1 Introduction

Tungsten can potentially be an excellent high-temperature structural material because of its attractive properties e.g., a high melting point of 3,420 °C, an elastic modulus of 407 GPa, a hardness of 9.75 GPa, good thermal conductivity, a low coefficient of thermal expansion, and a low vapor pressure.¹⁾ However, it is difficult to achieve the sintering of a conventional microcrystalline W powder.^2,3) Malewar et al.⁴⁾ has reported that a sintered density as high as 97.4 % was achieved at 1,790 °C for 900 min, using nanopowder of W, but the grain size in this sintered W was remarkably coarse i.e., 15-30 μm, compared to an initial powder size of 8 nm.

Recently, the properties of nanostructured materials as advanced engineering materials have received great attention. These properties include fracture toughness and a high degree of hardness.^5,6) To hinder the grain growth during sintering, it is necessary to fabricate nanostructured materials. In this respect, a high-frequency induction heated sintering method has been presented as an effective technique for making nanostructured materials at a lower temperature in a very short heating period.^7-9) The grain growth of the W matrix is blocked by the presence of an oxide particle at the grain boundary of W. The dispersed oxide makes the enhancement of creep resistance and high-temperature strength due to pinning effect of grain boundary and block sliding of the grain boundary.¹⁰⁾

This study reports the fabrication of nanopowders of W and Al₂O₃ from WO₃ and 2 Al powders by a mechanical ball mill and nanostructured W-Al₂O₃ composites. These materials were made within three minutes by a high-frequency induction heated sintering method and evaluated in terms of its hardness, fracture toughness, and microstructure.

2 Experimental Procedures

Powders of WO₃(99.8 purity, < 10-20 μm, Alfa Aesar, Inc.) and Al(99.5 purity, < 45 μm, Alfa Aesar, Inc.) were used as initial materials. A high-energy ball mill (a Pulverisette-5 planetary mill) was conducted for 4 h under an argon atmosphere after WO₃ and 2Al powders were mixed. WC-8Co hard materials were used as balls with 10 mm in diameter.

Milled powder(Al₂O₃-W) was placed in a graphite die using upper and lower graphite punches. High-frequency induction heated sintering equipment is shown schematically in Ref.11) The sintering process consists of four stages. In stage 1, the system was evacuated under a vacuum of 20 Pa. A uniaxial pressure of 80 MPa was applied to powder in stage 2. An induced current was introduced and maintained to 1,400 °C with a heating rate of 1,000 °C/minute in stage 3. And then the powder was turned off without holding time. The specimen was cooled to room temperature at the end of the stage.

Grain sizes of W and Al₂O₃ were calculated by Suryanarayana and Grant Norton’s equation.¹²⁾ Relative densities of the sintered specimen were evaluated after calculating the volume of the sample using Archimedes’ method. Microstructure information of W-Al₂O₃ composite was observed using SEM with EDS. Phases were evaluated using X-ray diffraction(XRD). Vickers hardness and fracture toughness were calculated, using indentations and crack length on the sintered specimen at a load of 196 N.

3 Results and Discussion

Fig. 1 displays X-ray diffraction patterns of initial powders. In Fig. 1(a) and (b), all peaks are WO₃ and Al, respectively. XRD pattern of ball milled powders is shown in Fig. 2. Here reactant peaks of WO₃ and Al were not observed. Instead, product peaks of W and Al₂O₃ were detected. In Fig. 2, full width at half-maximum of the diffraction peak is broad. This indicates that strain was introduced, and powders were refined during the milling. Reactant powders milled with high-energy balls were synthesized during the milling according to equation (1).

Fig. 1.

XRD patterns of raw powders : (a) WO₃ powder, (b) Al powder.

Fig. 2.

X-ray diffraction patterns of Al₂O₃+W powder milled by high energy ball milling for 4 hours.

The interaction between WO₃ and 2 Al in equation (1) is thermodynamically feasible as shown in Fig. 3. Fig. 4 displays a plot of Br cosθ versus sinθ to calculate powder sizes of W and Al₂O₃ by Suryanarayana and Norton’s formula.12) Powder sizes of W and Al₂O₃ were 28 and 40 nm, respectively. FE-SEM image and EDS of WAl₂O₃ powder milled for 4 hours are shown in Fig. 5. Milled powders consist of nanopowders with some agglomeration. In EDS, only W, Al and O peaks were detected without other peaks such as WC and Co, which is possible to introduce during the ball milling.

Fig. 3.

The Gibbs free energy variation with temperature by interaction of WO₃ with 2 Al.

Fig. 4.

Plot of Br cosθ versus sinθ for W and Al₂O₃ powder milled for 4h.

Fig. 5.

