Synthesis of W-Y₂O₃ alloys by freeze-drying and subsequent low temperature sintering: Microstructure refinement and second phase particles regulation

1. Introduction

Tungsten-based alloys, due to their excellent physical properties such as high melting point, good thermal conductivity, corrosion and wear resistance, are widely used in the field of cemented carbides and superalloys [[1], [2], [3], [4]]. In the field of superalloys, tungsten-based alloys are mainly used as aerospace materials and nuclear materials [5,6]. However, embrittlement, high ductile-brittle transition temperature (DBTT) and low density limit their applications [[7], [8], [9]].

In order to solve these problems to obtain tungsten-based materials with high performance, the microstructures and properties of conventional tungsten materials need to be improved and optimized. Current studies mainly focus on second-phase dispersion strengthening and fine-grained strengthening [7,10,11]. On the one hand, second phase particles with high thermal stability includes carbides (such as TiC and ZrC) [9,12] and oxides (such as Y₂O₃, La₂O₃ and Ce₂O₃) [[13], [14], [15]]. Rare-earth oxide particles can effectively pin the movement of W grain boundaries, refining and stabilizing W grain. At the same time, it can improve low temperature brittleness and low recrystallization temperature. Moreover, nanosized oxide dispersed strengthened (ODS) tungsten materials also have good creep resistance and radiation resistance at high temperature [[16], [17], [18]]. Among rare earth oxides, Yttrium oxide (Y₂O₃) is the best dispersion strengthening phase [19], so W-Y₂O₃ alloys has gained extensive attention.

On the other hand, according to the principle of fine grain strengthening, alloys with small grain size have good plasticity and toughness while maintaining high strength and hardness [5,20]. Moreover, the smaller the grain size, the more grain boundaries. More grain boundaries can reduce the concentration of impurities at grain boundaries and optimize their distribution [17]. Because of the high melting point of tungsten-based alloys, they are mainly obtained by powder metallurgy. Fine-grained tungsten-based alloys can be obtained by low temperature sintering of ultrafine nano powders with high sintering activity [10].

Former researchers mainly employed tungstic acid co-precipitation method [21,22] or ball milling method [[23], [24], [25], [26]] to prepare ultrafine W-Y₂O₃ composite powders. However, many impurities like nickel, iron, and soon are introduced in the process of ball milling meanwhile the agglomeration is serious [[27], [28], [29]]. In addition, the refinement of powders is not obvious in the later stage of ball milling, which wastes a lot of energy [24,30]. Although the powders obtained by tungstic acid co-precipitation achieve uniform doping at atomic level, there is a phenomenon of bimodal distribution of their tungsten grain size, which has a negative impact on subsequent sintering [2,22].

The smaller and more dispersed the second phase particles in W-Y₂O₃ alloys are, the better the dispersion strengthening is. Y₂O₃ particles at grain boundaries can refine W grains [27,31]. Because the interfacial strength of W-W is greater than that of W-Y₂O₃ [16,17], large Y₂O₃ particles at the grain boundaries are prone to stress concentration and cracking. In the related papers on ball milling, even if powders are very fine, the second phase particles grow up to submicron size after sintering, and almost all of them are distributed at grain boundaries [[23], [24], [25], [26]]. In addition, apart from focusing on the size and distribution of second phase particles, few literatures focus on the composition of second phase particles. Except for the second phase Y₂O₃ particles in W-Y₂O₃ alloys [2,14,[31], [32], [33], [34]], few researchers mentioned that the composition of second phase particles are Y₂WO₆ or Y₆WO₁₂ [5,35] and they did not investigate these W-Y-O ternary phases in depth. The second phase composition has an important influence on the interface structure and strength, so it is necessary to systematically explore the second phase composition.

