ultimately disappeared.

The hydrogen pressure also plays an important role in hydrogenation process. As shown in Fig.2, when the

NU

hydrogen pressure was decreased from 4MPa for sample 4 to 2 MPa for sample 5, Mg17Al12 intermetallic phase

MA

couldn't be hydrogenated completely while the matrix Mg phase had been transformed into MgH2 totally. Furthermore, even if the processing time was increased to 24 hours, there still existed obvious Mg17Al12 peak in the

D

patterns of sample 6 which was treated under 2 MPa hydrogen pressure.

PT E

Fig. 3 is the XRD patterns of sample 7 and sample 8 which are the dehydrogenated states of completely hydrogenated sample 4. After treated at 300℃ for 3 hours in vacuum, both MgH2 and Mg2Al3 phase remained and the

CE

dehydrogenation hadn't been activated. However, when the temperature was raised to 350℃, AZ91 alloy was totally

AC

dehydrogenated. In order to investigate effect of processing time on the dehydrogenation of MgH 2, XRD patterns of the hydrogenated AZ91 alloy dehydrogenated at 350℃ for 1 hour, 2 hours and 3 hours are measured as shown in Fig. 4. Compared with sample 8, the Mg peaks appeared and Mg3Al2 was totally transformed into Mg17Al12 after 1 hour of dehydrogenation, but MgH2 was still the absolutely dominant phase. Increasing the time to 2 hours, Mg peaks became the main peaks with some residual weak peaks of MgH2. Finally, the dehydrogenation was completed after 3hours. The grain size of sample 4 and sample 8 were estimated using X-ray diffraction. The three most intensive peaks were selected to measure the average grain size using the Scherrer formula [10]： 4

ACCEPTED MANUSCRIPT β(2θ) =

θ

Where β(2θ) is the half breadth of XRD peak (excluding instrumental broadening),

is the wavelength of the X-ray

radiation, θ is the Bragg angle, and d is the average grain size. The average grain size of hydrides in sample 4 was estimated to be about 16 nm, while the average grain size of the recombined AZ91 alloy in sample 8 was about 20 nm.

PT

The average grain size was also confirmed by observations of the microstructures using transmission electron

RI

microscopy (TEM).

SC

3.2 Microstructure evolution

Fig.5 presents the optical microstructures of the original ingot. It can be seen that the original AZ91 ingot has a

NU

very coarse grain size of 100–160 μm and the grain morphology is irregular with eutectic phase distributed along the

MA

grain boundary.

Fig.6 shows TEM images and the corresponding electron diffraction patterns of the hydrogenated powders in

D

sample 4. The diffraction rings is attributed to grains with different orientation. In the as-hydrogenated state, MgH2

PT E

was identified to be the dominant phase, and the formation of MgO may be resulted from dehydrogenation and oxidation of MgH2 during preparing TEM sample, especially in mechanical lapping. Fig. 6(d) shows the grain size By

CE

distribution for the powder, which is summarized from measuring 200 grain diameters in dark field images.

AC

TEM observation, the grains of the hydride powders are markedly refined to approximately 15 nm. The TEM images of the dehydrogenated powders in sample 8 are shown in Fig. 7. After dehydriding at 350℃ for 3 hours in vacuum, AZ91 alloy with Mg phase and Mg17Al12 phase was recombined. As a comparison, the average grain size in sample 8 were estimated to be 15 nm without no significant growth compared to sample 4. Obviously, the grain size was significantly reduced to 15 nm from 100-160 μm for the original AZ91 alloy after HDDR treatment. According to the Scherrer formulation and the XRD pattern, the grain size of the hydrogenated and dehydrogenated Mg alloy powders were calculated as about 16 and 20 nm, which are in agreement with the grain size 5

ACCEPTED MANUSCRIPT of TEM observations (15 nm). So the developed HDDR process is an effective method to refine the grain size of AZ91 Mg alloy to nanometer scale. It is emphasized that the equipment in our lab is able to process 200g Mg powders every time and all of the grain sizes of AZ91 alloy powders were effectively refined to 15nm under the optimum HDDR process parameters.