(a) FE-SEM image and (b) EDS analysis of W+ Al₂O₃ powder milled for 4h.

Al₂O₃ composite. Thermal expansion shows up to 900 °C as the induced current was applied. And then shrinkage displacement abruptly increased up to 1,200 °C. Shrinkage displacement is nearly constant above the temperature. This means that nearly full density is achieved at 1,400 °C. Fig. 7 displays X-ray diffraction pattern of W-Al₂O₃ composite heated to 1,400 °C, in which only W and Al₂O₃ peaks were detected. The corresponding relative density of this composite was 98 %.

Fig. 6.

Variations of temperature and shrinkage displacement with heating time during the sintering of W-Al₂O₃ powder.

Fig. 7.

X-ray diffraction patterns of Al₂O₃+W composite sintered using high-frequency induction heating.

The plots of sinθ versus Br cosθ to calculate grain size of W and Al₂O₃ are displayed in Fig. 8. Average grain sizes of W and Al₂O₃ in the composite are 148 and 55 nm, respectively. R. Malewar⁴⁾ has reported that a sintered density as high as 97.4 % was achieved at 1,790 °C for 900 min using nanopowder of W milled by a highenergy ball mill, but the grain size of this sintered W was remarkably coarse as 15-30 μm, compared to initial powder size of 8 nm. In our study, dense nanostructured W- Al₂O₃ was achieved in 1,400 °C and 3 min using nanopowder of W and Al₂O₃ milled by high-energy ball mill. And nanostructured W- Al₂O₃ composite was obtained in a shorter time and lower temperature. Reasons for highly dense nanostructured composite obtained within three minutes at low temperature are as follows. Three factors that contribute to rapid consolidation process can be explained. First, electrical field generated spark discharges that cleaned the surface of powders. Grain boundary diffusion can increase due to this cleaning. Atomic diffusion can enhance under the electric field due to electromigration.^13,14) Second, application of pressure during the sintering is helpful in removing pores in the specimen and increasing driving force. Third, high-energy ball mill produced defect, strain and finer powder facilitating sintering.15,16) So, the sintering temperature of milled powders is lower than that of un-milled powders.¹⁷⁾

Fig. 8.

Plot of Br cosθ versus sinθ for W (a) and Al₂O₃ (b) in the composite sintered at 1,400 °C.

Fig. 9 displays FE-SEM image and X-ray mapping of the sample sintered at 1,400 °C. Microstructure consists of ultra-fine grey phase and bright phase. Grey and bright phase in Fig. 9 are Al₂O₃ and W, respectively from mass contrast and X-ray mapping.

Fig. 9.

FE-SEM image and EDS analysis of W-Al₂O₃ composite : (a) FE-SEM image, (b) Al mapping, (c) W mapping, (d) O mapping.

Vickers hardness was evaluated on polished sections of the W-Al₂O₃ composite under a 196 N load. The calculated hardness value of W-Al₂O₃ composite sintered at 1,400 °C was 1,364 kg/mm². This value is higher than that of monolithic W reported as 9.75 GPa.¹⁾ Cracks propagated from indent corners as shown in Fig. 10(a). Length of these cracks can permit fracture toughness using the Niihara,s equation.¹⁸⁾ Fracture toughness value of the W-Al₂O₃ composite measured by this method is 7.1 MPa·m^1/2. Fracture toughness of the W-Al₂O₃ composite is higher than that of monolithic Al₂O₃ reported as 4MPa·m^1/2.19) A crack propagated in a branching(↑) and deflective manner(↓) in Fig. 10(b). Enhanced fracture toughness of W-Al₂O₃ composite is believed that W and Al₂O₃ in the composite may deter propagation of cracks and W and Al₂O₃ have nanostructured phases.

Fig. 10.

(a) Vickers hardness indent and (b) crack propagating in W-Al₂O₃ composite.

4 Conclusions

Nanopowders of W and Al₂O₃ were synthesized during the high-energy ball mill from WO₃ and 2Al powders. The nanostructured W-Al₂O₃ composite was achieved within three minutes from synthesized powder by highfrequency induction heated sintering. Average grain sizes of W and Al₂O₃ in composite sintered at 1,400 °C are 148 and 55 nm, respectively and the relative density of the composite was 98 %. The hardness and fracture toughness of the composite was 1,364 kg/mm², 7.1 MPa·m^1/2, respectively. Fracture toughness of the W-Al₂O₃ composite is higher than that of monolithic Al₂O₃ reported as 4 MPa·m^1/2. Enhanced fracture toughness of W-Al₂O₃ composite is believed that W and Al₂O₃ in the composite may deter propagation of cracks and W and Al₂O₃ have nanostructured phases.