Refinement process and mechanisms of tungsten powder by high energy ball milling

Accepted Manuscript Refinement process and mechanisms of tungsten powder by high energy ball milling

Yanxia Liang, Zaoming Wu, Engang Fu, Jinlong Du, Peipei Wang, Yunbiao Zhao, Yuanhang Qiu, Zhaoyi Hu PII: DOI: Reference:

S0263-4368(17)30114-2 doi: 10.1016/j.ijrmhm.2017.04.006 RMHM 4446

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

27 February 2017 18 April 2017 23 April 2017

Please cite this article as: Yanxia Liang, Zaoming Wu, Engang Fu, Jinlong Du, Peipei Wang, Yunbiao Zhao, Yuanhang Qiu, Zhaoyi Hu , Refinement process and mechanisms of tungsten powder by high energy ball milling. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Rmhm(2017), doi: 10.1016/j.ijrmhm.2017.04.006

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 Refinement process and mechanisms of tungsten powder by high energy ball milling Yanxia Liang a, Zaoming Wu a, Engang Fu a,*, Jinlong Du a, Peipei Wang a, Yunbiao Zhao a, Yuanhang Qiu a, Zhaoyi Hu a a

RI

PT

State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, P. R. China. Corresponding author: Engang Fu, [email protected] or [email protected]

SC

Yanxia Liang and Zaoming Wu contributed equally to the work.

NU

Abstract

The grain refinement process and mechanisms of tungsten powder during ball

MA

milling are explored based on different ball milling time. From the result of scanning electron microscope (SEM), the model for ball milling process is proposed to include

D

four stages: welding stage, squeezing stage, fracturing stage and dynamic balance

PT E

stage. Microstructure of tungsten powder examined by transmission electron microscope (TEM) shows that the tungsten powder ball milled for 50 hours has polycrystalline grains and most of them have long strip structure with high angle grain

CE

boundaries. Nanocrystalline tungsten powder with minimum grain size of 5 nm was

AC

obtained in the final stage of ball milling.

Keywords: morphology and microstructure; nanocrystalline tungsten; refinement process and mechanisms; ball milling; grain size

1. Introduction Tungsten (W) is one of the most alternative plasma-facing materials because of its high strength, high melting temperature, good thermal conductivity, low thermal expansion coefficients, low sputtering yield and low retention properties of hydrogen [1-5]. However, some shortcomings such as embrittlement and high recrystallization 1

ACCEPTED MANUSCRIPT temperature need to overcome to meet the requirements for materials used in nuclear fusion devices [6-10]. Nanograin structure is verified to improve the properties of tungsten [11-14]. Mechanical ball milling is one of methods that can be used to prepare ultrafine grain tungsten [15-17]. In contrast to the other methods such as wet chemical method

PT

[18-20], ball milling makes the preparation technique of nanocrystalline tungsten much more practical and effective. Furthermore, it has been known that mechanical

RI

ball milling has been used to synthesize a variety of alloy materials with the stable and metastable phases including intermediate phases, solid solutions and amorphous

SC

alloys [21]. Several researches about nanograin formation and ball milling influence factors are reported [11, 22-26]. Y.C. Wu used mechanical ball milling to fabricate

NU

W-TiN, W-Sc2O3, W-Lu2O3 and W-Sm2O3 powder with the grain size of 0.5-1 μm [27-30]. J.L. Fan applied this method to prepare W-Ni-Fe composite powder with the

MA

grain size of 2 μm and studied the influence of ball milling on powder characteristics [22]. E.J. Oda used ball milling to prepare W powder with nanograin structure [23].

D

E.Y. Lvanov investigated the formation of a nanocrystalline W-Re solid solution by

PT E

mechanical ball milling [21]. H. Kurishita employed mechanical alloying method to get W-TiC with the equiaxed grain size of 50-200 nm [31]. Ball milling can be used to fabricate nanograin materials and the process of grain refinement in terms of

CE

microstructure is studied, however, the fundamental mechanisms regarding the transformation of powder characteristics during ball milling are not exhaustively

AC

known. Especially few studies about mechanisms for the completed process of ball milling at a high speed are reported. In this study, the nanograin W powder with the minimum grain size of 5 nm was prepared by mechanical ball milling. The objective of this study is to clarify the pulverization process and nanograin formation mechanism of W powder in different stages of high energy ball milling by analyzing SEM and TEM images of W powder milled for different time. The parameters to fabricate the nanograin W by ball milling are optimized and the factors in controlling the ball milling process are discussed. 2

