Mechanical alloying and magnetic saturation of tungsten–nickel powders

Int. Journal of Refractory Metals and Hard Materials 31 (2012) 247–252

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Mechanical alloying and magnetic saturation of tungsten–nickel powders Kasonde Maweja a,⁎, T. Montong b, L. Moyo b, M.J. Phasha c a b c

Element Six Ltd, PO Box 560, Springs 1559, South Africa University of the Witwatersrand, School of Chemical and Metallurgical engineering, South Africa The Council for Scientific and Industrial Research, Materials Science and Manufacturing, Pretoria, South Africa

a r t i c l e

i n f o

Article history: Received 21 August 2011 Accepted 1 December 2011 Keywords: Tungsten Nickel High energy ball milling Mechanical alloying Magnetic saturation

a b s t r a c t Alloying mechanism and magnetic saturation of tungsten and W-40 wt.% Ni milled powders were investigated using XRD, SEM and saturation magnetisation techniques. Mechanical alloying was proceeded by deformation of FCC Ni toward FCT phase and BCC to BCT in W, hence formation of supersaturated tetragonal Ni(W) solid solution. Milling of pure W yielded a product comprised of magnetic BCT and non-magnetic nanocrystalline BCC W powders. The magnetic saturation of W increased at the early milling stage and decreased later due to the transition of the BCC W structure toward anisotropic close packed crystal structure and formation of nanograins with high specific surface. Magnetic saturation of W–Ni powders decreased with milling time but increased after forming a metastable tetragonal solid solution. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Enhanced mechanical properties, resistance of thermal shock and corrosion behavior are achieved by alloying Ni with W. These alloys received considerable attention as the substrates for coated high temperature superconductors (HTS) because they develop a very strong cube texture. However, a limiting factor of the Ni-alloy based HTS wires is their ferromagnetism with a Curie temperature of 364 °C and a saturation magnetisation of 57.5 emu g− 1 [1–3]. It has been demonstrated that mechanical alloying of Ni-3 at.% W by high energy ball milling was beneficial to developing higher degree of cube texture in Ni–W tapes and improved the mechanical strength of Ni [3]. Thompson et al. [1] reported lower magnetisation of Ni-based alloys than pure Ni. Wu et al. [4] and Fuster et al. [5] noticed that the crystal lattice parameters of pure BCC Mn powders processed by ball milling increased. The strained crystals were high-spin states, which traduced into higher magnetism properties [4,5]. Full dense W–Ni based alloys are typically processed by liquid phase sintering of powder compacts, which results in coarse final microstructures and some level of distortion [6,7]. Nanocrystalline metastable have been produced by ball milling of W-containing alloys and low sintering temperature was achieved [8–14]. Genç et al. [15] reported a maximum W solubility of 17.86 at.% in Ni achieved in Ni-30 wt.% W milled powders. Magnetic properties strongly depend on the chemical composition of the local environment of atoms and their electronic structure in the

⁎ Corresponding author. Tel.: + 27 83 365 0952 (mobile). E-mail address: [email protected] (K. Maweja). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.12.003