In order to solve the W-Y₂O₃ alloys problems mentioned above (including powder preparation, the distribution and composition of second phase particles, oxygen content), W-Y₂O₃ alloys were innovatively prepared by freeze-drying and subsequent low temperature sintering in this work. Freeze-drying is a physical method that can mix at atomic level without introducing additives or losing solutes. It has been used to prepare ultrafine powders such as Al₂O₃, WC-Co, Co₂AlO₄ and superconducting powders [[36], [37], [38], [39]]. Hereby W-Y₂O₃ composite powders precursor were prepared by freeze-drying method. The grain size of the powder prepared by freeze-drying is only 18.1 nm, and the corresponding grain size distribution is excellently narrow. After subsequent low temperature sintering, the sintered W-Y₂O₃ alloys have a smaller grain size of 510 nm while maintaining a comparatively high density of 97.8%. Lots of small Y₂WO₆ particles (< 20 nm) were found in W matrix while a few large Y₂O₃ particles distribute at W grain boundaries. Combining with the purification of grain boundary, the distribution and composition of second phase particles are studied. In addition, the Vickers microhardness of sintered W-Y₂O₃ alloys also reaches 656.6 ± 39.0 HV_0.2.

2. Experimental procedure

2.1. Preparation of powders

Ammonium metatungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O (AMT) and yttrium nitrate hexahydrate (Y(NO₃)₃•6H₂O) were used as raw materials to prepare W-2 wt% Y₂O₃ composite powders. At first, 20.00 g of ammonium metatungstate and 1.03 g of yttrium nitrate hexahydrate were dissolved in 200 mL deionized water while ultrasonic treatment was introduced to dissolve solutes more fully. Then, the solution was pre-frozen for 12 h in a constant temperature freezer at -40 °C. Next, after the freeze dryer temperature decreased and stabilized, the pre-frozen solution is put into freeze dryer to lyophilize for 48 h. After that, porous precursor powders were calcined in argon atmosphere at 500 °C for 30 min. The reduction in pure hydrogen atmosphere was carried out in two steps, that is, the first step at 600 °C for 2 h and the second step at 800 °C for 2 h. Finally, ultrafine W-Y₂O₃ composite powders were obtained by cooling slowly in hydrogen atmosphere to room temperature.

At the same time, W-2 wt% Y₂O₃ composite powders were prepared by ball milling method for comparison. Commercial pure W powders (99.9% purity, 0.6-1 μm in size) and Y₂O₃ powders (99.99% purity, 20-50 nm in size) were used as raw materials. W-Y₂O₃ composite powders were obtained by using planetary ball mill at a speed of 300 r/min for 20 h.

2.2. Low temperature sintering process

W-Y₂O₃ composite powders were placed in a mold having a diameter of 25 mm, and then the pressure of 20 MPa was applied to the die and kept for 10 min to obtain preformed samples. The pre-formed samples were then placed in a furnace with hydrogen atmosphere, holding for 2 h at 800 °C, then heating to 1600 °C and holding for 4 h. The sample of W-Y₂O₃ alloys sintered by ball milling powders was named as BM sample, and the sample of W-Y₂O₃ alloys sintered by freeze-drying powders was named FD sample.

2.3. Characterizations

The phase, microstructure and second phase particles distribution of composite powders and alloys were measured by X-ray diffraction (XRD, D/MAX-2500) with CuKα radiation, scanning electron microscopy (SEM, Hitachi Model No. S4800) equipped with EDX detector and transmission electron microscopy (TEM, JEM-2100) equipped with EDX detector. The density of sintered W-Y₂O₃ samples was measured by Archimedes method. Oxygen content was detected by inert gas pulse infrared thermal conductivity. The Vickers microhardness of sintered samples was tested for 20 s under 200 gf load and each sample was tested for twenty times to get an average hardness value.

3. Results and discussion

3.1. Characteristics of prepared W-Y₂O₃ composite nanopowders

Fig. 1 shows XRD patterns of the W-Y₂O₃ composite powders fabricated by freeze-drying method and balling milling method. It can be seen that the peak of Y₂O₃ phase are not detected due to the less addition of Y₂O₃. Freeze-dried powders only have α-W phase, indicating that the reduction of these powders has been completed. According to the calculation of Scherrer’s formula, the grain sizes of ball milling powder and freeze-dried powder are 14.1 and 19.1 nm, respectively. Furthermore, the full width at half maximum (FWHM) of peaks of freeze-dried powders is narrower than that of mechanical ball-milling powders, which indicates that the grain size of mechanical ball-milling powders is smaller.

