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The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling
Accepted Manuscript The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling
Z.M. Wu, Y.X. Liang, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu PII: DOI: Reference:
S0032-5910(18)30593-X doi:10.1016/j.powtec.2018.07.094 PTEC 13576
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
13 March 2018 4 June 2018 29 July 2018
Please cite this article as: Z.M. Wu, Y.X. Liang, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu , The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling. Ptec (2018), doi:10.1016/j.powtec.2018.07.094
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ACCEPTED MANUSCRIPT
The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling Z.M. Wu†, Y.X. Liang†, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu*
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State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking
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University, Beijing 100871, P. R. China
Tel: +86-10-62750612, Fax: +86-10-62751873
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Email: [email protected] or [email protected]
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*Corresponding author: Engang Fu
Present address: 201 Chengfu Road, Haidian District, Beijing, 100871, China.
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†: Zaoming Wu and Yanxia Liang contributed equally to the work.
ACCEPTED MANUSCRIPT Abstract: The nanocrystalline tungsten powder with the minimum grain size of around 5nm was fabricated by mechanical alloying. The powder refinement process and the influence of ball to powder ratio (BPR) on the morphology and microstructure of
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powder were investigated. The morphology and microstructure were characterized by
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X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM) and
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transmission electron microscope (TEM). Results revealed that the high energy ball milling process has four stages. Compared with the samples with the BPR of 10:1 and
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4:1 under the same ball milling conditions, the samples with the BPR of 15:1
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experiences the whole four stages after 60h ball milling. One key factor in determining the final morphology and microstructure of tungsten powder by ball
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the particle size is discussed.
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milling was revealed. The model to illustrate the fundamental mechanism for refining
Keywords:
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Morphology, Microstructure, Nanocrystalline tungsten, Refinement, Ball to powder
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ratio, Ball milling
ACCEPTED MANUSCRIPT 1. Introduction Tungsten is one of the most promising plasma facing materials (PFMs) in fusion environment of high-dose neutron radiation and high heat flux due to its excellent properties such as high melting point (3410℃), low thermal expansion coefficient,
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good thermal conductivity, low sputtering yield, low retention of hydrogen and high
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elevated temperature strength [1-6]. However, some other properties of tungsten
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restrict its applications in fusion reactor. For instance, tungsten with high ductile-to-brittle transition temperature (373~673K) shows the brittleness at low
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temperature; The grain size of tungsten is easy to grow up in high temperature
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environment due to its low recrystallization temperature (1423~1623K), which degrades its mechanical properties including strength and hardness ; in addition, long
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term neutron radiation results in serious degradation of materials, such as swelling
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and fracture induced by the nuclear transmutation and its products [7-12]. So it is very necessary to improve the properties of tungsten serving in the harsh environment.
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Research shows that the refinement of tungsten grain size can improve the
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mechanical properties and the radiation tolerance of tungsten based materials apparently. The nanocrystalline materials were first proposed by H. Gleiter in the early 1980s [13], and their properties (especially mechanical properties) were well studied due to their unique structural feature and outstanding performance. Currently, top-down method and bottom-up method are two major approaches to prepare nanocrystalline tungsten [14]. Top-down method is a direct way to refine the grain by severe plastic deformation repetitively of coarse grain solid materials, such as equal
ACCEPTED MANUSCRIPT channel angular extrusion (ECAE) [15-17], high-pressure torsion (HPT) [15,16,18], accumulative roll bonding (ARB) [19,20] and surface mechanical attrition treatment (SMAT) [21-25]. Bottom-up method adopts mechanical alloying (MA) [26-31] or other approaches such as wet chemical method [32-34] to obtain the nano-size
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precursor powder, then the precursor powder is sintered to fabricate the ultrafine grain
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or nanocrystalline bulk materials. This method usually refers to powder metallurgy
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(PM).
Q. Yan used wet chemical method to fabricate core-shell structure W-TiC and
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W-La2O3 powder with the particle size from 50nm to 200nm [32-34]. K. Ameyama
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adopted mechanical alloying (MA) process to ball mill W-10wt%Re powder and obtained nano-scale W-Re powder, which contains nano grain structure with the grain
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size of 10-20nm [26]. E. Oda used mechanical milling method to obtain 12nm wide
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and 25nm long nano-scale tungsten grain powder [27]. J. Chen et al. adapted MA method to refine the grain of W-15wt.%Nb alloy. They obtained tungsten powder with
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grain size of 28.3 nm after 45h ball milled at milling speed of 400rpm and ball to
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powder ratio (BPR) of 20:1 [31]. Many methods have been put forward to refine the grain size of tungsten powder or tungsten bulk materials. ECAE, HPT, ARB and SMAT are the methods to refine the bulk tungsten materials; however, they cannot be able to successfully form the tungsten grain with the size range from 5nm to 15nm. MA as a method to refine the powder adopts high energy ball milling to obtain the powder with nanograin as MA is able to give extremely large amount of strain and high deformation rate. Especially, it
ACCEPTED MANUSCRIPT is possible to obtain the powder with the minimum grain size of around 5nm, a theoretical value that equals to the equilibrium distance of two edge dislocations deduced by a formula proposed by T.G. Nieh [35]. As a typical MA, high energy ball milling process is a very complex dynamic
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process that possesses a lot of variables, such as the ball to powder ratio (BPR),
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milling speed, number and type of milling ball, the amount of process control agent,
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etc. High energy ball milling is a mechanical alloying process that can be used to fabricate nanocrystalline powder and other advanced materials due to its extremely
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large amount of strain and high deformation rate. It is very vital to optimize the
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technical parameters of high energy ball milling as each of them plays important role in determining the final properties of the powder.
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In this paper, experiments with the different BPRs were conducted to investigate
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the refinement process and the influence of BPR on morphology and microstructure of tungsten powder in the process of ball milling, and the fundamental mechanism for
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well.