PT

Therefore, mass production of nanocrystalline AZ91 alloy powders can be realized as long as the equipment

Discussion

SC

4.

RI

conditions were achieved.

NU

4.1 Mechanism for phase evolution

The main interest of this paper is the microstructural evolution of Mg alloys during HDDR processing rather

MA

than the hydrogenation/ desorption mechanism of Mg alloys. Moreover, a great number of papers have been

PT E

discuss the above experimental results.

D

published on the thermodynamics and kinetics of Mg–hydrogen reaction. So, the related literatures will be used to

AC

CE

The hydrogenation reaction of AZ91 alloys should be as follows [16]: g

(1)

2

g1 Al12

2

g

2

g2 Al

2

g

2

g Al 2

(2) (3)

Table 2 presents some reported thermodynamics and kinetics parameters of Mg–hydrogen reaction [16,21,22]. Chou K C et al [23] proposed a hydrogenation/desorption kinetic function as follow:

(4) Where

is the reacted fraction, that is, the amount of metal that becomes hydride in absorption experiments or the

amount of hydride that transforms to metal in desorption experiments; 6

E is the activation energy of the overall

ACCEPTED MANUSCRIPT reactions;

and

are partial pressure and plateau pressure of H2, respectively; t is the processing time; T is

the experimental temperature; R is the gas constant;

is a constant only related to the materials; KH presents the

equilibrium constant of the hydrogenation/ desorption reaction;R0 represents the sample geometry; Vm is a constant related to density.

PT

For Mg17Al12 phase the hydrogenation reaction is essentially a two-step process i.e. reaction (2) and (3). And

RI

there are two plateau pressures for the hydrogenation of Mg17Al12 at 623 K: one for reaction (2) (9.4 bar) and the

SC

other for corresponding to reaction (3) (14 bar), both of which are much larger than that for reaction (1) (6 bar). Combined with equation (4) it can be inferred that a higher hydrogen pressure is required for hydrogenation of

NU

Mg17Al12 to acquire a satisfying velocity compared with Mg matrix (Fig.2). Moreover, the further reaction from

MA

Mg2Al3 to MgH2 didn't take place in our experiments because the hydrogen pressure of 4 MPa was not high enough to accelerate it. Crivello et al [24] also reported that the reversible hydrogenation of Mg-Al alloys at 350℃ is

D

accomplished in three transformation steps in chronological order with the increase of hydrogen pressures: Mg matrix

PT E

firstly, Mg17Al12 phase secondly and Mg2Al3 finally. On the other hand, when the hydrogen pressure is enough, Mg17Al12 phases are much easier to be transformed into hydride than Mg matrix at the same temperature. For

CE

example, at the condition of 300℃, 4MPa and 12 hours, Mg17Al12 phases had been hydrogenated completely while

AC

the matrix Mg phase were not been transformed into MgH2 totally, shown as Fig. 1. Referenced to Table 2, the activation energy for the absorption process of Mg17Al12 is 81 kJmol-1H, which is lower than that of pure Mg (90-140 kJmol-1H). As a result, the hydrogenation velocity for Mg17Al12 phase is higher than that of Mg matrix according to equation (4). The lattice constants (a = 0.4517 nm and c = 0.3023 nm) of MgH2 in sample 7 is almost as the same as those of the pure MgH2 (a = 0.4517 nm and c = 0.3020 nm).Thus, 350℃, 4 MPa and 12 hours are considered as the optimized hydrogenation technique of AZ91 alloy. 7

ACCEPTED MANUSCRIPT Both reaction (1) and reaction (2) are exothermic ones, and their formation enthalpies of the hydride are -77.5 kJmol-1H2 and -74 kJmol-1H2 respectively, which means the intermetallic compound Mg17Al12 has a reduced enthalpy of hydrogenation compared with pure Mg. It can be deduced that MgH2 formed by a hydrogenation reaction of Mg17Al12 is more unstable and prone to be decomposed than that formed by pure Mg during the endothermic

PT

dehydrogenation. As shown in Fig.4, at the same dehydrogenation condition of 350℃in vacuum, Mg17Al12 phase was

RI

completely recombined by the reverse reaction (2) after 1 hour, but the accomplished recombination of Mg matrix

SC

needed 3 hours.