ACCEPTED MANUSCRIPT 2. Methods Commercially pure W powder (purity 99.9%) with an average particle size of 570 nm was used as initial powder to prepare nanograin W powder. Ball milling was carried out at room temperature by Fritsch P-7 planetary ball mill. The revolution speed was 500 rev·min-1 and transmission ratio between revolution speed and spin

PT

speed of vials was 1 to 2. Zirconium oxide (ZrO2) vials and ZrO2 balls were coated by tungsten layer to avoid the contamination on the powder. Analysis of ZrO2 ball

RI

surface and tungsten powder after ball milling was performed and the results (not

SC

shown here) show that the status of tungsten coating is as good as initial coating and no zirconium contaminates the milled tungsten powder. A weight ratio of ball to

NU

powder was 15 to 1. Pure ethanol (purity>99.997%) was chosen as process control agent (PCA) and atmosphere for milling was nitrogen (purity 99.999%).

MA

The powder ball milled for different time was characterized by scanning electron microscope (SEM, Nova NanoSEM430) and transmission electron microscope (TEM, Tecnai F30), respectively. The size distribution of tungsten particles was statistically

D

analyzed by a program of Nanomeasure. The SEM images can be enlarged in the

PT E

Nanomeasure software and the edge of each particle can be distinguished. The W powder which has clear edge is defined as a single particle in statistical analysis.

CE

Martin diameter which is the length of the area bisector of polygon particles in a horizontal measuring direction is used to define the particle size. Every statistical

AC

histogram illustrated more than 600 measurements for samples of W particles. Take one micrograph as an example. In order to get enough measurements for W particles, a low-magnification image (Fig. 1a) was chosen as a measurement source. By enlarging the image in Nanomeasure software, we measured the size of particles whose edge can be distinguished from the surrounding (Fig. 1b and 1c) and then made the statistical histogram.

3

ACCEPTED MANUSCRIPT

PT

Figure 1

RI

3. Results and discussion

SC

3.1 Interactions between W powder, grinding balls and surrounding wall in a vial During balling milling, two vials orbit the center of turnplate circularly and each

NU

vial spins on its own axis in opposite direction. Fig. 2 is photo of planetary ball mill (Fig. 2a) and a schematic diagram of a vial from top-view during ball milling process

MA

(Fig. 2b), which shows the movement situations of W powder and balls inside the vial. Under the effect of two centrifugal forces from revolution and spin, powder and balls

D

slide along the wall for a distance, and then fly off the wall and finally hit against the

PT E

wall again. This process is repeated as long as the vials revolve. After ball milling for a very short time, some of W powder adheres to the surface of grinding balls and vial

AC

CE

wall.

Figure 2

Two kinds of forces during ball milling acting on powder are impact force and shear friction. The impact force to powder is mainly resulted from three representative 4

ACCEPTED MANUSCRIPT situations (A, B and C) shown in Fig. 2. Situation A represents that the powder adhered to the wall is subjected to impact force from the balls when the balls hit against the wall of the vial; Situation B and C represent that the powder adhered to the balls is subjected to the impact force from balls with different speeds when the balls slide along the wall and fly off the wall. The shear friction to the powder is mainly

PT

resulted from three representative situations (D, E and F) shown in Fig. 2 as well. Situation D represents that the powder adhered to the wall is exerted by the shear

RI

friction from the balls sliding the wall. Situation E represents that power adhered to the surface of the ball is exerted by the shear friction when the rolling balls interact.

SC

Situation F represents that the powder is upon the shear friction from the wall of vial when the powder moves in the vial.

NU

The situations occur simultaneously and independently. The influence of ball milling parameters can be understood based on the situations outlined above. For

MA

example, revolution speed and ball-to-powder mass ratio are two major parameters during ball milling. The increase of revolution speed can increase the impact force

D

from situation A and thus make powder particles smaller. When ball-to-powder mass

PT E

ratio increases, more balls interact with powder and as a result the ball milling process will be more effective. As the contributions from impact force and shear friction to the change of powder are different in different stages of ball milling process, the

CE

followings will discuss the process of ball milling based on the model of interactions among powder, balls and vial wall.

