Preparation and Properties of ZrO₂/Mo Alloys

Introduction

Molybdenum alloys have been widely used in glass, insulation materials, rare earth industry and other fields due to their excellent performance of high temperature strength, resistance to oxidation and corrosion, and service life [1, 2]. However, their shortcomings such as the low recrystallization temperature and room temperature brittleness limit applications especially on the aerospace industry [3–5]. Some efforts have been tried to improve the properties including enhancing the ductility, increasing recrystallization temperature and high temperature strength and lowering the brittle-ductile transition temperature.

Presently, some people of this research focus on oxide addition [2]. Oxides are always added in the form of solid, by which distribution of the second phase is very concentrated. Hydrothermal method was introduced into the preparation of precursor particles, obtaining highly dispersed and uniform precursor particles [6–9]. Theoretically, it can effectively improve the strength of the alloy. In this paper, hydrothermal method was adopted to prepare the precursor of the ZrO₂/Mo alloys, and then the microstructures and mechanical properties of the alloy were mainly studied.

Experimental

Precursor was prepared by hydrothermal synthesis of (NH₄)₂Mo₄O₁₃·2H₂O and Zr(NO₃)₄·3H₂O(180 °C/18 h). After drying, roasting treatment (550 °C/4 h), hydrogen reduction occurred, and then the powders with different zirconia contents were obtained. The size of cool-pressed billet is 20 × 40mm . The dimension of sintered compact is 17×38 mm. The content of ZrO₂ was shown in Table 1.

Scanning electron microscope (SEM, VEGA-3 SBH) coupled with energy dispersive spectrometer (EDS) was used to observe the microstructures of samples before and after tensile tests.

Results and discussion

In Figure 1, the zirconia dispersedly located around molybdenum powder particles. In Figure 3, the size of powder and grains were measured. It can be seen that zirconia can reduce the adhesion of Mo particles and effectively hinder the growth of Mo particles. Moreover, the nanometer-sized zirconia can effectively restrain the growth of Mo grains during sintering process. Finally, the zirconia can increase the alloy strength. With increasing ZrO₂ addition, the size of Mo powder in alloy powder decreased to a certain extent (Figure 3). Actually, as the relevant data show, the granularity of ZrO₂ particles prepared by hydrothermal process can reach nanometer level [8, 9]. During subsequent preparation process, the dimension of ZrO₂ particles was retained in the resulting microstructure. Eventually, the nanoscale ZrO₂ particles were gained. Hydrothermal process will be adopted during the preparation process to decrease the accumulation of ZrO₂ particles. The ZrO₂ particles have a better effect of dispersion strengthening [10].

Figure 1:

SEM images (×5.0 kx) of molybdenum power: (a) Mob. Mo-1.0%  ZrO₂.

Sample 4*, a reference sample, was prepared by the traditional method (solid–solid). Figure 2 displays the microstructure images of the alloys, which were prepared by different methods. In Figure 2, the second phase in Sample 4* was heavily concentrated, while that in the alloys made by the new method was dispersed. This distribution of this second phase influences the mechanical properties. Sample 4* cannot be deformed in the hot processing. The second phase was small and dispersed in the alloys prepared by the new method. The second phase is bulky and concentrated in Sample 4*.

In Figure 2, the grains of the alloys prepared by the new method are much smaller. With the increase of the ZrO₂ addition, the grain size of alloy reduces. This will influence the mechanical properties of the alloys. The distribution of this second phase is caused by the different preparation methods. The alloy, which has prepared using the new method, will have a better distribution and smaller grain size.

Figure 2:

SEM photo of ZrO₂/Mo alloy: (a) 4* × 2.00 kx (Reference sample); (b) 5 × 10.00 kx (New method); (c) TEM of alloy.

Figure 3:

Size of ZrO₂/Mo: (a) powder size; (b)grain size.

In the experiment, the sintered samples were tested as showed in Table 1. With the increase of ZrO₂ addition, the density of the alloys reduces, but the relative density and hardness of the alloys increase. It can be seen from the TEM photographs of the alloys that the second phase in the nanometer size (Figure 2) is evenly distributed around the Mo grains. The larger ZrO₂ particles are located at grain boundaries or sub-boundaries, while most of the smaller ZrO₂ particles are located within grains. This distribution of ZrO₂ particles can restrain the accumulation and growth of Mo grains during sintering process (Figure 3). The nanometer ZrO₂ added can effectively restrain the accumulation and growth of Mo grains, and fine grains are obtained finally to improve the strength of the alloys. From the SEM photos (Figure 2), we can see that the grain size of the alloys was obviously refined with the increase of nanometer ZrO₂ addition. This can effectively improve the performance of the alloy. The second phase not only has the effect of dispersion strengthening but also achieves the result of refining grain. The mechanical properties are effectively improved.

The results of yield strength (σy), tensile properties and microhardness tests are summarized in Table 2. It can be observed that the addition of nano ZrO₂ can effectively improve the alloy strength. With the increase of ZrO₂ addition, the tensile strength and yield strength of the alloys have greatly improved. Compared with the pure molybdenum, the tensile strength and yield strength of the Mo alloy with 1.5wt% ZrO₂ have increased by 32.33%  and 53.76% , respectively. The added nano ZrO₂ particles can effectively increase the tensile strength and yield strength of the alloys. With the increase of ZrO₂ addition, the strength rises significantly.

Conclusion

The Mo powders doped with nano-sized ZrO₂ particles were prepared by hydrothermal synthesis, and the microstructures were investigated. With the increase of ZrO₂ addition, the granularity of the alloy powders decreases. In other word, the nano ZrO₂ particles can effectively refine the grain size of the powders. The microstructures of sintered billet were refined gradually with the increase of ZrO₂ addition. The larger ZrO₂ particles are located at grain boundaries or sub-boundaries, while most of the smaller ZrO₂ particles are located within grains. The added ZrO₂ particles not only refine the grains of matrix but also increase mechanical properties such as tensile strength, yield strength and hardness.