Synthesis and Characterization of W80Ni10Mo10 alloy produced by mechanical alloying

The present study aims at synthesis and characterization of nanostructured W80Ni10Mo10 (wt. %) alloy produced by mechanical alloying (MA). Elemental powders of tungsten (W), nickel (Ni), molybdenum (Mo) were subjected to mechanical attrition in a high energy planetary ball-mill using chrome steel as grinding media and toluene as a process control agent. The crystallite size and lattice strain of the nanostructured powders at different stages of milling (0 h to 20h) was calculated from the X-ray diffraction patterns (XRD). The crystallite size of W in W80Ni10Mo10 powder was reduced from 100 μm to 55 nm at 10 h and farther reduction to 40 nm at 20 h of milling with increase in lattice strain of 0.25% at 20 h of milling. The lattice parameter of tungsten showed initial expansion upto 0.03% at 10 h of milling and then contraction upto 0.04% at 20 h of milling. The scanning electron microscopy (SEM) also showed mixed morphology of W80Ni10Mo10 powders consisting spherical and elongated particles after 20 h of milling. SEM analysis also revealed that particle size reduced from 100 μm to 2 μm with an increase in the milling time from 0 to 20 hours. The dark-field Transmission Electron Microscopy (TEM) observations revealed that the crystallite size of W in milled W80Ni10Mo10 alloy is in good agreement with calculated crystallite size from XRD.


Introduction
T ungsten is a refractory metal that possesses high melting point (3410°C) and excellent mechanical strength at elevated temperature, highest density of all engineering materials (19.3 g/ml) and tensile elastic modulus of 411 GPa. T herefore it is desired for applications in electrical, electronic, nuclear and space vehicle equipment [1]. On the other hand tungsten suffers from relatively poor fabricability and high ductile-brittle transition temperature. In order to improve the properties and to increase the spectrum of applicability, much effort has been directed towards the development of tungsten alloys in recent years. T he work is essentially centered on the goals such as to improve fabricability, particularly at elevated temperatures and to lower the ductile brittle transition temperature of W based alloys. As a result of increasing demand for higher and better mechanical properties, amorphous and nanocrystalline tungsten alloys have attracted enormous attention in recent years [2,3]. Mechanical properties of nanostructured materials are known to be improved by the refinement of microstructures by mechanical alloying which is a convenient solid state synthesis alternative to melt spinning and similar rapid quenching techniques to develop crystalline and amorphous alloys [4]. Among the various solid state methods, high-energy ball milling has gained quite popularity in recent years because of its simplicity, ease scale up production, high productivity, relatively inexpensive equipment, and applicability to a wide variety of materials [5].
Present investigation aims at synthesis, characterization and optimization of the process parameters of the current W 80 Ni 10 Mo 10 alloy produced by mechanical alloying (MA) route.

Mate rials and me thods
A planetary ball mill (Fritsch Pulverisette P5) was used to mill elemental W, Ni, Mo powder at a mill speed of 300 r.p.m using chrome steel as grinding media and ball to powder weight ratio of 10:1. Milled samples were taken out after 1, 5, 10, 15, 20 h for characterization purpose. T he details of the selected alloy and milling parameters are presented in the table 1. A high resolution X-ray diffractometer (Make: Rigaku Japan, Model: Ultima IV) was used to record the X-ray diffraction pattern (XRD) of the mechanically alloyed powders at different stages of milling using Cu-k α radiation (λ=1.541874 Å). T he record was matched with the JCPDS data bank to track the evolution of phases during mechanical alloying [6]. T he crystal size and lattice strain was calculated by determining the peak position and broadening of peak from the X-ray diffraction pattern [7]. T he lattice parameter was calculated from the X-ray diffraction pattern after stripping of K α2 of XRD pattern using precise lattice parameter calculation method [7]. T he d values of the high angle diffraction peaks were extrapolated against the function cos 2 (θ)/sin (θ), cos 2 (θ) [6], to yield the precise measure of d at cos (θ) tending to 0. It was found that cos 2 (θ) function gave best possible fit. T he morphology and particle size of the mechanically alloyed (MA) powders at different stages of milling was observed under a scanning electron microscope (SEM) (Make: JEOL, Model: JSM-6084LV). T he elemental compositional distribution of milled W 80 Ni 10 Mo 10 powder was analyzed by the energy dispersive X-ray (EDX) analysis attached with SEM. Crystallite size of nanostructured W in mechanically alloyed W 80 Ni 10 Mo 10 powder at 20 hr was detected by transmission electron microscope (T EM) (Make : JEOL, Japan, Model-JEM 2100). Selected area diffraction (SAD) patterns were obtained to identify the crystal structure using appropriate aperture and tilt. Reduction in crystallite size and plastic strain buildup is attributed to the increase in full-width at halfmaximum with increasing milling time [8]. Peak broadening analysis was done with the help of Scherrer equation after elimination of contributions from the strain and instrumental error [6].  T he variation of lattice parameter of W with milling time in W 80 Ni 10 Mo 10 alloy during mechanical alloying is shown in figure 3. It is evident that initially the lattice parameter of W expands upto 0.03% at 10 h and then contracts upto 0.04% at 20 h of milling. T his is due to the fact that the initial expansion of lattice parameter of W is attributed to the negative hydrostatic pressure exerted by the formation of nano-crystallites during mechanical alloying. Pabi. et. al has recently reported that a significant expansion of Nb lattice during ball milling of pure Nb [9]. On the other hand, the alloying elements Mo and Ni undergoes to form the substitutional solid solution with W, as their atomic radius is marginally lower than that of atomic radius of W which leads to contraction of the lattice after 10 hours of milling [10]. Initially upto 10 hours of milling the reduction in crystallite size is predominant whereas from 10 to 20 hours of milling formation of solid solution (by alloying of Mo and Ni with W) dominates over crystallite size reduction.

Scanning Electron Microscope (SEM) analysis of milled powders
T he SEM micrographs in figure 4 illustrate the change in particle morphology and size from ~100 µm to ~2 µm at different milling time. T he W, Ni and Mo particles consist of spherical shape at 0 h which changes to elongated shape after 20 h of milling. T he elongated nanoparticles at 20 h of milling are not conducive for flowability due to large interparticle friction therefore responsible for poor green densification property.

Conclusion
From the details synthesis and structural characterizations of the present alloy, the following conclusions can be drawn:  Mechanical alloying is an effective route for synthesis of W 80 Ni 10 Mo 10 alloy powder.  Crystallite size of tungsten in W 80 Ni 10 Mo 10 gradually decreases with increasing milling time and it records 40 nm at 20 h of milling have been calculated by using Scherrer formula from the XRD patterns.  T he lattice parameter of tungsten initially expands upto 0.03% at 10 h and then contracts upto 0.04% at 20 h of milling.  T he apparent particle size of powders gradually reduces from 100 µm (manually blended) to 2 µm (20 h) by SEM analysis of milled powders.  T he presence of nanocrystalline BCC-W phase with 30-40 nm in size at 20 h of milling is confirmed from the bright field T EM image and corresponding SAD pattern.