SEM and TEM images of freeze-dried powders and mechanical ball-milled powders are shown in Fig. 2. It is observed from Fig. 2(a) and (b) that the agglomeration of powders prepared by ball milling is very severe, and crystal grains are in close contact with each other. During the process of balling milling, the plastic deformation and fracture effect caused by friction and impact from balls and jar can lead to grain refinement, but also result in the formation of large irregular particles [25]. Because of the irregular shape of some large particles, large voids between large particles are hard to eliminate during sintering, which finally affects the mechanical properties of W-Y₂O₃ alloys. Compared with ball milled powders, the freeze-dried powders of Fig. 2(c) and (d) are also fine, and the average grain size is about 18.1 nm, which is consistent with the results calculated by Scherrer’s formula based on XRD data. Moreover, the dispersion of their grains is extremely high and almost no agglomeration is observed mean while the distribution of grain size (10-25 nm) is extremely narrow, which means that freeze-dried powders has high sintering activity. In previous studies, some researchers obtained a bimodal distribution of W grain size in the W-Y₂O₃ composite powders with particle sizes of 250 nm and 50 nm using conventional wet chemical methods [22]. The W-Y₂O₃ composite powders having a size of 0.1-1 μm was obtained by a novel wet chemical method [34]. In addition, the researchers also obtained W-Y₂O₃ composite powders with the size of 160 nm by nano-in-situ composite method [27]. Comparing with these previous studies, it can be seen that the size of freeze-dried W-Y₂O₃ composite powders is excellent fine. The HRTEM image of the W-Y₂O₃ composite powders prepared by freeze-drying method is shown in Fig. 3. It can be observed that Y₂O₃ is coated by W. The formation of Y₂O₃ within W grains is attributed to the homogeneous doping of the initial precursor powder atoms. Moreover, the Y₂O₃ particles coated by W does not easily grow up after the subsequent sintering due to the blocking effect of W.

The oxygen content of W-Y₂O₃ composite powders prepared by balling milling and freeze-drying are 2.32% and 2.26%, respectively. After excluding the oxygen in Y₂O₃ phase, the net oxygen (oxygen impurity) content of these two powders is 1.90% and 1.84%, respectively. Oxygen impurities usually form a thin layer of tungsten oxide on the surface of W grains [40,41]. Freeze-dried powders with high dispersion and fine size have a large surface area, which leads to a high oxygen content. For ball milling powder, on the one hand, impurities such as nickel, iron, and oxygen are introduced during ball milling [27]. On the other hand, after ball milling, multiple defects and high internal energy make W-Y₂O₃ composite powders highly active and easy to be oxidized after contact with air. It is worth noting that the impurity oxygen (excluding oxygen in Y₂O₃ phase) content of these two kinds of powders is even close to that in the Y₂O₃ addition. If the high oxygen content of these powders is not controlled, the mechanical properties of sintered W-Y₂O₃ alloys will deteriorate. Finally, the summary of characteristics of W-Y₂O₃ composite powders prepared by these two methods is summarized in Table 1.