bulk materials [16–24]. It was observed that the magnetic saturation of zinc ferrite powder processed by high energy ball milling increased with milling time between 0 and 5 h [25]. It has also been reported that the magnetic saturation of Ni powders mechanically alloyed with Fe, Mo and Nb slightly increased at the early stage of milling before it decreased as a result of the electronic interactions between magnetic and non-magnetic elements [26–28]. Mechanical milling of a supersaturated W-40 wt.% Ni powder mixture and its magnetic saturation were investigated in the present work. The effects of high energy milling on the alloying process were investigated by means of X-ray diffraction (XRD) analysis of changes in crystallite sizes and lattice strains. Diffusion of atoms between powder particles was analyzed by means of scanning electron microscopy (SEM) techniques. The effects of mechanical milling on magnetic saturation of pure W powder were also determined for a better understanding of the effects on the properties of W-40 wt.% Ni system. Composition of powder mixtures of 60 wt.% W-40 wt.% Ni was selected to form a targeted single phase 4Ni: W product for use as refractory binder for hard materials, which melt at 1495 °C. Mixtures of brittle intermetallics, which have higher melting temperatures, are expected to form at the higher W contents. 2. Material and experiments Milling experiments of 100 g of W powder at ~99.5% purity (average particle size of 1–2 μm) and that of 100 g powder mixtures of 60 wt.%W and 40 wt.% Ni (~99.5% purity, average particle size of 2.2–3 μm) prepared in a glove box filled with argon were conducted in a Retsch PM400 MA-type planetary high energy ball mill. The milling media consisted of 5 mm diameter 3Cr12 stainless steel balls. Milling speed of 350 rpm, ball-to-powder ratio of 10:1 and milling durations of 12, 24

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and 48 h were used. The particle size distribution of the powders was determined using the Malvern in hexane or de-ionized water. Lattice strains and crystallite sizes evolutions including possible crystallographic transformations of powders during mechanical milling were analyzed using an X'Pert Philips powder diffractometer with Ni filtered Co-Kα radiation. The crystallite sizes (L) and lattice strains (e) were determined by means of the modified Williamson–Hall method [29].

λ δð2Þ ¼ þ 4e sinθ L

ð1Þ

a particle size [micronmetres]

248

60 50 40

20

3. Results and discussion 3.1. Particle size distribution and microstructure The particle size distribution parameters of milled pure W powder presented minima at 12 h (Fig. 1a), which indicates that fracturing of the particles prevailed at the early stage of milling. The d10, d50 and d90 of the powders were smaller than those of the starting powders and those of the products milled longer than 12 h. The increased particle size of W powders milled for longer times was ascribed to the increase in the degree of cold welding between finer ductile particles. Thus, cold welding became the dominant process at longer milling durations until steady state condition is reached. On the other hand, the particle size distribution parameters of the milled W-40 wt.% Ni powder mixtures followed different trends (Fig. 1b) than pure W powder. The particle size parameters increased at the early stage of milling, this is because the larger Ni particles were more ductile than W. Thus, the harder W particles were either dispersed into or binded by the Ni particles. This process delayed the fracturing and grain refinement of W particles that were observed at the early stage of milling of pure W powders. Therefore, cold welding is evident at the early stages of milling W in the presence of ductile Ni powder. The d10, d50 and d90 trends of W-40 wt.% Ni milled powders were characterized by a cyclic development between cold welding and fracturing of the particles. BSE–SEM micrographs of non milled W-40 wt.% Ni powder mixture and those milled for 12 h, 24 h and 48 h are shown in Fig. 2(a) through to 2(d) respectively. Fig. 2(b)–(d) illustrates the mixing of the two constituents and the cold welding of the grains after milling. The BSE–SEM homogeneous contrast of the cross sections of the particles suggests the formation of a solid solution, which has been analyzed by XRD and EDX techniques. Typical EDX graph of cross sections of particles milled for 24 or 48 h is shown in Fig. 2(e). The average composition of the grains of the homogeneous products determined

d50 µm

10 0

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milling time [h]

b particle size [micronmetres]

where θ and δ(2θ) are the diffraction angle and the corresponding full width at half maximum (FWHM) of the peak, and λ is the wavelength of the X-ray. The milled samples were cold mounted on epoxy resin and were polished using a series of SiC grit papers to cut through the crosssection of the powder particles. The microstructures of particles were analyzed in a Jeol JSM-7500F field Emission SEM equipped with EDX detector. A commercial saturation induction measuring system (LDJ Electronics Inc.) was employed. The test pieces, which consisted of powder samples or piles compacted at an ambient temperature under 40 MPa in a uniaxial press, were inserted into a high-intensity permanent magnet field (~0.75 T), and then quickly withdrawn. Compaction of piles reduced the risk of spilling of powder particles inside the testing chamber during the extraction out of the magnetic field. The response of a search coil was displayed on a magnetic multimeter in terms of magnetic moment (saturation). The system was calibrated using pure cobalt and pure nickel references. Each measurement was repeated twice to check consistency of results.