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refining the particle size and the influence of BPR during ball milling is discussed as
2. Experimental procedures 2.1. The preparation of nanocrystalline tungsten powder The nanocrystalline tungsten powder was fabricated by MA method of high energy ball milling in this study. Commercial pure tungsten powder with the average particle size of about 390nm and a purity of 99.9% was used as initial powder. The
ACCEPTED MANUSCRIPT samples were mechanically alloyed in Fritsch pulverisette 7 premium line planetary mill with the two vessels made of zirconium oxide. The balls of diameter 1 mm were also made of zirconium oxide. The Fritsch pulverisette 7 premium line planetary mill possesses two pots and each pot volume is 80 ml. The 1 mm diameter balls were
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adopted here due to its high surface area compared with larger balls. It increases the
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milling efficiency of ball milling. Tungsten powder and grinding balls were weighed
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by the precision balance (with the accuracy of 0.0001g) respectively to make samples with the BPR (the ratio of milling balls mass to powder mass) of 4:1, 10:1 and 15:1.
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In order to avoid agglomeration of the powder, the wet milling method was utilized by
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adding ethanol with 8 wt. % of the total weight of balls and powder as process control agent (PCA). The samples were milled for the total time of 60h with the rotate speed
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of 500rpm in the purified N2 gas (purity 99.999%). The experimental conditions of
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ball milled tungsten powder with different BPR are shown in Table 1. The procedure to make scanning electron microscopy (SEM) and transmission
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electron microscopy (TEM) samples of tungsten powder is as follows: (1) ball milling
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tungsten powder samples were taken out into centrifuge tube every 5 hours in glove box filled with N2 gas; (2) 1 ml ethanol was added to the samples made at the different milling time and with the different BPRs to dilute the samples; (3) the diluted samples were ultrasonic dispersed for 40 minutes; (4) one drop of solution was added by dropper into conductive carbon glue and ultra-thin carbon film to make SEM samples and TEM samples, respectively; (5) the SEM and TEM samples were put into vacuum drying oven to dry for 20 minutes at the temperature of 30℃.
ACCEPTED MANUSCRIPT 2.2. Microstructural characterization The morphology and microstructure for different powder refinement stages were characterized by the secondary electrons images of FEI Nano SEM 430 field-emission scanning electron microscope (FE-SEM), BF (BF) images and dark field (DF) images
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of FEI Tecnai F30 transmission electron microscope (TEM), and high resolution TEM
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(HRTEM). The Empyrean high resolution powder X-ray diffractometer (XRD) was
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utilized to characterize the grain size of tungsten powder before and after ball milling. The particle size was obtained from the SEM images of tungsten powder at different
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milling time. The software Nanomeasurer was used on the SEM images to make
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statistics of particle size. In order to obtain the reliable particle size, plenty of SEM images of each sample were randomly selected and the particle size was recorded on
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different positions. More than 1000 particles for each sample were used to determine
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the Martin particle size based on the number percentage. The grain size of ball-milled tungsten powder was obtained by the DF TEM image. We chose one diffraction spot
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in the (110) crystal direction and entangled this point with the aperture of an objective
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lens to obtain the dark-field TEM image and grain size. 3. Results and discussions 3.1. Initial tungsten powder Fig.1a shows the SEM image for the morphology of initial tungsten powder. It can be seen that tungsten powder possesses polyhedron type morphology and equiaxed structure. The typical grain is marked by the red arrow in Fig. 1a. The edges and corners of grains are very clear, and the initial tungsten powder appears
ACCEPTED MANUSCRIPT agglomeration. Fig. 1b shows the particle size distribution of initial tungsten powder. Most of initial tungsten powder (90%) has the particle size smaller than 1 μm and the average particle size is about 390nm. 3.2. The morphology of tungsten powder in ball milling process
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Fig.2 shows the SEM images of tungsten powder milled at different milling time
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with the BPR of 15:1. It can be seen that 60 hours of ball milling (Fig. 2e), plenty of
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nano-scale particles are present in the samples, and the formation of nano-scale particles through the whole ball milling process can be clearly seen in Fig.2a-2e.
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Fig. 1a shows that the initial tungsten powder has polyhedron type morphology
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and equiaxed structure, and the average particle size is about 390nm. Under the impact force and shear force exerted by the grinding balls and the vessel’s wall, the
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tungsten powder experiences compressional deformation during ball milling, at the
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same time, two or more tungsten powder particles weld together to form one much larger particle, and the tungsten powder appear welding due to the Van der Waals
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force as the tungsten atoms become close. In the range of attractive force, the Van der
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Waals force increases with the decrease of distance of atoms. Then, the particle gradually becomes flake structure due to the accumulation of deformation induced by being hit constantly as shown in Fig. 2a. The average particle size is about 460nm. With the increase of ball milling time, the flake structure of tungsten powder becomes much thinner and larger as shown from Fig. 2a to Fig. 2b, and the average particle size is about 2.0 micron after 20h ball milling shown in Fig. 2b. After weld stage, the tungsten powder undergoes unrecoverable deformation through the impact of milling
ACCEPTED MANUSCRIPT balls. The volume of tungsten particle keeps the same after weld stage. The particle becomes flake structure in the milling process and the thickness is decreased, as a result, the tungsten particles size in other dimension should be increased. The micro cracks start to appear on the flake structure tungsten powder and the tungsten powder
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is gradually smashed when the slice-layer tungsten is very thin. The nano-scale
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particles fall off from the slice-layer structure tungsten as shown in Fig. 2c. Then the
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tungsten powder is further refined observed from 40h to 50h ball milling tungsten as shown from Fig. 2c to Fig. 2d. The ductile-brittle transition temperature (DBTT) of
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tungsten is in the range of 373 K~673 K and it shows the brittleness of high strain
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below DBTT. The DBTT is the crossing point of yield strength and fracture strength. The temperature of tungsten powder in the milling process is lower than DBTT (In
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order to avoid the increase of temperature, the mill was running for 10 min and then
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stopped for 10 min, and repeated, so the temperature of the mill is measured to be around room temperature in the whole milling process.). It means the tungsten powder
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would break under the high strain induced by the impact of milling balls because it
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reaches fracture stage first. This indicates that the tungsten powder would fracture easily due to the high stress induced by the milling balls as the temperature during ball milling is below DBTT. Those finely-sized particles come from the fracturing of slice-layer structure tungsten particles. With the increase of the ball milling time, approximate spherical nano-scale tungsten particles with the average size of about 50 nm were formed shown in Fig. 2e with 60h ball milling. Although it is found that the nano-scale tungsten particles appear severe agglomeration due to high surface energy