From the XRD patterns of sample 8, the lattice constants of the recovered Mg matrix(a= 0.3189 nm，c= 0.5189

NU

nm)are the same as those of the original AZ91 alloy (a=0.3209 nm，c=0.5210 nm) within the experimental errors, so

MA

it can be deduced that the dehydrogenation–recombination reactions had been completed.

4.2 Mechanism for microstructural evolution

PT E

D

The grain size of AZ91 powders changes from 100 μm to 15 nm after HD (hydrogenation-disproportionation) processes and without grain growth after DR (desorption-recombination) processes, which demonstrates that the

CE

major grain refinement happens in HD processes and the grain size of the nanocrystallites did not grow up during subsequent DR processes.

AC

As shown in Fig.8, it is assumed that Mg particles are spherical and monocrystalline for simplification based on the fact that the particle sizes of the experimental powders are about the same as the grain sizes of the original alloy in this paper. Besides, the composition of the particles is considered as pure Mg for simplification due to the smaller content of Mg17Al12 phase in the heat-treated AZ91 alloys. According to the phase diagram for Mg-H system, at the temperatures below about 566℃, the phases are different with hydrogen concentration. For Mg-x at.% H system at 350℃, when x is less than Cα,eq (saturated concentration of hydrogen in α phase about 0.006) the system forms a solid solution (α phase), above Cα,eq a hydride 8

ACCEPTED MANUSCRIPT phase (β phase) precipitates, and the concentration of hydrogen in β phase, Cβ,eq is 0.667. Combined with the unreacted core model [25] and the work of Kuo-Chih Chou et al [23], the hydrogenation process for a particle can be described as the following eight steps: (i) Transfer of hydrogen gas (H2) to the surface of the metal particle; (ii) Physisorption of hydrogen molecules (H2)on the solid surface; (iii) Dissociation of hydrogen

PT

molecules and chemisorption of hydrogen atoms(H); (iv)Surface penetration of hydrogen atoms (H) and formation of

RI

solid solution (Mg(H), α phase); (v) Concentration saturation in α phase and nucleation of hydride (MgH2, β phase);

SC

(vi) Growth of hydride grains mainly following the spherical direction by diffusion of hydrogen atoms through metal;(vii)Formation of a closed hydride layer; (viii)Continuous growth of hydride grains following the radius

NU

direction by diffusion of hydrogen atoms through the hydride layer.

MA

Of all the above steps, the sixth step, the diffusion of hydrogen atoms through metal, is the rate-controlling reaction step before the completely closed hydride layer is formed. Once the hydride layer has closed, the eighth step,

D

i.e., the diffusion of hydrogen atoms through hydride layer, becomes the rate-controlling step. Many literature

PT E

data[16,21] indicate that the hydrogen diffusion rate in MgH2 is many orders of magnitude lower than in Mg, which is helpful to explain why most of the absorption curves obey an exponential growth with a rapider increase at the

CE

initial stage and a slower absorption rate at later stage.

AC

As for microstructural evolution during the hydrogenation process, the nucleation of MgH2 phase is heterogeneous and occurs preferentially at the sites with crystal defects (the activated sites), and the nucleation rate of MgH2 phase should be proportional to the number of crystal defects in the surface of Mg particles and the hydrogen partial pressure in gas phase. Then these nucleuses grow following both the spherical direction by the diffusion of hydrogen atoms through metal and the radius direction by the diffusion of hydrogen atoms through the hydride layer, and the growth velocities in both directions are all consistent with the hydrogen partial pressure in gas phase while the velocity in the spherical direction is much more rapid than that in the radius direction, which will result in 9

ACCEPTED MANUSCRIPT formation of a closed hydride shell covering a metallic core. And the thickness of the closed hydride shell is correspondent with the growth velocity in the radius direction and inversely proportional to the growth rate in the spherical direction. Subsequently, the hydride grains slowly grow following the radius direction by the diffusion of hydrogen atoms through the hydride layer until the Mg particle is hydrogenated totally. As a result, the

PT

monocrystalline Mg grain is also refined to many tiny MgH2 grains.