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milling time [h] Fig. 1. Effect of milling time on the particle size distribution parameters of (a) pure W and (b) mixture of W-40 wt.% Ni powders.

by EDX was 64 ± 2 wt.% W and 33 ± 3 wt.% Ni. Iron contamination of milled powders by the milling media was lower than 5 wt.%. The element distributions in powder milled for 24 h were uniform within grains as shown in Fig. 3. Small elongated particles of W-rich product were observed in the powders milled for 12 h (bright contrast in Fig. 2b). 3.2. Crystal structure evolution of W and W-40 wt.% Ni milled powders The intensity of the XRD peaks of W, shown in Fig. 4, systematically decreased and broadened with increasing milling times due to the straining and the refinement of the crystallites. The XRD pattern of the powders milled for 48 h was a characteristic of nanocrystalline materials with nearly flattened peaks. However, upon milling for 24 and 48 h, a small peak emerged at the 2θ angle of ~ 52°. The appearance of this new peak was attributed to deformation along (011) of tungsten BCC lattice induced by ball milling, implying compression along lattice parameters a and b as evidenced by shifting of peak position toward higher angles. Hence, the resulting tetragonal lattice deformation indicates the beginning of transformation toward more closely-packed structures such as HCP, FCC or orthorhombic. The ICSD data [30] suggests that the new peak is the (022) peak of an orthorhombic phase. The intensity of this new peak increased with further milling which implies more W particles being deformed to a metastable phase. On the other hand, the effect of milling on the XRD patterns of W40 wt.% Ni powder mixtures is presented in Fig. 5, showing that the starting powders contained BCC W and FCC Ni. Upon 12 h milling, the diffraction peaks of Ni (111) plane have shifted to lower 2θ angles due to the expansion of the lattice parameter a of FCC Ni from 0.3519 nm to 0.3577 nm. If the unit cell volume of Ni is kept constant, then the observed lattice expansion implies phase transformation toward face-centered tetragonal (FCT) structure with lattice parameters a = b = 0.3577 and c = 0.3406 nm, as was observed by Bolokang and Phasha after cold pressing milled Ni powder [31]. The d-spacing

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Fig. 2. BSE–SEM micrographs of (a) W-40 wt.% Ni powder mixture, (b) milled for 12 h, (c) 24 h, (d) milled for 48 h illustrating the mixing of the two constituents and the cold welding of the grains after milling and (e) the area EDX graph of product milled for 24 h . (Dark areas contained resin).

of Ni calculated using the Ni(111), peak at 2θ–52.243°, increased from 0.20317 nm to 0.20930 nm. After 24 h milling, the intensity of (101) peak became stronger in addition to a broad peak between 51 and 52° positions as a result of crystal structure coherency of FCT Ni and BCT(body centered tetragonal) W metastable phases forming tetragonal solid solution. This solid solution is persistent after 48 h alongside nanocrystalline BCC tungsten. The presence of FCC Ni helps in wetting the BCC W crystal so that the grain refinement and subsequent deformation were delayed as noticed in Fig. 1, and discussed in Section 3.1. It is thus Ni that got deformed first into FCT structure and then BCC W particles that got refined experienced deformation that yielded metastable BCT lattice via Bain's deformation

mechanism. Other W particles only got refined but did not find the opportunity to deform, hence resulted in nanostructured BCC W. The mechanical alloying process was complete after 48 h. The persistence of W XRD peaks in the patterns of the powders milled for 48 h, although very small, and their slight shift to the right is indicative of compressive strain that yields slight lattice reduction as was also the case of pure W illustrated in Fig. 4. Therefore the two new peaks that appeared at 42 and 52° in the XRD patterns of the W–Ni milled powders (Fig. 5), were ascribed to the formation of FCT Ni and also BCT W (2θ = 52°) after 48 h. It is therefore evident from the SEM and XRD analyses that a supersaturated solid solution of Ni(W) had been formed after milling