ACCEPTED MANUSCRIPT of nano-scale tungsten particles shown in Fig. 2e, it has no effect on the following sintering process by using obtained ball milling tungsten powder. Fig.3 shows the schematic diagram of tungsten powder refinement process by ball milling based on the observation of morphology change with milling time. The
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process of ball milling on tungsten powder includes four stages which are welding
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stage, squeezing stage, fracturing stage and dynamic balance stage. (1) welding stage
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and squeezing stage: Tungsten powder weld together first and then squeeze to become flake structure. As a result, the tungsten powder becomes thin and large illustrated by
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the process from a to d in Fig. 3. (2) fracturing stage: with the impact of the milling
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ball, the tungsten powder is fractured and smashed, and the nano-scale particles fall off from the slice-layer structure tungsten illustrated by process e in Fig. 3. (3)
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dynamic balance stage: the tungsten powder particle size barely changes and reaches
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an equilibrium shown in process f in Fig. 3. 3.3 The influence of BPR on the morphology and microstructure of ball milling
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tungsten powder
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Fig.4 shows the SEM images of tungsten powder ball milled with the BPR of 4:1 at different milling time. It is obvious that the tungsten powder experiences welding stage and squeezing stage only and the tungsten powder becomes larger and thinner in the whole milling process as shown from Fig.4a to Fig.4c, and the average particle size reaches about 2.4 micron. No nano-scale particles are introduced by separating from the powder in the ball milling process. Fig.5 shows the SEM images of tungsten powder at different milling time with
ACCEPTED MANUSCRIPT the BPR of 10:1. The tungsten powder undergoes first three stages: (1) welding stage and squeezing stage: the ball milled tungsten powder particles go through welding stage and gradually becomes flake structure due to the accumulation of deformation by being hit constantly as shown in Fig. 5a, and the particle size gradually increases
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and the average particle size reaches 1.35 micron. With the increase of milling time,
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the flake structure tungsten powder becomes much thinner and larger as shown from
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Fig. 5a to Fig. 5b, and the average particle size reaches about 1.45 micron. (2) fracturing stage: the flake structure of tungsten powder becomes very thin and is
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partially smashed into nano-scale particle as shown in Fig. 5c, and the average
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particle size is about 110nm.
It is clearly seen that when the BPR of 4:1, the tungsten powder experiences the
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welding stage and squeezing stage only in the whole milling process, and the tungsten
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particle size gradually increases. When the BPR is 10:1, the tungsten powder undergoes first three stages. The tungsten particle size increases at ball milling time
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from 0h to 30h, and then the tungsten powder experiences fracturing stage. In this
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stage, nano-scale particles fall from the tungsten powder. When the BPR is 15:1, the tungsten powder experiences four milling stages, and finally the particle size reaches an equilibrium status. The reason why only BPR 15:1 experiences whole stages as the milling efficiency is the highest and energy provided by the planetary mill is the largest. The tungsten powder reaches fracture stage and dynamic balance stage faster than BPR of 10:1 and 4:1. The tungsten powder refinement process corresponding to the changes of tungsten particle size at different milling time is shown in Fig. 6. The
ACCEPTED MANUSCRIPT comparison of the process of ball milling stages for the samples with different BPRs is shown in Table 2. The mechanical alloying is a very complex dynamic process that possesses a lot of variables. In order to understand the influence of BPR on the morphology and
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alloying process and corresponding model are discussed.
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microstructure of tungsten powder, and its intrinsic mechanism, the mechanical
Several models for mechanical milling of powder are proposed [36,37]. A ball
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milling model that includes many technical parameters of ball milling and is suitable
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for planetary mill was put forward. This model was purposed by A. S. Kurlov and A. I. Gusev [38,39]. They consider two ways of the total energy consumed during ball
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milling (Emill). One is the energy consumed for the rupture of interatomic bonds in one particle in the initial powder (Erupt) and the other one is the energy consumed for
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creating the additional surface appearing upon the fragmentation of one particle in the
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initial powder (Esurf). This means that the total energy consumed for ball milling is E mill N ( E rupt E surf ) ,
where, N is the number of initial tungsten powder particles.
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In this case, the total energy consumed for ball milling equals to the energy provided by planetary mill expressed by
E mill t ，where 3
κ is a constant related to the given
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mill, ω is the angular speed of mill rotation and t is the milling time. Emill is a function of the mass of initial powder M, the initial average linear size of particles Din, the final particle size D(t,M) and the Burgers vector based on the expression above. The details regarding the derivation process can be clearly seen in references [35,36]. meanwhile, the powder particle size as a function of initial powder mass M, initial particle size Din and the applied energy Emill, is expressed as follows:
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where
M A B [ln( D in / 2 b )]
max
[ t /( t )][ M /( M p )]
(1)
t M A B ln( D in / 2 b ) max [ t /( t )][ M /( M p )] / D in 3
max [ t /( t )][ M /( M p )] 8 a k N b m ( R C r ) 3
2
2
64 3 ( r / R c )
1 2
RC
64 16 r / R c
4
(2)
2
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where D(t,M) is the final particle size through MA method of high energy ball milling and is determined by many parameters, such as material varieties including
, the numbers of milling ball Nb and the mass of milling ball
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M , D in , D t , M , t , M
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m and other constant parameters. For a certain material the parameters of A and B are
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constant.
In current study, the ball to powder ratio (BPR) is used as a variable to modify
A
B [ln( D in / 2 b )]
max
[ t /( t )][ 1 /( 1 p / M )]
t BPR A B ln( D in / 2 b ) max [ t /( t )][ 1 /( 1 p / M )] / D in '
3
8 a k ( R C r ) 3
2
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'
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D (t , M )
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the equation (1) as follows to illustrate the influence of BPR on tungsten powder.
2
1 2
RC
64 3 ( r / R c ) 64 16 r / R c
(3)
4
2
(4)
where, τ and p are the normalizing parameters. The model can be simplified as
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follows and C1、C2、C3 are constant. D ( t , M , BPR )
[ C 1 C 2 ( t , M )] C 3 BPR [ C 1 C 2 ( t , M )] / D in
(5)
In this paper, the milling time t is the same (60h), and the influence of the mass of initial tungsten powder on the microstrain can be negligible as the parameter p is very small, namely, in the later stage of ball milling we assume the ε is the same for different BPRs, so the equation (5) can be modified as:
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[C 1 C 2 ] C 3 BPR [ C 1 C 2 ] / D in
(6)
The formula describes the relationship between the final D and BPR. The bigger the BPR is, the smaller the particle size is. This model can reasonably qualitatively
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analyze the result of the powder size with different BPR at ball milling time of 60h and gives the reason why BPR of 15:1 has the most significant effect on the particle
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size of the powder.