RI

In fact, there are a large number of defects or even micro cracks in the starting Mg particles which is prepared by

SC

casting and mechanical cutting. From these defects or micro cracks hydrogen atoms can easily diffuse into the interior of an Mg particle. Therefore, the primary hydrides may nucleate not only at the surface but also at the interior

NU

of an Mg particle (shown in Fig.9), which will result in a rapid increase of nucleation rate of MgH2 grains. Finally, g alloys.

MA

the average grain size of the hydride is about 15 nm refined from 100μm of

Seen from the view of thermodynamics and kinetics, the hydrogenation and dehydrogenation are two inverse

D

processes. However, in nanocrystalline MgH2 particle there exist a lot of grain boundaries which are the fast diffusion

PT E

channels of H atoms. Mg grains may nucleate at the surface of every MgH2 grain so it is scarcely possible to form a closed Mg layer surrounded an unreacted MgH2 core as that occurred during hydrogenation process. On the other

CE

hand, because the defects in nanocrystalline grain are much less than that in their coarse-grained counterparts, the

Conclusion

AC

grains of the dehydrogenated alloys and the hydrides are about the same in sizes.

1)

Grain size of the AZ91 alloy powders was significantly refined from 100μm to 15 nm.

2)

The optimal HDDR condition for acquiring nanocrystalline AZ91 Mg alloy powders is hydrogenation at 350℃ under 4MPa for 12 hours, and then dehydrogenation at 350℃ for 3 hours in vacuum.

3)

A modified unreacted core model was introduced to describe the mechanism of grain refinement of during HDDR process. 10

ACCEPTED MANUSCRIPT

Acknowledgements The authors thank Program for New Century Excellent Talents in University (NCET-12-1040), National Natural Science Foundation of China (51504162, 50901048 and 51174143), Natural Science Foundation of Shanxi Province

RI

Shanxi Province Talent Project (201605D211015) for their financial supports.

PT

(2015011033, 2015021073), Research Project Supported by Shanxi Scholarship Council of China (2016-029), and

Reference

magnesium alloys, J. Alloy. Compd. 554 (2013) 142-146.

SC

[1] J. Jia, J. F. Fan, B. S. Xu, H. B. Dong, Microstructure and properties of the super-hydrophobic films fabricated on

NU

[2] J. F. Nie, Y. M. Zhu, J. Z. Liu, X. Y. Fang, Periodic segregation of solute atoms in fully coherent twin boundaries, Science 340 (2013) 957–960.

[3] K. Lu, The future of metals, Science 328 (2010) 319–320.

MA

[4] F. D. Dumitru, O. F. Higuera-Cobos, J. M. Cabrera, ZK60 alloy processed by ECAP: Microstructural, physical and mechanical characterization, Mater. Sci. Eng. A 594 (2014) 32-39. [5] R. Jahadi, M. Sedighi, H. Jahed, ECAP effect on the micro-structure and mechanical properties of AM30 magnesium alloy, Mater. Sci. Eng. A 593 (2014) 178-184.

D

[6] B. Kim, S. M. Baek, H. Y. Jeong, J. G. Lee, S. S. Park, Grain refinement and reduced yield asymmetry of extruded Mg–5Sn–1Zn alloy by Al addition, J. Alloy. Compd. 660 (2016) 304-309.

PT E

[7] Q. D. Wang, Y. J. Chen, J. B. Lin, L. J. Zhang, C. Q. Zhai, Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method, Mater. Lett. 61 (2007) 4599-4602. [8] M. Pozuelo, Y.W.Chang,J.-M.Yang, Enhanced compressive strength of an extruded nanostructured Mg–10Al alloy, Mater. Sci. Eng. A 594 (2014) 203–211.