AC

3.2 Four stages of W powder during mechanical ball milling Mechanical ball milling is an effective method to make the powder smash. The process of ball milling starts with the mixture of W powder and grinding balls. After ball milling for 50 hours, the powder removed from vials has nanoscale particles with the size range from 10 nm to 120 nm. The whole ball milling process deduced from SEM results includes four stages: welding stage, squeezing stage, fracturing stage and dynamic balance stage. The particle characteristics of W powder are various in different stages. 3.2.1 Welding stage 5

ACCEPTED MANUSCRIPT Fig. 3 shows SEM micrographs and statistical histograms of original W powder and W powder milled for 5 hours. The original W powder shown in Fig. 3a and Fig. 3b has polygon structure. Fig. 3a is a low-magnification image for an overall observation of original powder. Although some small particles are in aggregation status, each particle can be distinguished and every particle in aggregate was counted in histogram analysis. The statistical histogram of original particle size in Fig. 3c

PT

shows that it is a unimodal distribution and the particles with the size from 0.2 μm to

RI

1 μm are in the majority. The particles with the size over 1μm account for 14%. After ball milling for 5 hours, Fig. 3d and Fig. 3e shows that W powder has sheet shape and

SC

particles weld together (as the arrows show) in the early period of ball milling. Fig. 3d is a low-magnification image for an overall observation of welding features after 5

NU

hours milling. As shown in statistical histogram (Fig. 3f), the distribution of particle size is more random than that of original powder. The particles with the size from 0.4

MA

μm to 1.2 μm are in the majority and particles with the size of over 1μm account for

AC

CE

PT E

D

46.7%.

Figure 3

The shape of W powder changes from polygon to sheet and powder particles weld together after ball milling for 5 hours. The average particle size slightly increases. In this stage, impact force in the situation A, B and C shown in Fig. 2 acts 6

ACCEPTED MANUSCRIPT on the overlapping particles, and thus the internal energy of W powder particle increases, at the same time, the atoms from overlapping particles migrate and mix together. As a result, the shape of W powder changes from polygon to sheet under the action of impact force. However, W powder does not get enough energy to completely mix in this stage, and thus only welding occurs between overlapping particles. The

PT

smaller the particle size is, the larger the surface area is, and thus easier to weld together to form bigger particle.

RI

3.2.2 Squeezing stage

When the shape of W powder changes from polygon to sheet, the second stage

SC

occurs. Fig. 4 shows SEM images of W powder ball milled for 10 hours, 15 hours, and 20 hours and corresponding statistical histograms of particle size. Fig. 4a shows

NU

that after ball milling for 10 hours, the morphology of W powder changes from sheet to lamellate and the overlap of particles almost disappear. The statistical histogram

MA

(Fig. 4b) shows that particles with the size from 0.2 μm to 1.6 μm are in the majority, and the maximum size is 2.4 μm. Fig. 4c shows SEM images of W powder milled for

D

15 hours. They are also in form of lamella and the edges of particles are irregular. As

PT E

shown in statistical histogram (Fig. 4d), particles smaller than 1 μm are in the majority, and the maximum size is 6.5 μm. Fig. 4e shows W powder milled for 20 hours. The surfaces of particles become rough and a lot of small particles adhere to the large

CE

lamellas. The statistical histogram (Fig. 4f) shows that particles smaller than 0.5 μm

AC

are in the majority, and the maximum size is 5.5 μm.

7

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Figure 4

CE

In this stage, the size of sheets of W powder gradually becomes large as milling time further increases. At the same time, some particles with small size appear. It is

AC

squeezing effect that plays a dominant role to make compression in axial of the particle and tension in radial direction so that the W powder sheets become large and thin. Shear friction from situation D and E shown in Fig. 2 also emerges in radial direction of the powder. Some granules peel off the powder surfaces and the powder surfaces become rough in appearance. In this stage, welds almost disappear due to diffusion mechanism of W atoms which is strengthened by accumulated energy. With the increase of milling time, lamellate particles become thinner and thinner until brittle fracture occurs and small particles appear. 3.2.3 Fracturing stage 8

ACCEPTED MANUSCRIPT When the lamellas of W powder are thin enough, they would go on to the fracturing stage. Fig. 5 shows SEM images and statistical histograms of W powder milled for 30 hours, 40 hours and 45 hours. Fig. 5a shows the sample ball milled for 30 hours has extremely thin lamellas and typical one is labeled by the red arrow. Many particles with small size in the range between 38 nm and 80 nm are adhered on the surface of the lamellas. The statistics (Fig. 5b) show that more than 65% of