3.2. Low temperate sintering characteristics of prepared W-Y₂O₃ composite nanopowders

In order to control the oxygen content and obtain high performance W-Y₂O₃ alloys, low temperature sintering under the hydrogen atmosphere was performed to the prepared W-Y₂O₃ composite nanopowders precursor. In order to better illustrate the distribution of phases, the atomic ordinal contrast image is obtained using backscattered electrons as signals. Fig. 4 represents the phase composition of microstructure of BM sample and FD sample in which white region is W-enriched matrix and the gray region is Y-enriched second phase particles. The average W grain size of BM sample in Fig. 4(a) is about 5.67 μm and its corresponding grain size distribution in Fig. 4(b) is wide. In previous studies using the ball milled W-Y₂O₃ composite nanopowders as precursor, the grain size (3-5 μm) was obtained by Zhou et al. [23] for the W-0.5 wt% Y₂O₃ alloys sintered by SPS at 1700 °C while the grain size of 1-2 μm was obtained by Li et al. for the W-2 wt% Y₂O₃ alloys sintered by SPS at 1200 °C [25]. This is similar to the grain size of W-Y₂O₃ alloys sintered under hydrogen atmosphere in our work. However, the oxygen content of BM sample is only 0.51%, indicating that the oxygen content has been effectively reduced. On the other hand, the size of Y₂O₃ particles (marked by black arrows) in Fig. 4(a) is 0.5-2 μm, and these Y₂O₃ particles are distributed at grain boundaries. W-2 wt% Y₂O₃ alloys with the Y₂O₃ particles size of 300 nm-1 μm has been obtained in Ref. [26]. In addition, it can be observed that there are many pores (marked by white arrows), resulting in a decrease in the density of W-Y₂O₃ alloys.

It is observed from Fig. 4(c) and (d) that FD sample has smaller grain and narrower grain size distribution than BM sample. The average grain size is only about 510 nm, and the corresponding grain size mainly distributes between 300 nm and 700 nm. In recent literatures, Zhao et al. obtained W-Y₂O₃ alloys with the grain size of 2-5 μm by wet chemical method and subsequent SPS [34]. The W-Y₂O₃ alloys of 5-10 μm was also obtained by sol-gel method and subsequent traditional consolidation sintering [42]. In addition, an improved wet chemical method and subsequent SPS were used to synthesize W-Y₂O₃ alloys with the grain size of 0.76 μm by Dong et al. [5,40]. Compared to these related papers, the freeze-drying and subsequent low temperature sintering is advantageous in refining W grain size of W-Y₂O₃ alloys. The fine grain structure of FD sample is mainly due to the pinning effect of second phase particles, which is closely related to the size and distribution of second phase particles. Combining the gray points in Fig. 4(c) amplified locally by using secondary electrons as signals, it can be observed that there are not only a few large second phase particles (marked by white dotted circular frames) of 100-300 nm at grain boundaries, but also many small particles (marked by black arrows) of about 10 nm at grain boundaries. It is reported that although the size of Y₂O₃ particles within W grains is only 2-10 nm by the improved wet chemical method, the size of Y₂O₃ particles at the grain boundaries is 100-300 nm [5,20]. In addition, submicron Y₂O₃ particles (400-800 nm) obtained by a new wet chemical method almost distribute at W grain boundaries [[32], [33], [34]]. However, the large number second phase particles of about 10 nm in our FD sample still exist at grain boundaries, significantly refining W grains. In order to better explain the detailed characteristics of these second phase particles, FD sample are further characterized by TEM (see Fig. 5).

In Fig. 5(a), white particles (marked by white arrows) are distributed on the black matrix. Combined with EDS, these white particles are found to be enriched in Y component, proving to be second phase particles. It is further found from Fig. 5(b) that there are also many small particles (marked by white arrows) within W grains. Moreover, an interesting phenomenon was found in Fig. 5(c). The large white particle was proved to be Y₂O₃ by EDX in the black box region, but many small particles (about 2-10 nm) were dispersed in it. These second phase particles were characterized by HRTEM (Fig. 6). As can be seen from the HRTEM images in Fig. 6(a) and (b), these spherical particles are Y₆WO₁₂ particles. Y₆WO₁₂ particles within Y₂O₃ matrix has never been reported in the previous studies on the W-Y₂O₃ alloys. At the same time, there is also a W-Y-O diffusion layer (about 5-10 nm) between Y₂O₃ and W matrix, as shown in Fig. 6(c) and (d). Because the lattice in the HRTEM image of diffusion layer is incomplete, the specific phase needs to be determined in the following work. On the other hand, the second phase particles within W grains are shown in Fig. 6(e) and (f) and proved to be Y₂WO₆ particles.