CE

[9] Lian-Yi Chen, Jia-Quan Xu, Hongseok Choi, Marta Pozuelo, Xiaolong Ma, Sanjit Bhowmick, Jenn-Ming Yang, Processing and properties of magnesium containing a dense uniform dispersion of nanoparticles, Nature 528 (2015) 539-543.

AC

[10] H.J. Choi, Y. Kim, J.H. Shin, D.H. Bae, Deformation behavior of magnesium in the grain size spectrum from nano- to micrometer, Mater. Sci. Eng. A 527(2010) 1565–1570. [11] K. Hono, H. Sepehri-Amin, Strategy for high-coercivity Nd–Fe–B magnets, Scripta Mater. 67 (2012) 530-535. [12] Y. Zhang, J. Han, S. Liu, F. Wan, H. Tian, X. Zhang, C. Wang, J. Yang, Y. Yang, Coercivity enhancement by grain refinement for anisotropic Nd2Fe14B-type magnetic powders, Scripta Mater. 110 (2016) 57-60. [13] H. Takamura, T. Miyashita, A. Kamegawa, M. Okada, Grain size refinement in Mg–Al-based alloy by hydrogen treatment, J. Alloy. Compd. 356-357 (2003) 804-808. [14] M. Takanori, K. Yuko, KamegawaAtsunori, T. Hitoshi, O. Masuo, Grain Size Refinements of Mg Alloys (AZ61, AZ91, ZK60) by HDDR Treatment, Mater. Trans. 45 (2004) 384-387. [15] S. Hong-fei, F. Wa, F. Wen-bin, Producing nanocrystalline bulk Mg–3Al–Zn alloy by powder metallurgy assisted hydriding–dehydriding, J. Alloy. Compd. 509 (2011) 8171-8175. [16] A. Andreasen, Hydrogenation properties of Mg–Al alloys, Int. J. Hydrogen Energ. 33 (2008) 7489-7497. 11

ACCEPTED MANUSCRIPT [17] S. Bouaricha, J. P. Dodelet, D. Guay, J. Huot, S. Boily, R. Schulz, Hydriding behavior of Mg–Al and leached Mg–Al compounds prepared by high-energy ball-milling, J. Alloy. Compd. 297 (2000) 282–293. [18] Y. Fan, X. Peng, T. Su, H. Bala, B. Liu, Modifying microstructures and hydrogen storage properties of 85mass% Mg–10mass% Ni–5mass% La alloy by ultra-high pressure, J. Alloy. Compd. 596 (2014) 113-117. [19] H. Fu, W. Wu, Y. Dou, B. Liu, H. Li, Q. Peng, Hydrogen diffusion kinetics and structural integrity of superhigh pressure Mg-5 wt%Ni alloys with dendrite interface, J. Power Sources 320 (2016) 212-221. [20] S. Niaz, T. Manzoor, A. H. Pandith, Hydrogen storage: Materials, methods and perspectives, Renew. Sust. Energ. Rev. 50 (2015) 457-469. [21] K. Zeng, T. Klassen, W. Oelerich, R. Bormann, Critical assessment and thermodynamic modeling of the Mg–H

PT

system, Int. J. Hydrogen Energ. 24 (1999) 989–1004.

[22] S. Bouaricha, J. P. Dodelet, D. Guay, J. Huot, S. Boily, R. Schulz, Hydriding behavior of Mg–Al and leached

RI

Mg–Al compounds prepared by high energy ball-milling, J. Alloy. Compd. 297 (2000) 282–293. [23] K. Chou, Q. Li, Q. Lin, L. Jiang, K. Xu, Kinetics of absorption and desorption of hydrogen in alloy powder, Int. J. Hydrogen Energ. 30 (2005) 301-309.

SC

[24] J. C. Crivello, T. Nobuki, T. Kuji, Improvement of Mg–Al alloys for hydrogen storage applications, Int. J. Hydrogen Energ. 34 (2009) 1937-1943.

