Fabrication and characterization of nanocrystalline Nb–W–Mo–Zr alloy powder by ball milling
Release time:
2025-07-28
1. Introduction
Niobium has attracted a considerable interest for many applications such as steel production, superalloy production(as alloy additions), and transport industry (aircraft turbine engines, magnetically levitated trains, and automobiles) owing to its high melting point (2741 K), a relatively low density (8.57 g/cm3), and excellent low temperature ductility [1]. About 75% of the niobium metal is used as a microalloying element in low-alloyed steels, while the other 20–25% is employed as an addition in Nickel-based superalloys and heat-resisting steels. Only 1–2% is used as high temperature materials[2]. Niobium-based alloys are considered to be the most promisinghigh temperature structural materials replacing the nickel-based superalloy, which has the maximum operating temperatures of about 1273 K [3]. In order to ameliorate the strength of niobium at elevated temperatures, many approaches have been employed such as solid solution strengthening (W and Mo), and composite strengthening with intermetallic compounds (Nb3Al, Nb3Ir and Nb5Si3) or carbide phase (TiC, ZrC and HfC) [4–7].
The low-alloys of the Nb–W–Mo–Zr system, exhibiting a combination of high strength with low-temperature ductility, presents an optimum content of alloying elements. Generally, the Nb–W–Mo system alloys are produced by common metal working processes such as vacuum-arc melting, forging, hot rolling, cold rolling, and long-time recrystallizing annealing process [3,5–10].These processes have exhibited disadvantages such as the capability to produce the complicated shapes, the poor material utilization, and causing pollution[2].The preparation of alloy powder has been considered to be the most difficult aspect of processing niobium alloys by powder methods albeit several studies have demonstrated that the processing of niobium parts via near net-shaped is the most feasible method [1,2]. Considering the limitations such as the high melting points and reactive properties of the alloys, high-purity powders can only be produced by expensive procedures such as the hydride-dehydride process, and centrifugal atomization equipped with electron beam or plasma heat source [1].
Nanocrystalline powders (NP), exhibiting a large specific surface area and a high defect density (especially in milled powders), have been reported to sinter at much lower temperatures of about 0.2–0.3 Tm (Tm is melting temperature) with a high sintering rate [11–13]. Samples prepared from nanocrystalline powders usually posses a characteristic microstructure indicating their potential for various applications [11]. High energy ball milling is a simple but useful process for preparing the ultrafine powders. In this process, elemental powders are collided by high speed mill balls, suffering a severe plastic deformation, and forming a high density of lattice defects and dislocations [14–16]. These lattice defects and dislocations as well as momentary increase in temperature of particles trapped between colliding balls accelerate the diffusion of components, rendering the multi-component powders into homogeneous alloy powders [17]. Thus mechanical alloying process (MA) can produce the ultrafine nanocrystalline refractory alloy powders at room temperature. The several reports have demonstrated the production of Nb–Si, Nb–Cr, and Nb–Al composite powders via MA. However, the fabrication of Nb–W–Mo–Zr system alloy powders via that process has not been reported so far [18–21].
In this work, the ultrafine nanocrystalline Nb–W–Mo–Zr alloy powder is prepared by mechanical milling. The influence of the milling time and velocity on size, shape, dispersion, and crystal structure of the particles is investigated.
2. Experimental procedures
Elemental niobium (≥99.8%; ≤44 μm), tungsten (≥99.9%; ≤3–5 μm), molybdenum (≥99.9%; ≤1–2 μm), and zirconium (≥99.99%;≤4–6 μm) powders were used as received without any furtherpurification.
The mixture powders with the compositions of Nb-5 wt.% W-2 wt.% Mo-1 wt.% Zr (Nb521) were put into the hardened chromium steel vial containing WC hard metal balls (6, 8,and 10 mm in diameter and 20%, 50%, and 30% in weight, respectively). All material handlings (including weighing and loading) were performed in high purity argon filled glove box, with low oxygen and water vapor content.The ball charge was 55% of a maximum charge in the container and the ball-to-powder-weight ratio was 20 to 1. The milling was carried out, nominally at room temperature using a QM-QX4L type planetary mill machine with the selected velocity of 250 rpm (route 1) and 450 rpm (route 2) for 2–60 h, respectively.
