1.   Introduction

Molybdenum (Mo) is a typical body-centered cubic refractory metal with high melting point temperature (2610°C), excellent electric and thermal conductivity, and satisfactory mechanical strength at high temperatures [1–4]. Due to these exceptional performances, Mo-based alloys are very attractive for various important fields such as metallurgy, machinery, nuclear fusion, and electronic packaging [5–7]. Nevertheless, accompanied by the increasingly high use requirements of molybdenum parts, coarse grains, intrinsic room temperature brittleness, and low recrystallization temperature seriously limit their further development [8–9].

At present, adding some solid solution elements, adding second phase particles, and reducing impurity are common and effective approaches to address the above problems [10–12]. Indeed, adding second phase particles can efficaciously refine Mo grains, improve the deformation ability, and enhance the deep processing characteristics of Mo brittleness [13–15]. For decades, the doping of rare earth oxides (Y₂O₃, La₂O₃, ZrO₂, etc.) into Mo alloys has been become a research hotspot, while carbides are rarely involved [8,16–18]. These rare earth oxides at grain boundaries (GBs) can effectively pin and hinder the movement of Mo GBs, thereby refining Mo grains [19–20]. On the basics of the fine-grain strengthening principle, ultrafine Mo alloys can achieve an outstanding match between high strength, hardness, and good plastic ductility [21–23]. Moreover, some second phase particles within Mo grains can also pin and accumulate dislocations, so as to further improve material strength [24–26].

The powder metallurgy process can be used to prepare high melting point superalloys, such as copper alloy [27–28], nickel alloy [29–30], tungsten alloy [31], high entropy alloy [32], steel [33–35], molybdenum alloy, and so on. For preparing the oxide dispersion strengthened Mo alloys (ODS-Mo) powders, ball milling is the most significant preparation technology of composite powder precursor for engineering promotion, which is due to its simple and easy mass production process [4,17,36]. Unfortunately, the ball-milled ODS-Mo still possesses a relatively large grain size, which restricts the further application of Mo alloys [9,19,23]. Moreover, most oxide particles are relatively large and mainly located at Mo GBs [11,37]. These large oxide particles at Mo GBs are not conducive to load transfer, resulting in stress concentration and crack initiation because of the deformation incongruity with the Mo matrix [9,38].

In many other alloy systems [39–42], the composite doping of carbide and oxide has achieved excellent results in terms of grain size and mechanical properties. For example, in the W alloy of the same family as Mo, TiC particles have been added to the oxide dispersion strengthened W (ODS-W) alloys many times to refine grain structure and further refine W grains [41–42], which provides us an inspiration. Moreover, it is also proved that some free Ti in TiC can adsorb oxygen impurities to form some TiO_x and purify W matrix, and can react with Y₂O₃ to form Y–Ti–O ternary particles with finer size [39,41,43]. Actually, some researchers found that composite oxide doped Mo alloys such as Mo–Y₂O₃–CeO₂ alloy [15], Mo–Y₂O₃–ZrO₂ alloy [17], and Mo–La₂O₃–Y₂O₃ alloy [44] possess higher comprehensive mechanical properties compared to single oxide doped Mo alloys. However, the co-doping of oxide and carbide into Mo alloys is rarely involved. Compared with oxide (La₂O₃, Y₂O₃, etc.), carbide (TiC, ZrC, etc.) tends to form a coherent interface with the Mo matrix and thus effectively strengthen the Mo matrix [2,9,18].

Based on the background mentioned above, a minor TiC additive, as the common second phase of carbides, was introduced into Mo–Y₂O₃ alloys in this work. Accompanied by TiC addition, the sizes of Mo grains and oxide particles were effectively refined. The corresponding mechanism was discussed in detail. Furthermore, the microstructure effect on properties (hardness) was also explored.