PT

particles have the size smaller than 0.1 μm and the maximum size of the particle is

RI

around 1.3 μm. As milling time further increases to 40 hours shown in Fig. 5c and Fig. 5d, W lamellas are broken into small particles with the irregular shape and they

SC

aggregate into big particle with the size smaller than 200 nm. After ball milling for 45 hours, W lamellas completely disappear shown in Fig. 5e and they are replaced by

NU

fragments with the size smaller than 500 nm. The statistical histogram (Fig. 5f) shows

AC

CE

PT E

D

MA

that more than 90% of particles have the size between 50 nm and 250 nm.

9

Figure 5

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

In fracturing state, the morphology of W powder changes from lamella to

AC

fragment, and the size of particle decreases significantly with the increase of milling time. In addition to squeezing effect, the effect from the shear friction in the situation D and E shown in Fig. 2 is more significant in this stage. The shear friction and impact force act on the extremely thin lamellas to generate the cracks due to stress concentration, and thus the particles are smashed into small fragments. When the ball milling time reaches to 45 hours, the fracturing stage completely finishes and all W lamellas are thinned and smashed into small pieces with the size below 500 nm due to the accumulation of enough energy on the lamellas. 3.2.4 Dynamic balance stage 10

ACCEPTED MANUSCRIPT As milling time further increases to 50 hours, the size of W powder is decreased to less than 100 nm shown in Fig. 6a and Fig. 6b and eventually ball milling process proceeds to a dynamic balance stage. In this stage, when the ball milling time is more than 50 hours, the particle size cannot be reduced further and remains within a few tens of nanometer as shown in Fig. 6c and Fig. 6e. The statistical distributions of

PT

particle size are normal shown in Fig. 6d and Fig. 6f. Fig. 7 is a high resolution SEM image of W powder milled for 60 hours and one cluster as a typical example labeled

RI

by the red circle consists of dozens of small particles with the size of about 40 nm. It indicates that the small particles are seriously agglomerated into clusters after ball

AC

CE

PT E

D

MA

NU

SC

milling more than 50 hours.

Figure 6

11

NU

Figure 7

SC

RI

PT

ACCEPTED MANUSCRIPT

In this stage, the particle size reaches less than 100 nm after 50 hours’ ball

MA

milling. The particles with such small size have high surface activity which results in the agglomeration of the particles. As ball-milling time increases, the process of

D

separation and agglomeration of the particles occurs simultaneously which is also

PT E

implied by the statistical histograms. According to the energy balance condition, the relation between the critical stress and the particle size can be given as [32]:

3E i 8( 1   2 )R

CE



Where  is the critical stress, E = 411 GPa and   0.28 are elastic modulus and

AC

Poisson’s radio of tungsten, i = 44.6 mJ/m2 is free surface energy of tungsten [33] and R is the radius of particles. Greater stress is required to make the grain size smaller. When the average particle size is 36 nm (R = 18 nm), the critical stress provided on the particles by ball milling is σ  2.48 GPa . It looks that the particle size cannot decrease after 50 hours’ ball milling from SEM observation, however, the surface energy of particles increases constantly with ball milling process and it therefore leads to the agglomeration of the particles. Fig. 8 is a schematic diagram of whole pulverization process of W particles. It shows that original particles are welded to a sheet under the effect of impact force, 12

ACCEPTED MANUSCRIPT and then the sheet becomes thinner and larger induced by squeezing effect. As ball milling time further increases, cracks arise from stress concentration and thus the sheet is broken into small fragments. The fragments are broken into much smaller pieces after ball milling with more time, and then they are in the process of dynamic

MA

NU

SC

RI

PT

balance stage with the stable size of particles.

D

Figure 8

PT E

Fig. 9 shows the relationship between the average size of W particles and ball milling time. In the welding stage, the average size of particles increases as several original powder welds into a bigger sheet particle. In squeezing stage, the sheet

CE

particles are firstly squeezed to flat sheets with large size and then small particles on the sheet surface appear and start to drop out. Therefore, the average size increases

AC

firstly and then decreases in this stage. In fracturing stage, the thin lamellas smash into small pieces and the particle size continues to decrease and reaches the smallest size less than 100 nm. In the dynamic balance stage, the average size of W particles remains under 100 nm without decreasing after ball milling for 50 hours and the process of separation and agglomeration of the particles access into balance.