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The intrinsic mechanism for the BPR’s effect on the powder during ball milling
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can be summarized as follows: the BPR determines the collision frequency and the velocity of the milling ball. The larger the BPR is, the higher the stress frequency due
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to collison frequency and the stress intensity due to the velocity of milling ball. As a result, the energy consumed for each powder particle is more when BPR is larger. As
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a consequence, it leads to different deformation degrees, and finally decides the particle size of the powder. The modified model reasonably considers the essential
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influence of BPR on milling effect, and illustrates the conclusions of the effect of BPR on powder in ball milling process.
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3.4. The grain size refinement of ball milling tungsten. Fig. 7 shows the TEM images of tungsten powder with different BPRs ball milled at 60h. Fig. 7a and Fig. 7b show the TEM images of tungsten powder with the BPR of 4:1. The tungsten powder appears flake structure with the particle size of about 3 micron (Fig. 7a) and no nano-scale particles formed in the milling process (Fig. 7b). Fig. 7b also shows many dislocation lines marked by the yellow arrows, and this indicates the initial stage of the formation process of tungsten nanograin. Fig. 7c
ACCEPTED MANUSCRIPT and Fig. 7d show the TEM images of tungsten powder with the BPR of 10:1 ball milled at 60h. Fig. 7c is BF TEM image and shows that many nanograins can be clearly seen marked by the yellow arrows. Fig. 7d shows the DF image from the same location shown in Fig. 7c. The grain size of three tungsten powders is 22nm, 23nm and 30nm. Fig. 7e and Fig. 7f show the BF and DF TEM images of tungsten powder
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from the same location with the BPR of 15:1 ball milled at 60h. It can be seen that
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many nanoscale grains marked by yellow arrows appear embedded in the small
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particle. Fig. 7f shows nanograin of tungsten powder with the grain size of 5-15nm marked by yellow arrows. Especially, the tungsten nanograin with the grain size of
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5nm is marked with red arrow, which indicates the tungsten grain with the minimum size of 5nm was successfully fabricated, a value approaches theoretical grain size by
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this method. The minimum tungsten nanograin of theoretical value is 5 nm, which is the critical equilibrium distance between the dislocation pileup consisting of two edge
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dislocations [40]. It is noted that these several nanograin tungsten powders show
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bright contrast in the DF image as they satisfy the diffraction condition. If one tilts an angle to another zone axis, the bright contrast will appear in different locations with
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the size of 5nm-15nm at the particle. This indicates that the bright contrasts are the nanograin tungsten that satisfies the diffraction condition, and the dark contrast area is
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nanograin tungsten as well. Fig. 8 is the comparison of diffraction peak of (110) from XRD patterns of initial tungsten powder and powder ball milled at 60h with different BPRs. In contrast to the initial tungsten powder, XRD results show that the diffraction peak width of ball milled tungsten powder broadens and the peak height decreases. This indicates the tungsten grain size decreases after 60h ball milling based on the calculation from
ACCEPTED MANUSCRIPT Scherrer equation. With the increase of BPR, the magnitude of peak broadening and decrease of peak height are more apparent, which indicates that BPR larger, grain size smaller under the same ball milling time. This result is consistent with the conclusions
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from Fig. 7 that the tungsten nanograin size decreases with the increase of BPR value.
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4. Conclusions
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The tungsten powder refinement process is understood based on the observations from experiments and includes four stages: (1) welding stage, (2)
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squeezing stage, (3) fracturing stage, and (4) dynamic balance stage. Only the samples
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with the BPR of 15:1 experience the whole process in high energy ball milling with ball milling time of 60h in contrast to the BPR of 10:1 and 4:1. As a result, the
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nanocrystalline tungsten powder with the grain size ranging from 5nm to 15nm was
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successfully fabricated by ball milling with the BPR of 15:1, and the minimum tungsten nanograin of theoretical value of 5nm was obtained for the first time by this
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method. The modified model illustrates that with the increase of BPR, the energy
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transfer to powder and the stress applied to powder increase, and thus the smaller particle size and grain size of tungsten powder are obtained.
Acknowledgments The authors thank Prof. D.G. Xia for his insightful discussion. This work was supported by National Magnetic Confinement Fusion Energy Research Project with number of 2015GB121004 from Ministry of Science and Technology of China and by
ACCEPTED MANUSCRIPT grants with number of 11375018 and 11528508 from National Science and Foundation of China (NSFC). The authors acknowledge the support by the key technology of nuclear energy, 2014, CAS Interdisciplinary Innovation Team.
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Additional Information
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Zaoming Wu and Yanxia Liang contributed equally to the work.
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Competing financial interests: The authors declare no competing financial interest.
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[22] K. Lu, J. Lu, Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment, Mat. Sci. Eng. A. 375-377 (2004) 38-45. [23] B. Sumit, S. Satyam, U. Tamas, C. Kaushik, Elucidating microstructural evolution and strengthening mechanisms in nanocrystalline surface induced by surface mechanical attrition treatment of stainless steel. Acta Mater. 122 (2017)138-151. [24] L. Zhou, G. Liu, X.L. Ma, K. Lu, Strain-induced refinement in a steel with
ACCEPTED MANUSCRIPT spheroidal cementite subjected to surface mechanical attrition treatment, Acta. Mater. 56 (2008) 78-87. [25] M.T. Tsai, J.C. Huang, W.Y. Tsai, T.H. Chou, C.F. Chen; T.H. Li, J.S.C. Jang, Effects of ultrasonic surface mechanical attrition treatment on microstructures and
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ACCEPTED MANUSCRIPT The graphical abstract shows the powder refinement process and its schematic diagram. The changes of particle size with different BPR at different milling time was also shown. The nanocrystalline tungsten powder with grain size ranging from 5-15
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nm was successfully fabricated by ball milling with the BPR of 15:1.