Following the milling, the powders were characterized using laser particle size analyzer (LPSA-LMS 30), X-ray diffractometry with Cu Kα radiation(XRD-MAC Science M21X), and scanning electron microscopy (SEM-ZEISS ULTRA 55).Crystallite size and lattice strain of specimens were calculated from XRD patterns using the Williamson–Hall method as follows [22].
Where B, θ, λ, D, and ε are full width at half maximum (FWHM),peak position, the wave length (=0.15406 nm), crystallite size and lattice strain, respectively.
3. Results and discussion
3.1. Particle size distribution
Fig. 1 shows the variation of average particles size as a function of milling time for Nb521 powder mixture milled by routes 1 and 2. It is evident that the average particle size of starting powder mixture is about 14 μm.
Comparing with the growing trend depicted by route 1, the average particle size of Nb521 powder mixture in route 2 reaches a maximum at 2 h milling time and then stabilizes between 20–60 h. This trend is similar to other studies regarding the effect of milling time on the particles size of composite powders [23–26]. During this process, the primary ductile niobium particles suffer cold welding following the work hardening resulting in the activation of fracture mechanism. When the rate of cold welding and fracturing processes reaches equilibrium, the steady state is achieved. The phenomenon in route 1 implies that the cold welding rather than work hardening is the main process because of the lower milling energy involved.
3.2. Morphological changes
FESEM is employed to examine the variation in the surface morphologies of the powder samples after being ball-milled. Fig. 2 presents the morphology of as-received powder particles. It is vividly discernible in Fig. 2 that the initial niobium powder consists of large irregular shaped particles of various sizes. The tungsten particles are polyhedral in shape with a mean size of about 3–5 μm. The molybdenum powder exhibits a smaller particle size (1–2 μm) with an irregular-rounded morphology and a high tendency for agglomeration. The zirconium particles appear similar to that of the niobium powder but with much smaller particle size.
Figs. 3 and 4 represent the morphological variations of Nb521powder mixture milled through route 1 and route 2, respectively.For route 1, it is visible that there is a continuous increase in the average particle size (Fig. 3). After 2 h milling, the most of particles remain fine in size and several large flake like particles are formed with a maximum size of about 200 μm. With further milling, the percentage of flake like particles increases as well as the maximum size decreases. It seems that the cold welding is the dominant mechanism during milling due to the ductility of Nb powders and low energy produced by the milling balls. In route 1, the slow velocity does not allow the crack between milling balls and powder particles to be fierce enough for the flake like particles to be fractured continuously.
In route 2, the powder particles exhibit remarkable changes in size and shape. The transformation in size and shape can be categorized into three stages. First is the formation of large flake particles, followed by smaller ellipsoidal particles, and finally ultrafine irregularrounded particles (Fig. 4). Due to ductile nature of the Nb powder,welding seems to be the dominating mechanism during the first stage, and thus the 2 h milled particles are larger in size and flatten in shape (Fig. 4(a)). Following the 10 h milling, these plate-like particles are work-hardened resulting in the activation of fracture mechanism (Fig. 4(b)). It is visible in Fig. 4(c and d) that the ellipsoidal morphology remains intact even after 20–40 h milling,however, the quantity of large particles and the average particle size decreases. It implies that the large ellipsoidal particles are crushed by intense impacts. Further milling up to 60 h results in the predominantly ultrafine equiaxed and irregular-rounded in shape particles with a narrow size distribution range (Fig. 4(e, f)). During the milling process, the energy needed to crush the particles increases with decreasing the particle size as well as work hardening [23,27]. Therefore, the particles in lower limit size cannot fracture even though prolonging the milling time. With the further continuation of milling, the stable equiaxed ultrafine particles of about 2 μm in size with a narrow size distribution are formed.
Fig. 5 denotes the frequency particle size distributions of Nb521 particles milled in different routes for diverse time. It can be seen from Fig. 5(a) that for route 1, as the milling time increasing, not only the frequency distribution peak shifts to larger particle size regions, but also the particle size distributions narrow. However, compare to Fig. 5(a), Fig. 5(b) reveals an adverse variation tendency of frequency distribution peak in route 2, supporting the SEM observations in Figs. 3 and 4.