2.   Experimental

The raw material of Mo alloys were commercial Mo powders of 0.6–1 μm, nano Y₂O₃ powders of 20–50 nm, and nano TiC powders of 20–50 nm. The purity and manufacturer of all raw materials were >99.8% and Aladdin, respectively. Every ball mill tank was added into 200 g Mo–1wt%Y₂O₃–0.5wt%TiC composite powders, then vacuumed, and finally filled with pure argon. The vacuum pumping and argon filling were repeated three times. The materials of the ball mill tank and milling balls were tungsten carbide YG8. The milling balls to powders weight ratio was 10:1. Then the ball milling process in a planetary ball mill was carried out at a rotation speed of 400 r/min for 12 h, thus obtaining Mo–Y₂O₃–TiC composite powders. Next 20 g of ball-milled Mo–Y₂O₃–TiC composite powders were pressed into small cylinders (diameter, d = 25 mm), followed by hydrogen sintering at 1600°C. Meanwhile, pure Mo and Mo–Y₂O₃ alloys were fabricated and used as a reference sample under the same conditions of ball milling and sintering.

The phases of powders and alloys were detected by X-ray diffraction (XRD, D/MAX-2500). The morphology of powders and alloys were examined by scanning electron microscopy (SEM, JSM-7800F) and transmission electron microscopy (TEM, JEM-2100) equipped with energy dispersive spectrometers (EDS). Nano Measurer 1.2 was used to measure and calculate the sizes of Mo grain and second phase particles. The sample densities were done based on the Archimedes’ principle. Every sample surface was polished before the Vickers hardness test, and it was tested at different 20 positions under a load of 1.96 N for 20 s, thus getting an average hardness value.

3.   Results and discussion

Fig. 1(a)–(c) represents the SEM images of ball-milled pure Mo, Mo–Y₂O₃, and Mo–Y₂O₃–TiC composite powders. It can be seen that ball-milled powders display irregular ellipsoidal morphologies, and the corresponding average particle sizes of pure Mo, Mo–Y₂O₃, and Mo–Y₂O₃–TiC composite powders are 1.92, 1.69, and 1.42 μm, respectively. In particular, each particle is composed of many fine grains, and the agglomeration phenomenon can be observed in Fig. 1(d). Fig. 2(a) shows the XRD patterns of ball-milled pure Mo, Mo–Y₂O₃, and Mo–Y₂O₃–TiC composite powders. There are no peaks corresponding to Y₂O₃ and TiC second phase, which is chiefly attributed to their little addition and the XRD test accuracy [12,45]. The XRD patterns only have pure Mo phase and are free of any peaks corresponding to the impurity phase, implying the ball-milled powders are not oxidized during the powder preparation process [46–47]. On account of the grain refinement and micro-stressing, the crystallinity sizes of pure Mo, Mo–Y₂O₃, and Mo–Y₂O₃–TiC composite powders are 31, 26, and 25 nm, respectively, which were calculated using the Hall method [12,19,48]. These ultrafine nano grains possess high sintering activity, that is, they can give consideration to ultrafine grains and high density at relatively low sintering temperature [37,49–50].

Figure  1.  SEM images of (a) ball-milled pure Mo powder, (b) Mo–Y₂O₃ composite powders, and (c, d) Mo–Y₂O₃–TiC composite powders.

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Figure  2.  XRD patterns of (a) ball-milled pure Mo, Mo–Y₂O₃ composite powders, and Mo–Y₂O₃–TiC composite powders and (b) sintered pure Mo, Mo–Y₂O₃ alloys, and Mo–Y₂O₃–TiC alloys.

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As exhibited in Fig. 2(b), the low temperature sintered pure Mo, Mo–Y₂O₃, Mo–Y₂O₃–TiC alloys also contain single Mo phase, which is similar to the powder XRD patterns. As shown in Fig. 3, all pure Mo, Mo–Y₂O₃, and Mo–Y₂O₃–TiC sintered alloys represent typical crystal sugar-like intergranular fracture morphology. The average grain size of pure Mo (see Fig. 3(a) and (b)) is approximately 10.42 μm. The Y₂O₃ particles could effectively refine Mo grains from 10.42 to 3.12 μm (see Fig. 3(c) and (d)). Thus one can see that minor Y₂O₃ additive is an effective rare earth oxide phase to refine Mo grains. Actually, Y₂O₃ has been proved to be the best additive phase in tungsten alloys of the same family as molybdenum. However, the large grain size of Mo–Y₂O₃ still needs to be further optimized. Interestingly, accompanied by further TiC addition, the Mo grains are further refined to 1.36 μm (see Fig. 3(e) and (f)). The corresponding grain size is less than half of the original Mo–Y₂O₃ alloys, which is a huge leap.