ACCEPTED MANUSCRIPT Highlights
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The effect of ball to powder ratio on W powder refinement process was revealed. The ball-milled W powder refinement process includes four stages. Only the W powder with ball to powder ratio of 15:1 experiences the whole stage. The final particle and grain size decrease with the increase of ball to powder ratio. Nanocrystalline W powder with grain size of 5 nm-15 nm was fabricated.
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Figure 1
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Z.M. Wu, Y.X. Liang, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu PII: DOI: Reference:
S0032-5910(18)30593-X doi:10.1016/j.powtec.2018.07.094 PTEC 13576
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
13 March 2018 4 June 2018 29 July 2018
Please cite this article as: Z.M. Wu, Y.X. Liang, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu , The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling. Ptec (2018), doi:10.1016/j.powtec.2018.07.094
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ACCEPTED MANUSCRIPT
The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling Z.M. Wu†, Y.X. Liang†, Y. Fan, P.P. Wang, J.L. Du, Y.B. Zhao, E.G. Fu*
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State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking
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University, Beijing 100871, P. R. China
Tel: +86-10-62750612, Fax: +86-10-62751873
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Email: [email protected] or [email protected]
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*Corresponding author: Engang Fu
Present address: 201 Chengfu Road, Haidian District, Beijing, 100871, China.
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†: Zaoming Wu and Yanxia Liang contributed equally to the work.
ACCEPTED MANUSCRIPT Abstract: The nanocrystalline tungsten powder with the minimum grain size of around 5nm was fabricated by mechanical alloying. The powder refinement process and the influence of ball to powder ratio (BPR) on the morphology and microstructure of
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powder were investigated. The morphology and microstructure were characterized by
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X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM) and
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transmission electron microscope (TEM). Results revealed that the high energy ball milling process has four stages. Compared with the samples with the BPR of 10:1 and
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4:1 under the same ball milling conditions, the samples with the BPR of 15:1
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experiences the whole four stages after 60h ball milling. One key factor in determining the final morphology and microstructure of tungsten powder by ball
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the particle size is discussed.
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milling was revealed. The model to illustrate the fundamental mechanism for refining
Keywords:
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Morphology, Microstructure, Nanocrystalline tungsten, Refinement, Ball to powder
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ratio, Ball milling
ACCEPTED MANUSCRIPT 1. Introduction Tungsten is one of the most promising plasma facing materials (PFMs) in fusion environment of high-dose neutron radiation and high heat flux due to its excellent properties such as high melting point (3410℃), low thermal expansion coefficient,
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good thermal conductivity, low sputtering yield, low retention of hydrogen and high
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elevated temperature strength [1-6]. However, some other properties of tungsten
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restrict its applications in fusion reactor. For instance, tungsten with high ductile-to-brittle transition temperature (373~673K) shows the brittleness at low
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temperature; The grain size of tungsten is easy to grow up in high temperature
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environment due to its low recrystallization temperature (1423~1623K), which degrades its mechanical properties including strength and hardness ; in addition, long
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term neutron radiation results in serious degradation of materials, such as swelling
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and fracture induced by the nuclear transmutation and its products [7-12]. So it is very necessary to improve the properties of tungsten serving in the harsh environment.
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Research shows that the refinement of tungsten grain size can improve the
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mechanical properties and the radiation tolerance of tungsten based materials apparently. The nanocrystalline materials were first proposed by H. Gleiter in the early 1980s [13], and their properties (especially mechanical properties) were well studied due to their unique structural feature and outstanding performance. Currently, top-down method and bottom-up method are two major approaches to prepare nanocrystalline tungsten [14]. Top-down method is a direct way to refine the grain by severe plastic deformation repetitively of coarse grain solid materials, such as equal
ACCEPTED MANUSCRIPT channel angular extrusion (ECAE) [15-17], high-pressure torsion (HPT) [15,16,18], accumulative roll bonding (ARB) [19,20] and surface mechanical attrition treatment (SMAT) [21-25]. Bottom-up method adopts mechanical alloying (MA) [26-31] or other approaches such as wet chemical method [32-34] to obtain the nano-size
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precursor powder, then the precursor powder is sintered to fabricate the ultrafine grain
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or nanocrystalline bulk materials. This method usually refers to powder metallurgy
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(PM).
Q. Yan used wet chemical method to fabricate core-shell structure W-TiC and
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W-La2O3 powder with the particle size from 50nm to 200nm [32-34]. K. Ameyama
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adopted mechanical alloying (MA) process to ball mill W-10wt%Re powder and obtained nano-scale W-Re powder, which contains nano grain structure with the grain
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size of 10-20nm [26]. E. Oda used mechanical milling method to obtain 12nm wide
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and 25nm long nano-scale tungsten grain powder [27]. J. Chen et al. adapted MA method to refine the grain of W-15wt.%Nb alloy. They obtained tungsten powder with
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grain size of 28.3 nm after 45h ball milled at milling speed of 400rpm and ball to
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powder ratio (BPR) of 20:1 [31]. Many methods have been put forward to refine the grain size of tungsten powder or tungsten bulk materials. ECAE, HPT, ARB and SMAT are the methods to refine the bulk tungsten materials; however, they cannot be able to successfully form the tungsten grain with the size range from 5nm to 15nm. MA as a method to refine the powder adopts high energy ball milling to obtain the powder with nanograin as MA is able to give extremely large amount of strain and high deformation rate. Especially, it
ACCEPTED MANUSCRIPT is possible to obtain the powder with the minimum grain size of around 5nm, a theoretical value that equals to the equilibrium distance of two edge dislocations deduced by a formula proposed by T.G. Nieh [35]. As a typical MA, high energy ball milling process is a very complex dynamic
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process that possesses a lot of variables, such as the ball to powder ratio (BPR),
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milling speed, number and type of milling ball, the amount of process control agent,
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etc. High energy ball milling is a mechanical alloying process that can be used to fabricate nanocrystalline powder and other advanced materials due to its extremely
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large amount of strain and high deformation rate. It is very vital to optimize the
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technical parameters of high energy ball milling as each of them plays important role in determining the final properties of the powder.
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In this paper, experiments with the different BPRs were conducted to investigate
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the refinement process and the influence of BPR on morphology and microstructure of tungsten powder in the process of ball milling, and the fundamental mechanism for
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well.