What’s the reason for the formation of Y₆WO₁₂ and Y₂WO₆? In the related previous studies, Y₆WO₁₂ or Y₂WO₆ are obtained by wet chemical method and subsequent SPS sintering. Yar et al. consider that these W-Y-O ternary phase particles may have two formation stages, i.e. in powder preparation stage or in high temperature sintering stage, but they did not make in-depth analysis [35]. Dong et al. believe that W atoms can be incorporated in Y₂O₃ lattice to form Y₆WO₁₂ phase at the high temperature sintering stage, and the formation of W-Y-O phase depends on the Y₂O₃ particle size [5]. Compared with SPS sintering, the FD sample in our work was sintered at 1600 °C for 4 h in a hydrogen atmosphere, so that the W atoms had sufficient time to diffuse into the small Y₂O₃ particles to form Y₆WO₁₂ or Y₂WO₆ particles. On the other hand, the diffusion distance of W is limited, so a W-Y-O diffusion layer is formed on the surface of submicron Y₂O₃ particles.

Compared with Y₂O₃ particles, the advantages of the formation of Y₆WO₁₂ and Y₂WO₆ particles are illustrated from a new angle. In the grain boundary strengthening principle of W-ZrC alloys, ZrC particles at the grain boundaries adsorb oxygen impurities to form W-ZrC-O phase, which purifies and strengthens grain boundaries [12,43]. By comparison, the formation of Y₂WO₆ (WO₃·Y₂O₃) and Y₆WO₁₂ (3WO₃·Y₂O₃) particles [35,44] has similar strengthening mechanism. On the one hand, Y₂O₃ particles can absorb nearby oxygen impurities in the W matrix to form Y₂WO₆ (WO₃·Y₂O₃) particles, which purifies and strengthens W grain boundaries and grain interior. On the other hand, Y₆WO₁₂ (3WO₃·Y₂O₃) particles with high oxygen content mean that more oxygen impurities are absorbed within Y₂O₃ particles interior. This also means that the oxygen content of W matrix is also relatively reduced. The oxygen content of FD sample is 0.51% and after excluding oxygen in Y₂O₃ phase, its net oxygen content is 0.058%, which is the same as that of BM sample. However, due to the formation of these W-Y-O ternary phases, the actual net oxygen content is much lower than 0.058%. It's also worth noting that because of the formation of W-Y-O ternary phases, the content of oxide exceeds 1 wt%, so the density of FD sample should be higher [35]. From the above discussion, it can be seen that the formation of the Y₆WO₁₂ and Y₂WO₆ phases has many advantages.

Y₆WO₁₂ particles are distributed in Y₂O₃ matrix and Y₂WO₆ particles are distributed in W matrix. This new phenomenon can also be explained from oxygen content. Compared with Y₂WO₆ (WO₃·Y₂O₃), Y₆WO₁₂ (3WO₃·Y₂O₃) has more WO₃, which means more nearby oxygen impurities are adsorbed. Y₂O₃ particles having high oxygen content can adsorb oxygen impurities nearby [7], so Y₆WO₁₂ particles are easy to form in Y₂O₃ matrix. In contrast, W matrix has a low oxygen content, which leads to the formation of Y₂WO₆ particles.