Figure  3.  (a) SEM image and (b) the corresponding BSE image of pure Mo; (c) SEM image and (d) the corresponding BSE image of Mo–Y₂O₃ alloys; (e) SEM image and (f) the corresponding BSE image of Mo–Y₂O₃–TiC alloys.

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The reason for Mo grain refinement after adding TiC was further explored. As displayed in the black area of Fig. 3(d), the Y₂O₃ particles (420 nm) in Mo–Y₂O₃ alloys are mainly distributed at Mo GBs. By comparison, the size of second phase particles is reduced to 230 nm with the TiC introduction, as shown in the black area of Fig. 3(f). In general, the second phase particle with small size and excellent dispersion can effectively pin the movement of Mo grains, resulting in grain refinement of Mo–Y₂O₃–TiC alloys. Compared with relevant literature about traditional ODS-Mo, their oxide particle size at GBs is usually larger than 400 nm [1,8,51–52], which is also much larger than that of the second phase particles in our prepared Mo–Y₂O₃–TiC alloys.

The Mo–Y₂O₃–TiC alloys were further characterized by TEM. Fig. 4(a) and (b) shows the TEM images of Mo–Y₂O₃–TiC alloys. There are many white “skylights” (marked by white arrows) on the black matrix, which were confirmed as second phase particles by EDS. Some inserted EDS results in Fig. 4(a) and (b) indicate that the particles at GBs are rich in Ti, Y, C, O, and Mo elements. The Ti–Y–C–O phase should be a mixture of TiO_x, TiC_x, YO_x, and Ti_xY_yO_z, which was proved in W alloys, the same family as Mo [39–42]. Furthermore, we observed that Mo matrix and large Y₂O₃ particles exist Mo–Y–O diffusion layers (<50 nm) in our previous work [9]. Thus, the Ti–Y–C–O (Mo) phase was chosen to represent these large second phase particles. The second phase particles at GBs play a major role in grain refinement, which hinders and pins GB movement [2,7]. The TiC additive of Mo–Y₂O₃–TiC alloys contacts Y₂O₃ to form a small Ti–Y–C–O (Mo) phase, which limits the fusion of single Y₂O₃ into a larger size to a certain extent. A suitable multicomponent second phase can restrict the growth of each other’s oxides, which has been reported in our previous work about tungsten alloys [53]. Compared with the single Y₂O₃ particles, the mixed Ti–Y–C–O particles can more effectively refine Mo grains, which is also consistent with the grain size results in Fig. 3.

Figure  4.  (a, b) TEM images of Mo–Y₂O₃–TiC alloys, inserting the EDS results of Mo–Ti–Y–C–O particles; (c–e) high resolution transmission electron microscopy (HRTEM) images of nano Y₂O₃, TiC_x, and TiO₂ particles within Mo grains, and their corresponding fractional Fourier transform (FFT) images with calibrated crystal plane, angle, and Z axis.

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A lot of nano particles (<100 nm) were also observed within Mo grains, and some of them are Mo–Ti–Y–C–O phase, as listed in Fig. 4(b). Moreover, it can be seen from Fig. 4(c)–(e) that some nano particles within Mo grains are identified as Y₂O₃ (PDF#88-2162), TiC_x (PDF#51-0628), and TiO₂ (PDF#76-1937) phases. In other words, a small amount of original TiC and Y₂O₃ addition can also exist alone. Interestingly, some active Ti within TiC can adsorb nearby oxygen impurities to form a new TiO_x strengthening phase, thereby purifying and strengthening Mo matrix [18,47,54–55]. This provides indirect evidence of TiC purification. The mismatch δ was calculated under an ideal condition, that is, the crystal plane of the second phase is parallel to the (110) crystal plane of Mo (the most representative crystal plane), which has been observed in our previous works [7,9,47]. Specifically, the nano Y₂O₃ (440), TiC ($00\bar{2}$), and TiO₂ ($\bar{4}10$) may form coherent or semi-coherent interfaces with Mo (110), and their corresponding calculated mismatch δ values are 14.5%, 2.7%, and 3.6%, respectively.