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refining the particle size and the influence of BPR during ball milling is discussed as
2. Experimental procedures 2.1. The preparation of nanocrystalline tungsten powder The nanocrystalline tungsten powder was fabricated by MA method of high energy ball milling in this study. Commercial pure tungsten powder with the average particle size of about 390nm and a purity of 99.9% was used as initial powder. The
ACCEPTED MANUSCRIPT samples were mechanically alloyed in Fritsch pulverisette 7 premium line planetary mill with the two vessels made of zirconium oxide. The balls of diameter 1 mm were also made of zirconium oxide. The Fritsch pulverisette 7 premium line planetary mill possesses two pots and each pot volume is 80 ml. The 1 mm diameter balls were
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adopted here due to its high surface area compared with larger balls. It increases the
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milling efficiency of ball milling. Tungsten powder and grinding balls were weighed
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by the precision balance (with the accuracy of 0.0001g) respectively to make samples with the BPR (the ratio of milling balls mass to powder mass) of 4:1, 10:1 and 15:1.
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In order to avoid agglomeration of the powder, the wet milling method was utilized by
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adding ethanol with 8 wt. % of the total weight of balls and powder as process control agent (PCA). The samples were milled for the total time of 60h with the rotate speed
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of 500rpm in the purified N2 gas (purity 99.999%). The experimental conditions of
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ball milled tungsten powder with different BPR are shown in Table 1. The procedure to make scanning electron microscopy (SEM) and transmission
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electron microscopy (TEM) samples of tungsten powder is as follows: (1) ball milling
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tungsten powder samples were taken out into centrifuge tube every 5 hours in glove box filled with N2 gas; (2) 1 ml ethanol was added to the samples made at the different milling time and with the different BPRs to dilute the samples; (3) the diluted samples were ultrasonic dispersed for 40 minutes; (4) one drop of solution was added by dropper into conductive carbon glue and ultra-thin carbon film to make SEM samples and TEM samples, respectively; (5) the SEM and TEM samples were put into vacuum drying oven to dry for 20 minutes at the temperature of 30℃.
ACCEPTED MANUSCRIPT 2.2. Microstructural characterization The morphology and microstructure for different powder refinement stages were characterized by the secondary electrons images of FEI Nano SEM 430 field-emission scanning electron microscope (FE-SEM), BF (BF) images and dark field (DF) images
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of FEI Tecnai F30 transmission electron microscope (TEM), and high resolution TEM
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(HRTEM). The Empyrean high resolution powder X-ray diffractometer (XRD) was
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utilized to characterize the grain size of tungsten powder before and after ball milling. The particle size was obtained from the SEM images of tungsten powder at different
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milling time. The software Nanomeasurer was used on the SEM images to make
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statistics of particle size. In order to obtain the reliable particle size, plenty of SEM images of each sample were randomly selected and the particle size was recorded on
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different positions. More than 1000 particles for each sample were used to determine
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the Martin particle size based on the number percentage. The grain size of ball-milled tungsten powder was obtained by the DF TEM image. We chose one diffraction spot
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in the (110) crystal direction and entangled this point with the aperture of an objective
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lens to obtain the dark-field TEM image and grain size. 3. Results and discussions 3.1. Initial tungsten powder Fig.1a shows the SEM image for the morphology of initial tungsten powder. It can be seen that tungsten powder possesses polyhedron type morphology and equiaxed structure. The typical grain is marked by the red arrow in Fig. 1a. The edges and corners of grains are very clear, and the initial tungsten powder appears
ACCEPTED MANUSCRIPT agglomeration. Fig. 1b shows the particle size distribution of initial tungsten powder. Most of initial tungsten powder (90%) has the particle size smaller than 1 μm and the average particle size is about 390nm. 3.2. The morphology of tungsten powder in ball milling process
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Fig.2 shows the SEM images of tungsten powder milled at different milling time
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with the BPR of 15:1. It can be seen that 60 hours of ball milling (Fig. 2e), plenty of
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nano-scale particles are present in the samples, and the formation of nano-scale particles through the whole ball milling process can be clearly seen in Fig.2a-2e.
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Fig. 1a shows that the initial tungsten powder has polyhedron type morphology
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and equiaxed structure, and the average particle size is about 390nm. Under the impact force and shear force exerted by the grinding balls and the vessel’s wall, the
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tungsten powder experiences compressional deformation during ball milling, at the
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same time, two or more tungsten powder particles weld together to form one much larger particle, and the tungsten powder appear welding due to the Van der Waals
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force as the tungsten atoms become close. In the range of attractive force, the Van der
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Waals force increases with the decrease of distance of atoms. Then, the particle gradually becomes flake structure due to the accumulation of deformation induced by being hit constantly as shown in Fig. 2a. The average particle size is about 460nm. With the increase of ball milling time, the flake structure of tungsten powder becomes much thinner and larger as shown from Fig. 2a to Fig. 2b, and the average particle size is about 2.0 micron after 20h ball milling shown in Fig. 2b. After weld stage, the tungsten powder undergoes unrecoverable deformation through the impact of milling
ACCEPTED MANUSCRIPT balls. The volume of tungsten particle keeps the same after weld stage. The particle becomes flake structure in the milling process and the thickness is decreased, as a result, the tungsten particles size in other dimension should be increased. The micro cracks start to appear on the flake structure tungsten powder and the tungsten powder
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is gradually smashed when the slice-layer tungsten is very thin. The nano-scale
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particles fall off from the slice-layer structure tungsten as shown in Fig. 2c. Then the
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tungsten powder is further refined observed from 40h to 50h ball milling tungsten as shown from Fig. 2c to Fig. 2d. The ductile-brittle transition temperature (DBTT) of
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tungsten is in the range of 373 K~673 K and it shows the brittleness of high strain
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below DBTT. The DBTT is the crossing point of yield strength and fracture strength. The temperature of tungsten powder in the milling process is lower than DBTT (In
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order to avoid the increase of temperature, the mill was running for 10 min and then
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stopped for 10 min, and repeated, so the temperature of the mill is measured to be around room temperature in the whole milling process.). It means the tungsten powder