13

Figure 9

SC

RI

PT

ACCEPTED MANUSCRIPT

NU

3.3 Microstructure of nanocrystalline W powder

Fig. 10a shows TEM bright field image of typical W particle from the powder

MA

milled for 50 hours. The grains shown in dark contrast observed under in-zone condition have long strip structures with the size smaller than 20 nm. If one tilts the

D

sample into another axis zone, other grains which satisfy the diffraction condition will

PT E

show similar features. Fig. 10b is the corresponding selected area electron diffraction (SAED) pattern of this sample. The observed diffraction pattern consists of concentric rings that indicate the polycrystalline nature of the powder, and the ring profile shows

CE

that the particle contains nanoscale grains with high angle grain boundaries [34]. Fig. 10c is a high resolution TEM image of one tungsten grain. It has the size about 5 nm,

AC

and its lattice spacing is 0.224 nm indicating that this single crystalline grain has W (110) plane.

14

ACCEPTED MANUSCRIPT

PT

Figure 10

RI

During ball milling process, plenty of dislocations are formed due to the effect of

SC

stress from impact force and shear friction. Previous research shows little recovery occurs in the metal if the temperature is less than around ten percent of its melting

NU

temperature during ball milling [23]. In current study, dislocations generated by the stress remain as the temperature during ball milling is less than ten percent of tungsten

MA

melting point temperature. As a result, each grain is divided into several volume elements by dislocation accumulation and rearrangement [35]. With the increase of

D

dislocation density in the volume element, the dislocations slip to the boundaries of

PT E

volume element, and interact to form grain boundaries along them. Eventually one grain is divided into several small grains through dislocation rearrangement. The smallest grain size in the sample ball milled for 50 hours measured in TEM is around

CE

5 nm. Actually, the tungsten grain with minimum size of 5 nm was also observed in the sample ball milled for 60 hours [36].

AC

4. Conclusions

The morphology in terms of particle shape and size and the microstructure in terms of grain shape and size in tungsten powder processed by ball milling are revealed in this study. The impact force and shear friction act on tungsten powder during ball milling. The model for refinement process and mechanism is proposed to have four stages as follows: in welding stage, overlapping W particles weld under the act of impact force, and the shape of W powder changes to sheet; In squeezing stage, W sheet gets thinner as energy accumulates; In fracturing stage, the particles were smashed into small fragments and the particle size reaches less than 100 nm; In 15

ACCEPTED MANUSCRIPT dynamic balance stage, breaking and connecting of W powder with the stable size are in balance. The nanoscale grains of W powder with size less than 20 nm were

AC

CE

PT E

D

MA

NU

SC

RI

PT

obtained by ball milling and the minimum grain size of 5 nm was observed.

16

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by International Thermonuclear Experimental Reactor (ITER) Program with the award number of 2015GB121004 from Ministry of Science and Technology of China and by grants with the numbers of 11375018 and 11528508 from National Natural Science Foundation of China (NSFC). E.F. appreciates the

PT

support from The Recruitment Program of Global Youth Experts in China and the Instrumental Analysis Fund of Peking University. The authors acknowledge the

RI

support by the key technology of nuclear energy, 2014, CAS Interdisciplinary

SC

Innovation Team.