NU

[25] X. Liu, F. Song, Z. Wen, A novel dimensionless form of unreacted shrinking core model for solid conversion

AC

CE

PT E

D

MA

during chemical looping combustion, Fuel 129 (2014) 231-237.

12

ACCEPTED MANUSCRIPT Figure captions Fig. 1. XRD patterns of the original powders and powders treated at different temperatures. Fig. 2. Effect of hydrogen pressure on hydrogenation of AZ91 alloy. Fig. 3. XRD patterns of sample 4 dehydrogenated at 300℃ and 350℃ for 3h in vacuum.

PT

Fig. 4. XRD patterns of the AZ91D alloy dehydrogenated at 350℃ for 1h, 2h and 3h in vacuum. Fig. 5. OM images of the original ingot.

RI

Fig. 6. TEM images of hydrides in sample 4. (a) The bright field; (b) the dark field; (c) The corresponding electron

SC

diffraction patterns; (d) A histogram indicating the grain size distribution for the sample.

NU

Fig. 7. TEM images of sample 8. (a) The bright field; (b) the dark field; (c) The corresponding electron diffraction patterns. (d) A histogram indicating the grain size distribution for the sample.

MA

Fig. 8. A schematic diagram of the major process during hydrogenation. Fig. 9. Another schematic diagram of the major process during hydrogenation.

PT E

D

Table 1 Samples of AZ31 powders treated by HDDR.

AC

CE

Table 2 Thermodynamics and kinetics parameters of Mg–hydrogen reaction [16, 21, 22].

13

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Figure 1

14

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Figure 2

15

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Figure 3

16

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Figure 4

17

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Figure 5

18

PT

ACCEPTED MANUSCRIPT

Figure 6c

MA

NU

SC

RI

Figure 6a

AC

CE

PT E

D

Figure 6b

19

Figure 6d

Figure 7a

PT E

D

MA

NU

SC

Figure 7c

RI

PT

ACCEPTED MANUSCRIPT

Figure 7d

AC

CE

Figure 7b

20

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

Figure 8

21

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

Figure 9

22

ACCEPTED MANUSCRIPT Table 1 Samples of AZ31 powders treated by HDDR. Hydrogen

Hydrogenation

hydrogenation

dehydrogenation

dehydrogenation

pressure

temperature

time

temperature

time

1

4MPa

250℃

12h

--

--

2

4MPa

300℃

12h

--

--

3

4MPa

300℃

24h

--

--

4

4MPa

350℃

12h

5

2MPa

350℃

12h

6

2MPa

350℃

24h

7

4MPa

350℃

12h

8

4MPa

350℃

12h

9

4MPa

350℃

10

4MPa

350℃

PT

samples

--

--

--

RI

--

--

300℃

3h

350℃

3h

12h

350℃

1h

12h

350℃

2h

AC

CE

PT E

D

MA

NU

SC

--

23

ACCEPTED MANUSCRIPT

Table 2 Thermodynamics and kinetics parameters of Mg–hydrogen reaction [13, 19, 20].

enthalpy change

entropy change

activation energy

Plateau pressure

Hf, kJmol-1H2

Sf, Jmol-1K-1H

Qe, kJmol-1H

at 623K, bar

reactions

Mg-H abs

90-140 139

6

PT

-77.5 Mg-H des

160±10 81

144

Mg17Al12-H des Mg2Al3-H abs 123

NU

-62.7 Mg2Al3-H des

30.33

MA

13.38

AC

CE

PT E

D

Mg-H solid Solution

SC

-74

RI

Mg17Al12-H abs

24

9.4

160

14

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

Graphical abstract

25

ACCEPTED MANUSCRIPT Highlights  Grain size of the AZ91 alloy powders was significantly refined from 100μm to 15nm.  The optimal HDDR technology for nano Mg alloy powders is obtained

PT

 A modified unreacted core model of grain refinement mechanism was

AC

CE

PT E

D

MA

NU

SC

RI

proposed.

26