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would break under the high strain induced by the impact of milling balls because it
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reaches fracture stage first. This indicates that the tungsten powder would fracture easily due to the high stress induced by the milling balls as the temperature during ball milling is below DBTT. Those finely-sized particles come from the fracturing of slice-layer structure tungsten particles. With the increase of the ball milling time, approximate spherical nano-scale tungsten particles with the average size of about 50 nm were formed shown in Fig. 2e with 60h ball milling. Although it is found that the nano-scale tungsten particles appear severe agglomeration due to high surface energy
ACCEPTED MANUSCRIPT of nano-scale tungsten particles shown in Fig. 2e, it has no effect on the following sintering process by using obtained ball milling tungsten powder. Fig.3 shows the schematic diagram of tungsten powder refinement process by ball milling based on the observation of morphology change with milling time. The
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process of ball milling on tungsten powder includes four stages which are welding
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stage, squeezing stage, fracturing stage and dynamic balance stage. (1) welding stage
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and squeezing stage: Tungsten powder weld together first and then squeeze to become flake structure. As a result, the tungsten powder becomes thin and large illustrated by
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the process from a to d in Fig. 3. (2) fracturing stage: with the impact of the milling
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ball, the tungsten powder is fractured and smashed, and the nano-scale particles fall off from the slice-layer structure tungsten illustrated by process e in Fig. 3. (3)
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dynamic balance stage: the tungsten powder particle size barely changes and reaches
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an equilibrium shown in process f in Fig. 3. 3.3 The influence of BPR on the morphology and microstructure of ball milling
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tungsten powder
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Fig.4 shows the SEM images of tungsten powder ball milled with the BPR of 4:1 at different milling time. It is obvious that the tungsten powder experiences welding stage and squeezing stage only and the tungsten powder becomes larger and thinner in the whole milling process as shown from Fig.4a to Fig.4c, and the average particle size reaches about 2.4 micron. No nano-scale particles are introduced by separating from the powder in the ball milling process. Fig.5 shows the SEM images of tungsten powder at different milling time with
ACCEPTED MANUSCRIPT the BPR of 10:1. The tungsten powder undergoes first three stages: (1) welding stage and squeezing stage: the ball milled tungsten powder particles go through welding stage and gradually becomes flake structure due to the accumulation of deformation by being hit constantly as shown in Fig. 5a, and the particle size gradually increases
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and the average particle size reaches 1.35 micron. With the increase of milling time,
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the flake structure tungsten powder becomes much thinner and larger as shown from
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Fig. 5a to Fig. 5b, and the average particle size reaches about 1.45 micron. (2) fracturing stage: the flake structure of tungsten powder becomes very thin and is
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partially smashed into nano-scale particle as shown in Fig. 5c, and the average
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particle size is about 110nm.
It is clearly seen that when the BPR of 4:1, the tungsten powder experiences the
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welding stage and squeezing stage only in the whole milling process, and the tungsten
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particle size gradually increases. When the BPR is 10:1, the tungsten powder undergoes first three stages. The tungsten particle size increases at ball milling time
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from 0h to 30h, and then the tungsten powder experiences fracturing stage. In this
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stage, nano-scale particles fall from the tungsten powder. When the BPR is 15:1, the tungsten powder experiences four milling stages, and finally the particle size reaches an equilibrium status. The reason why only BPR 15:1 experiences whole stages as the milling efficiency is the highest and energy provided by the planetary mill is the largest. The tungsten powder reaches fracture stage and dynamic balance stage faster than BPR of 10:1 and 4:1. The tungsten powder refinement process corresponding to the changes of tungsten particle size at different milling time is shown in Fig. 6. The
ACCEPTED MANUSCRIPT comparison of the process of ball milling stages for the samples with different BPRs is shown in Table 2. The mechanical alloying is a very complex dynamic process that possesses a lot of variables. In order to understand the influence of BPR on the morphology and
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alloying process and corresponding model are discussed.
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microstructure of tungsten powder, and its intrinsic mechanism, the mechanical
Several models for mechanical milling of powder are proposed [36,37]. A ball
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milling model that includes many technical parameters of ball milling and is suitable
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for planetary mill was put forward. This model was purposed by A. S. Kurlov and A. I. Gusev [38,39]. They consider two ways of the total energy consumed during ball
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milling (Emill). One is the energy consumed for the rupture of interatomic bonds in one particle in the initial powder (Erupt) and the other one is the energy consumed for
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creating the additional surface appearing upon the fragmentation of one particle in the
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initial powder (Esurf). This means that the total energy consumed for ball milling is E mill N ( E rupt E surf ) ,
where, N is the number of initial tungsten powder particles.
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In this case, the total energy consumed for ball milling equals to the energy provided by planetary mill expressed by
E mill t ，where 3
κ is a constant related to the given
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mill, ω is the angular speed of mill rotation and t is the milling time. Emill is a function of the mass of initial powder M, the initial average linear size of particles Din, the final particle size D(t,M) and the Burgers vector based on the expression above. The details regarding the derivation process can be clearly seen in references [35,36]. meanwhile, the powder particle size as a function of initial powder mass M, initial particle size Din and the applied energy Emill, is expressed as follows:
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where
M A B [ln( D in / 2 b )]
max
[ t /( t )][ M /( M p )]
(1)
t M A B ln( D in / 2 b ) max [ t /( t )][ M /( M p )] / D in 3
max [ t /( t )][ M /( M p )] 8 a k N b m ( R C r ) 3
2
2
64 3 ( r / R c )
1 2
RC
64 16 r / R c
4
(2)
2
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where D(t,M) is the final particle size through MA method of high energy ball milling and is determined by many parameters, such as material varieties including
, the numbers of milling ball Nb and the mass of milling ball
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M , D in , D t , M , t , M
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m and other constant parameters. For a certain material the parameters of A and B are
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constant.
In current study, the ball to powder ratio (BPR) is used as a variable to modify
A
B [ln( D in / 2 b )]
max
[ t /( t )][ 1 /( 1 p / M )]
t BPR A B ln( D in / 2 b ) max [ t /( t )][ 1 /( 1 p / M )] / D in '
3
8 a k ( R C r ) 3
2
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'
D
D (t , M )
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the equation (1) as follows to illustrate the influence of BPR on tungsten powder.