The formation of submicron Y₂O₃ particles in the FD sample is explained. In addition to the fusion growth of Y₂O₃ dispersed at grain boundaries during sintering, another reason for the formation of a few submicron Y₂O₃ particles may be that the original precursor powders of Y component are not ideal uniform mixing. When pre-freezing with a refrigerator, the cooling rate is slower, resulting in the rate of solute movement is faster than that of water solidification. A part of deionized water solidifies evenly into ice crystals first in the solution to separate solutes [45]. When solute concentration reaches their critical value, they solidify with deionized water and distributes in the gap between ice crystals [46,47]. Because of the different solubility and addition of AMT and YN₃O₉, AMT and YN₃O₉ cannot reach the critical value at the same time. However, due to the limitation of surrounding ice crystals [45], solutes may be segregated in a small region. The precursor powders enriched in Y component precipitate numerous large Y₂O₃ particles and these Y₂O₃ particles are easy to aggregate and grow up to submicron during subsequent low temperature sintering due to their concentrated distribution. Meanwhile, the W component within Y₂O₃ particles reacts with the surrounding Y₂O₃ and oxygen impurities, thus forming Y₆WO₁₂ particles. Through the above analysis, the reasons for the composition, size and distribution of second phase particles in FD sample are well explained. Finally, the schematic diagram of freeze-drying and subsequent low temperature sintering is shown in Fig. 7, which also shows the formation process of Y₆WO₁₂, Y₂WO₆ and the diffusion layer between W and Y₂O₃.

The Vickers microhardness of BM sample and FD sample are 351.0 ± 100.7 HV_0.2 and 656.6 ± 39.0 HV_0.2, respectively. It can be seen that the hardness of FD sample is much higher than that of BM sample. The Vickers microhardness mainly reflects the grain size and relative density of W based alloys [20,27,40]. On the one hand, the grain size of BM sample is more than ten times that of FD sample. According to the principle of fine grain strengthening [31,48], the hardness of refined grain is higher. On the other hand, compared with the 84.1% relative densities of BM sample, the FD sample has a higher relative density of 97.8%. What’s more, the uniformly dispersed second phase particles also contribute to the significant increase of hardness of FD sample. In previous study on the W-Y₂O₃ alloy prepared by mechanical ball milling and sintering, the hardness of 476.5 ± 13.5 HV was obtained when the grain size of W-Y₂O₃ alloys was 3-5 μm with a density of 96.8% [23]. While maintaining a high density of 99%, the hardness of sintered powder after ball milling is 525 HV, and its grain size is 3.7 μm [19]. W-Y₂O₃ composite powders prepared by conventional wet chemical method has a density of 96.28% after SPS sintering, but large grain size of 5.82 μm makes its hardness only 352.92 HV_0.2 [40]. Although the hardness of W-Y₂O₃ alloys obtained by sol-gel method and subsequent SPS sintering is about 600 HV, the density is only 93.8% [49]. Compared with these studies, FD sample in our work has smaller grain size, higher density and hardness, which also implies the advantages of freeze-drying and low temperature sintering method developed in our work. The characteristics of BM sample and FD sample are summarized in Table 2.

4. Conclusion

The W-Y₂O₃ alloys were successfully prepared by freeze-drying and subsequent low temperature sintering in our work. The average grain size of W-Y₂O₃ composite nano powders synthesized by freeze-drying method is only 18.1 nm. After low temperature sintering of these composite nanopowders, the formed W-Y₂O₃ alloys possess a smaller W grain size of 510 nm while maintaining a comparatively higher density of 97.8%. In addition to a few large Y₂O₃ particles (about 100-300 nm) at grain boundaries, more Y₂WO₆ particles (about 5-20 nm) distribute both within W grains and at W grain boundaries. Simultaneously, an interesting phenomenon that many Y₆WO₁₂ (<10 nm) particles distribute within large Y₂O₃ particles was found. The formation of these ternary phases indicates that some oxygen impurities in the W matrix can be adsorbed by these particles, resulting in the purification of W matrix and the strengthening of phase boundaries. The Vickers microhardness is as high as 656.6 ± 39.0 HV_0.2. Above results indicate that the process of freeze-drying and subsequent low temperature sintering is a promising way to fabricate high performance oxide-dispersion-strengthened tungsten based alloys with ultra-fine grain.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 51822404 and 51574178), the Science and Technology Program of Tianjin (No. 18YFZCGX00070), the Natural Science Foundation of Tianjin (No. 18JCYBJC17900) and the Seed Foundation of Tianjin University (Nos. 2018XRX-0005 and 2019XYF-0066).

---