NU

References

AC

CE

PT E

D

MA

[1] Kurishita H, Kobayashi S, Nakai K, et al. Development of ultra-fine grained W–(0.25–0.8) wt.%TiC and its superior resistance to neutron and 3 MeV He-ion irradiations. Journal of Nuclear Materials. 2008; 377: 34-40. [2] Norajitra P, Boccaccini L V, Diegele E, et al. Development of a helium-cooled divertor concept: design-related requirements on materials and fabrication technology. Journal of Nuclear Materials. 2004; 329–333: 1594–1598. [3] Kawai M, Kikuchi K, Kurishita H, Li J F, Furusaka M. Fabrication of a tantalum-clad tungsten target for KENS. Journal of Nuclear Materials. 2001; 296: 312-320. [4] Kawai M, Furusaka M, Kikuchi K, et al. R&D of a MW-class solid-target for a spallation neutron source. Journal of Nuclear Materials. 2003; 318: 38-55. [5] Ameyama K, Oda E, Fujiwara H. Superplasticity and high temperature deformation behaviour in nano grain tungsten compacts. Materialwissenschaft und Werkstofftech. 2008; 39: 336-339. [6] Mathaudhu S N, deRosset A J, Hartwig K T, Kecskes L J. Microstructures and recrystallization behavior of severely hot-deformed tungsten. Materials Science and Engineering A. 2009; 503: 28-31. [7] Kim Y, Lee K H, Kim E, Cheong D, Hong S H. Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process. International Journal of Refractory Metals and Hard Materials. 2009; 27: 842-846. [8] Philipps V. Tungsten as material for plasma-facing components in fusion devices. Journal of Nuclear Materials. 2011; 415: S2-S9. [9] Smid I, Akiba M, Vieider G, Plochl L. Development of tungsten armor and bonding to copper for plasma-interactive components. Journal of Nuclear Materials. 1998; 253–263: 160-172. [10] Seki Y, Ezato K, Suzuki S, et al. A study of plasma facing tungsten components 1

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

with electrical discharge machined surface exposed to cyclic thermal loads. Fusion Engineering and Design.2016; 109-111: 1148-1152. [11] Jia K, Fischer T E, Gallois B. Microstructure, hardness and toughness of nanotructure and conventional WC-Co composites. Nanostructured Materials. 1998; 10: 875-891. [12] Gleiter H. Nanocrystalline materials. Progress in Materials Science. 1989; 33: 223-315. [13] Liu W. B. et al. Strain-induced refinement and thermal stability of a nanocrystalline steel produced by surface mechanical attrition treatment. Materials Science and Engineering A. 2013; 568: 176-183. [14] Tao N R, Wang Z B, Tong W P, et al. An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment. Acta Materialia. 2002; 50: 4603-4616. [15] Zhang F L, Wang C Y, Zhu M. Nanostructured WC/Co composite powder prepared by high energy ball milling. Scripta Materialia. 2003; 49: 1123–1128. [16] Tang H G, Ma X F, Zhao W, Yan X W, Hong R J. Preparation of W–Al alloys by mechanical alloying. Journal of Alloys and Compounds.2002; 347: 228–230. [17] Cai S G, Ma X F, Tang H G, et al. Preparation of W–Al–Mo ternary alloys by mechanical alloying. Journal of Alloys and Compounds. 2007; 430: 77–80. [18] Xia, M. et al. Bulk tungsten with uniformly dispersed La2O3 nanoparticles sintered from Co-precipitated La2O3/W Nanoparticles. Journal of Nuclear Materials. 2013; 434: 85-89. [19] Xu L, Yan Q Z, Xia M, Zhu L X. Preparation of La2O3 doped ultra-fine W powders by hydrothermal-hydrogen reduction process. International Journal of Refractory Metals and Hard Materials. 2013; 36: 238-242. [20] Xia, M. et al. Synthesis of TiC/W core–shell nanoparticles by precipitate-coating process. Journal of Nuclear Materials.2012; 430: 216-220. [21] Ivanov E Y, Suryanarayana C, Bryskin B D. Synthesis of a nanocrystalline W–25 wt.% Re alloy by mechanical alloying. Materials Science and Engineering A. 1998; 251: 225-261. [22] Han Y, Fan J L, Liu T, et al. Influence of ball milling on powder characteristics and feedstock rheological behaviours of injection moulded ultrafine 98W–1Ni–1Fe. Powder Metallurgy. 2013; 56: 135-14 (2013). [23] Oda E, Fujiwara H, Ameyama K. Nano grain formation in tungsten by severe plastic deformation-mechanical milling process. Materials Transactions. 2008; 49: 54-57. [24] Tan X Y, Li P, Luo L M, et al. Effect of mechanical milling on the microstructure of tungsten under He+ irradiation condition. Fusion Engineering and Design. 2015; 100: 571-575. [25] Gong X, Fan J L, Ding F, Song M, Huang B. Y. Effect of tungsten content on microstructure and quasi-static tensile fracture characteristics of rapidly hot-extruded W–Ni–Fe Alloys. Journal of Refractory Metals and Hard Materials. 2012; 30: 71-77. [26] Wu X L, Ma E, Zhu Y T. Deformation defects in nanocrystalline nickel. Journal 2