2
1 2
RC
64 3 ( r / R c ) 64 16 r / R c
(3)
4
2
(4)
where, τ and p are the normalizing parameters. The model can be simplified as
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follows and C1、C2、C3 are constant. D ( t , M , BPR )
[ C 1 C 2 ( t , M )] C 3 BPR [ C 1 C 2 ( t , M )] / D in
(5)
In this paper, the milling time t is the same (60h), and the influence of the mass of initial tungsten powder on the microstrain can be negligible as the parameter p is very small, namely, in the later stage of ball milling we assume the ε is the same for different BPRs, so the equation (5) can be modified as:
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[C 1 C 2 ] C 3 BPR [ C 1 C 2 ] / D in
(6)
The formula describes the relationship between the final D and BPR. The bigger the BPR is, the smaller the particle size is. This model can reasonably qualitatively
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analyze the result of the powder size with different BPR at ball milling time of 60h and gives the reason why BPR of 15:1 has the most significant effect on the particle
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size of the powder.
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The intrinsic mechanism for the BPR’s effect on the powder during ball milling
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can be summarized as follows: the BPR determines the collision frequency and the velocity of the milling ball. The larger the BPR is, the higher the stress frequency due
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to collison frequency and the stress intensity due to the velocity of milling ball. As a result, the energy consumed for each powder particle is more when BPR is larger. As
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a consequence, it leads to different deformation degrees, and finally decides the particle size of the powder. The modified model reasonably considers the essential
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influence of BPR on milling effect, and illustrates the conclusions of the effect of BPR on powder in ball milling process.
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3.4. The grain size refinement of ball milling tungsten. Fig. 7 shows the TEM images of tungsten powder with different BPRs ball milled at 60h. Fig. 7a and Fig. 7b show the TEM images of tungsten powder with the BPR of 4:1. The tungsten powder appears flake structure with the particle size of about 3 micron (Fig. 7a) and no nano-scale particles formed in the milling process (Fig. 7b). Fig. 7b also shows many dislocation lines marked by the yellow arrows, and this indicates the initial stage of the formation process of tungsten nanograin. Fig. 7c
ACCEPTED MANUSCRIPT and Fig. 7d show the TEM images of tungsten powder with the BPR of 10:1 ball milled at 60h. Fig. 7c is BF TEM image and shows that many nanograins can be clearly seen marked by the yellow arrows. Fig. 7d shows the DF image from the same location shown in Fig. 7c. The grain size of three tungsten powders is 22nm, 23nm and 30nm. Fig. 7e and Fig. 7f show the BF and DF TEM images of tungsten powder
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from the same location with the BPR of 15:1 ball milled at 60h. It can be seen that
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many nanoscale grains marked by yellow arrows appear embedded in the small
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particle. Fig. 7f shows nanograin of tungsten powder with the grain size of 5-15nm marked by yellow arrows. Especially, the tungsten nanograin with the grain size of
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5nm is marked with red arrow, which indicates the tungsten grain with the minimum size of 5nm was successfully fabricated, a value approaches theoretical grain size by
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this method. The minimum tungsten nanograin of theoretical value is 5 nm, which is the critical equilibrium distance between the dislocation pileup consisting of two edge
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dislocations [40]. It is noted that these several nanograin tungsten powders show
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bright contrast in the DF image as they satisfy the diffraction condition. If one tilts an angle to another zone axis, the bright contrast will appear in different locations with
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the size of 5nm-15nm at the particle. This indicates that the bright contrasts are the nanograin tungsten that satisfies the diffraction condition, and the dark contrast area is
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nanograin tungsten as well. Fig. 8 is the comparison of diffraction peak of (110) from XRD patterns of initial tungsten powder and powder ball milled at 60h with different BPRs. In contrast to the initial tungsten powder, XRD results show that the diffraction peak width of ball milled tungsten powder broadens and the peak height decreases. This indicates the tungsten grain size decreases after 60h ball milling based on the calculation from
ACCEPTED MANUSCRIPT Scherrer equation. With the increase of BPR, the magnitude of peak broadening and decrease of peak height are more apparent, which indicates that BPR larger, grain size smaller under the same ball milling time. This result is consistent with the conclusions
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from Fig. 7 that the tungsten nanograin size decreases with the increase of BPR value.
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4. Conclusions
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The tungsten powder refinement process is understood based on the observations from experiments and includes four stages: (1) welding stage, (2)
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squeezing stage, (3) fracturing stage, and (4) dynamic balance stage. Only the samples
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with the BPR of 15:1 experience the whole process in high energy ball milling with ball milling time of 60h in contrast to the BPR of 10:1 and 4:1. As a result, the
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nanocrystalline tungsten powder with the grain size ranging from 5nm to 15nm was
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successfully fabricated by ball milling with the BPR of 15:1, and the minimum tungsten nanograin of theoretical value of 5nm was obtained for the first time by this
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method. The modified model illustrates that with the increase of BPR, the energy
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transfer to powder and the stress applied to powder increase, and thus the smaller particle size and grain size of tungsten powder are obtained.
Acknowledgments The authors thank Prof. D.G. Xia for his insightful discussion. This work was supported by National Magnetic Confinement Fusion Energy Research Project with number of 2015GB121004 from Ministry of Science and Technology of China and by
ACCEPTED MANUSCRIPT grants with number of 11375018 and 11528508 from National Science and Foundation of China (NSFC). The authors acknowledge the support by the key technology of nuclear energy, 2014, CAS Interdisciplinary Innovation Team.
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Additional Information
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Zaoming Wu and Yanxia Liang contributed equally to the work.
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Competing financial interests: The authors declare no competing financial interest.
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ACCEPTED MANUSCRIPT The graphical abstract shows the powder refinement process and its schematic diagram. The changes of particle size with different BPR at different milling time was also shown. The nanocrystalline tungsten powder with grain size ranging from 5-15
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nm was successfully fabricated by ball milling with the BPR of 15:1.
ACCEPTED MANUSCRIPT Highlights
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The effect of ball to powder ratio on W powder refinement process was revealed. The ball-milled W powder refinement process includes four stages. Only the W powder with ball to powder ratio of 15:1 experiences the whole stage. The final particle and grain size decrease with the increase of ball to powder ratio. Nanocrystalline W powder with grain size of 5 nm-15 nm was fabricated.
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Figure 1
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