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Fig. 3. Typical EDX mapping showing uniform distribution of the elements in the selected area of agglomerated W-40 wt.% Ni powder particles milled for 24 h. (Scale bar = 10 μm).

for 24 to 48 h. The product consisted of tetragonal Ni(W) solid solution and nanostructured BCC W. The sequential disordering of first Ni and then of W crystal lattices during high energy ball milling was therefore similar to that reported by Yang et al. [13] during ion irradiation of multilayered Ni–W systems at room temperature. The crystallite size of W milled in the presence of Ni decreased faster than that of pure W as shown in Fig. 6 unlike the variation of powder particle size discussed in Section 3.1. Therefore, crystallite size or lattice strains vary independently of powder particle size upon

milling. The lattice strain of W milled with Ni increased at shorter milling time and reached a maximum value after 24 h, which correspond to the simultaneous appearance of the Ni(W) solid solution and nanostructured W products. Therefore the relaxation of lattice strain contributed to the driving force for dissolution of W atoms in metastable Ni lattices. Non dissolved tungsten was converted to nanostructured material. A solubility of ~32 at.% W was achieved in the metastable FCC Ni(W) prepared by ball milling of W-40 wt.% Ni powders in a Retsch

Fig. 4. Evolution of the XRD patterns of W milled powders.

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Fig. 5. XRD patterns of the W-40 wt.% Ni powder mixtures milled for different times.

homogenous dispersion was obtained in hexane. It was therefore hypothesized that the electrostatic properties of W fine particles formed after 12 h milling were increased, which increased their hydrophobic character. The magnetic saturation of pure W and W–Ni powders evolved with the milling time, as shown in Fig. 7. However, the trends were different for the two types of powders. The magnetic saturation of W-40 wt.% Ni powders decreased with increasing milling time due to interference of non-magnetic W particles around particles of ferromagnetic Ni as a result of homogenisation, as shown in Fig. 2. The presence of non-magnetic W particles with 0 μT m 3 kg − 1 decreased the proportion of Ni magnetic atoms, thereby reducing the density of the magnetic atom interactions. According to Miracle [32] this reduction would lead to lower exchange interaction between Ni atoms and the suppression of the Ni metal moment by the charge transfer to the d-band and by the d–d hybridization which decreases the number of polarization d-states. However, upon forming metastable tetragonal solid solution, the magnetic saturation increased. On the other hand, the magnetic saturation of pure W powder increased from 0 μT m 3 kg − 1 to about 15 μT m 3 kg − 1 at early milling stages and reached maximum after 24 h. The increased saturation magnetisation of W could be ascribed to mixed effects induced by ball milling such as formation of the nanograins, the development of

PM400 MA-type planetary high energy ball mill, which is higher than 17.8 at.% W reported by Genç et al. [15] by mechanical alloying of Ni30 wt.% W powder mixtures in a Spex Mixer/Mix. The difference in maximum solubility achieved could be ascribed to the compositions of the starting mixtures. The powder mixture used in the present study contained twice the amount of W than that used by Genç et al. [15]. In accordance to Hume-Rothery rules on solid solutions, the two ought to have similar crystal structure. Therefore, the formation of Ni(W) solid solution implies that the transformation of ductile FCC Ni to FCT and hard BCC W to BCT lattice acts as a driving force. Extension of the Ni–W solubility during ion milling was ascribed to a biased collapsing of first Ni and then W lattices, yielding disordered FCC Ni structures when they could not accommodate more partner atoms than the respective solubilities [13]. From the published data [14], the cohesive energy of W is 8.90 eV/atom, which is more negative than that of Ni, being 4.44 eV/atom, suggesting that the W lattice is more stable and strongly bonded than that of Ni. 3.3. Magnetic saturation of the mechanically alloyed powders It was observed that pure W powder particles milled for 12 h were not evenly dispersed in de-ionized water used for Malvern particle size analysis. They clustered at the bottom of the flask. However, a W(L)

0.7

Crystallite size L[A]

W-Ni(e)

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Milling time [h] Fig. 6. Crystallite sizes (L) and lattice strains (e) of pure W and W-40 wt.% Ni powders processed by high energy ball milling.