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

of Materials Science. 2007; 42: 1427-1432. [27] Wang S, Luo L M, Tan X Y, et al. Influence of TiN nanoparticles on the microstructure and properties of W matrix materials prepared by spark plasma sintering. Journal of Nuclear Materials. 2014; 454: 114-118. [28] Chen H Y, Luo L M, Chen J B, et al. Effect of Sc2O3 particles on the microstructure and properties of tungsten alloy prepared by spark plasma sintering. Journal of Nuclear Materials. 2015; 462: 496–501. [29] Zhang J, Luo L M, Zhu X Y, et al. Effect of doped Lu2O3 on the microstructures and properties of tungsten alloy prepared by spark plasma sintering. Journal of Nuclear Materials. 2015; 456: 316–320. [30] Zhu X Y, Zhang J, Luo L M, et al. Microstructure and properties of W/Sm2O3 composites prepared spark plasma sintering. Advanced Powder Technology. 2015; 26, 640–643. [31] Kurishita H, Kobayashi S, Nakai K, et al. Current status of ultra-fine grained W–TiC development for use in irradiation environments. Physica Scripta. 2007; T128: 76-80. [32] Sun J, Deng Z J, Li Z H, Tu M J. Fracture strength of spheroidal carbide particle. International Journal of Fracture. 1990; 42: 39-42. [33] Gao Z Y, Sun W, Hu Y H, Liu X W. Surface energies and appearances of commonly exposed surfaces of scheelite crystal. Transactions of Nonferrous Metals Society of China. 2013; 23: 2147-2152. [34] Du J L, et al. What determines the interfacial configuration of Nb/Al2O3 and Nb/MgO interface. Scientific Reports. 2016; 6: 33931. [35] Hansen N, Huang X. Microstructure and flow stress of polycrystals and single crystals. Acta Materialia. 1998; 46: 1827-1836. [36] Wu Z M, Liang Y X, Wang P P, et al. unpublished work.

Additional Information Yanxia Liang and Zaoming Wu contributed equally to the work. Competing financial interests: The authors declare no competing financial interest.

3

ACCEPTED MANUSCRIPT

Figure Captions

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 1. An example of particle size measurement. (a) low magnification SEM image of W powder milled for 10h, (b) the measured image in the Nanomeasure software, and (c) enlarged image of the red square in (b) and every measured particle is marked by number. Figure 2. Photo of planetary ball mill and schematic diagram of a vial from top-view in the process of ball milling. Powder and grinding balls are included in the vial represented by black and blue drawings. The vial has two kinds of motions: revolution and spin and their directions are opposite shown by blue arrows. The movement of powder and balls includes two processes: sliding along and flying to the wall and their directions are shown in red arrows. Figure 3. SEM images of (a) and (b) for original W powder, and (d) and (e) for W powder ball milled for 5h. Statistical histograms of (c) original W powder and (f) W powder ball milled for 5h Figure 4. SEM images of W powder milled for (a) 10h, (c) 15h and (e) 20h and statistical histograms of W powder milled for (b) 10h, (d) 15h and (f) 20h Figure 5. SEM images of W powder milled for (a) 30h, (c) 40h and (e) 45h and statistical histograms of W powder milled for (b) 30h, (d) 40h and (f) 45h Figure 6. SEM images of W powder milled for (a) 50h, (c) 55h and (e) 60h and statistical histograms of W powder milled for (b) 50h, (d) 55h and (f) 60h Figure 7. High resolution SEM image of W powder milled for 60h Figure 8. Schematic diagram of W particles pulverization Figure 9. Relationship between the average size of W particle and ball milling time Figure 10. TEM and SAED images of W powder milled for 50h. (a) TEM bright image, (b) SAED image and (c) high resolution image of a small grain

4

ACCEPTED MANUSCRIPT Highlights

PT

RI SC NU MA D



PT E



CE



Provide an insight into interactions between tungsten powder and surrounding in a vial during high energy ball milling. Reveal morphology and microstructure of tungsten powder in different stages during the whole ball milling process. Illustrate the mechanisms of tungsten powder in different stages by high energy ball milling. Nanocrystalline tungsten powder with grain size smaller than 20 nm was successfully fabricated and the powder with minimum grain size of 5 nm was obtained in final step of ball milling.

AC



5