Fig. 6 displays the typical particle morphology of samples mechanically milled in route 2 for 2, 10, 20, and 60 h, respectively. During the onset of the milling process, the ductile Nb particles undergo an intense cold welding and some of these are cold welded to microalloying particles to form a layer of wrappage (Fig. 6(a)). Due to further rigorous ball-milling, the hard additions powder particles undergo repeated collisions with Nb particles. Under the action of high plastic deformation followed by work hardening of Nb particles, the hard additions powder particles are collided into smaller pieces which piercing into Nb particles and distributed more evenly (Fig. 6(b, c)). The sequence welding, fracture, piercing, deforming, fracturing leads to the formation of homogenize Nb521 alloy powder (Fig. 6(d)).
3.3. Structural evolutions
The XRD patterns of Nb521 powder mixture produced from routes 1 and 2 at various milling times are depicted in Figs. 7 and 8. It is observed from Fig. 7 that the XRD peak of W and Mo are steady during the milling process that indicates that the route 1 has insignificant effect in forming the Nb521 alloy powder.
Fig. 8 has revealed that the Nb521 powder mixture exhibits a series of changes during route 2. Compared with the starting material, Nb phase demonstrates a lower and broaden diffraction peaks, while other elements have unchanged diffraction peaks after 2 h of ball milling. This phenomenon demonstrates that the powder mixture only undergoes a sub-microstructural changes of Nb phase owing to the severe plastic deformation of the Nb particles [23,24].
Ball milling for 10 h or more leads to a remarkable broadening of Nb diffraction peaks and decrease in the intensity of W and Mo diffraction peaks. The peaks of W and Mo disappear after 40 h of ball milling indicating the formation of a solid solution (or secondary solid solution) of W and Mo phase in Nb phase. It is well known that high velocity ball milling supplements an input of high energy to the powder system. During this process, a large number of flaws including dislocations and new grain boundary are generated rendering the diffusion between different components facile and in the occurrence of solid solution of W and Mo phase in Nb phase.
The appearance of WC peaks during both routes may be attributed to the fraying of hard metal balls. The absence of the diffraction peaks of Zr element is probably due to its small amount and weak X-ray scattering intensity.
Fig. 9 plots the crystallite size and lattice strain of Nb in route 2, respectively. According to the Williamson–Hall method, the crystallite size and lattice strain before milling were about 179.6 nm and 0.14%, respectively. The crystallite size decrease but lattice strain increase with milling time. After 40 h of ball milling, the two parameters appeared to approach a constant value at about 14 nm and 0.84%. The structural evolution mechanism of powders during high energy ball milling has been reported in many papers [14–17]. High energy ball milling provides the particles an intense plastic deformation at extremely high strain rate resulting in the creation of high density lattice defects and dislocations as well as recovery phenomena [16,17,28]. When the rate of former is higher, the dislocations increase resulting in a dislocation cell structure that ultimately creates low-angle grain boundaries. As the milling continues, low-angle grain boundaries transform to a whole nanocrystalline structure. In this stage, the crystallite size decreases and the lattice strain increases dramatically. The constant values of crystallite size and lattice strain reveal the balance of the creation and disappearance of dislocations.
4. Conclusions
An ultrafine nanocrystalline Nb–W–Mo–Zr alloy powder is produced by ball milling at room temperature. During this process, the particles undergo clod welding, plastic deforming, work hardening and recovery stages. Milling velocity is an influencing parameter in producing alloy powders. As the velocity reaches up to 250 rpm, cold welding is the dominant mechanism during milling and no solid solution has been observed even after milling for 60 h. The optimum milling conditions of 450 rpm for 60 h lead to nanocrystalline Nb–W–Mo–Zr alloy powder particles with a crystallite size of 14 nm.
Paper Citation Information
Int. Journal of Refractory Metals and Hard Materials 32 (2012) 45–50
Refractory metal elements mainly include Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, etc. The high temperature mechanical properties of RHEAs are better than those of nickel-based high temperature alloys and other traditional high temperature alloys, and they have excellent radiation resistance. Stardust Technology has developed more than 20 series of refractory high entropy alloy powder products, which are widely used in aerospace, nuclear energy and other industries with radioactive environments. Customization of composition and particle size is acceptable. Please contact Vicky Zhang for more customization services at +86-13318326185.
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