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Magnetic saturation [micro T m^3/kg]

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W W-Ni (powder)

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milling time [h] Fig. 7. Variation of magnetic saturation of W and W-40 wt.% Ni powders with the milling time.

strong cube textures and the transition toward the metastable close packed systems by shearing mechanism. Upon further milling to 48 h, the magnetic saturation decreased. Thus, the magnetic domains of the non-magnetic W were aligned, which lead to the increased magnetism, however, milling for more than 24 h has reversed the transformation and the texture developed earlier. In order to understand the origin for the observed anomalous magnetism induced by ball milling of W, further studies are necessary. Moreover, the contamination of the milled W powders by Fe could also be hypothesized to result in the observed increase in magnetic properties. However, an estimation of the theoretical magnetic saturation, by means of the linear dilution law [16], and considering the magnetic saturation of W is nil, that of Fe is 203 μT m 3 kg− 1 and the maximum Fe content determined by ICP-OES of 4 wt.% yield a momentum equal to 8 μT m3 kg− 1, which is about half the measured magnetic saturation of milled mixture of about 15 μT m 3 kg− 1. Contamination by iron would have had opposite effect on magnetic saturation of W–Ni milled powders, which decreased upon milling. The high magnetic saturation was therefore ascribed to the formation a ferromagnetic W metastable tetragonal phase at early milling stages. It was inferred from the magnetic saturation values that the dissolution of W in Ni was more significant than that of the contaminant Fe atoms in either of the constituents of Ni–W mixtures. Thus, resulting in the decrease of magnetic properties with milling time, unlike observed in milling of pure tungsten powders. However, at the initial stages of milling (b15 h), most of W particles were embedded into larger ductile particles of Ni, thus only Ni particles were subjected to strain which led to reduction in magnetism. Beyond 15 h, the W particles were now resurfaced and became exposed and experienced milling collisions. As a result, crystal transformation toward more magnetic phases began as evidenced by gradual increase in magnetic saturation of Ni–W at longer milling time (Fig. 7). Effect of compaction was evident at shorter milling time of W–Ni powder mixtures. For ductile metallic particles, compaction induces some level of long-range ordering (LRO), hence higher magnetic saturation compared to loose powder. However, compaction under 40 MPa did not affect magnetic saturation of milled W or that of W–Ni powders at long milling time. It inferred from these results that long-range ordering was induced after long milling W–Ni powders. The absence of effect of compaction on the magnetic saturation of pure W powders indicates that only orientation of magnetic zones in Ni or Ni(W) was affected due the compaction pressure.

of nanocrystalline BCC W and tetragonal Ni(W) solid solution. Formation of tetragonal solid solution occurred as a result of the collapsing of first Ni FCC crystal lattice into FCT and then followed by that of W from BCC to BCT. The driving force for mechanical alloying by dissolution of W atoms was ascribed to the relaxation of lattice strain of metastable Ni lattices. The magnetic saturation of W increased at the early milling stage and decreased later due to the transition of the BCC W structure toward anisotropic close packed crystal structure and formation of nanograins. Magnetic saturation of W–Ni powders decreased with milling time but began to increase upon forming metastable tetragonal solid solution. References [1] [2] [3] [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

4. Conclusion High energy ball milling of W yielded a product comprised of magnetic BCT and non-magnetic nanocrystalline BCC W powders. The W-40 wt.% Ni powder mixtures resulted in a product, which consisted

[28] [29] [30] [31] [32]

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