The second phase interfacial coherence of Mo alloy mainly depends on the particle size. There is an interface interaction between the second phase and Mo matrix during the sintering process. When the second phase size is small, part of the second phase is swallowed into the grain interior during sintering. Due to their small size, they tend to form coherent or semi-coherent interfaces with Mo matrix with less lattice distortion and greater probability [7,45,55–56]. Moreover, the second phase particles within Mo grains are hindered by the surrounding grain interior, so they are not easy to migrate and grow during subsequent sintering and maintain initial nano size. Moreover, according to the Powder Diffraction File (PDF) cards of these second phase particles, the appropriate crystal plane spacing to maintain the coherent interface/semi-coherent interface with Mo completely exists, which is also reflected in Fig. 4(c)–(e). By comparison, the second phase particles at Mo GBs will fuse and grow to large submicron size. However, due to their large particle size, the lattice distortion cannot resist the large interface energy, so the coherent/semi-coherent interface gradually changes a non-coherent interface. Compared to incoherent interfaces, semi-coherent/coherent interfaces can effectually stabilize and strengthen phase interface, thus limiting oxide growth [9,19]. In addition, the interaction between the dislocation strain field and the atomic semi-coherent/coherent strain field can lead to strengthening and hardening, which can improve material strength and resist crack initiation [2–3,7].

The Vickers hardness values of pure Mo/Mo–Y₂O₃/Mo–Y₂O₃–TiC alloys are HV_0.2 (245 ± 31), (370 ± 26), and (425 ± 25), respectively. Mo–Y₂O₃–TiC alloys possess the highest hardness in comparison to pure Mo and Mo–Y₂O₃ alloys. As listed in Table 1, the hardness of traditional ODS-Mo in relevant literature is usually less than HV 400, which is also lower than that of Mo–Y₂O₃–TiC alloys prepared in this work. The hardness of Mo alloys is mainly determined by grain size and relative density [7,52,59]. Moreover, the second phase also contributes to hardness to a certain extent [12,16,51]. On the one hand, as mentioned above, Mo–Y₂O₃–TiC alloys possess the finest grain size and can better resist deformation when pressed by external force [7,9,20]. On the other hand, the relative densities of pure Mo/Mo–Y₂O₃/Mo–Y₂O₃–TiC alloys are ~98.0%, ~97.4%, and ~97.5%, respectively, as shown in Table 1. It can be seen that all Mo alloys have similar high relative densities, which has little effect on hardness. Furthermore, the uniform and dispersed ultrafine second phase particles are also conducive to improving hardness. Integrating above evaluations, one can see that the co-doping of oxides and carbides developed in this work delivers a new insight to fabricate ultrafine Mo alloys with high performance.

Table  1.  Summary of pure Mo, Mo–Y₂O₃, Mo–Y₂O₃–TiC alloy characteristics in this work and relevant ODS-Mo in literature

Sample	Powder technology	Mo grain size / μm	Relative density / %	Oxide particle size / nm	Vickers hardness, HV	Refs.
Pure Mo	Ball-milling	10.42	~98.0	—	245 ± 31	This work
Mo–1wt%Y2O3	Ball-milling	3.12	~97.4	420	370 ± 26	This work
Mo–1wt%Y2O3–0.5wt%TiC	Ball-milling	1.36	~97.5	230	425 ± 25	This work
Pure Mo	Freeze-drying	8.96	99.0	—	258	[9]
Mo–1wt%TiOx	Ball-milling	4.44	>99	540	402	[12]
Mo–ZrO2 (0–1wt%)	Hydrothermal	50	97	<500	168–236	[57–58]
Mo–ZrO2 (0–1.5wt%)	Hydrothermal	20–60	95	—	300–350	[6]
Mo–Al2O3 (0–10vol%)	Chemical	<10	>99	~1000	160–350	[16]
Mo–Al2O3 (0–40vol%)	Chemical	10	95–97.5	3000–6000	384	[1]
Mo/ZrO2–Y2O3 (0–5vol%)	Ball-milling/chemical	<10	94–98	—	175–325	[17]
Mo–La2O3 (0.3wt%–2.5wt%)	Ball-milling	3–20	97.4–98.7	<700	—	[4]

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4.   Conclusion

The Y₂O₃ and TiC particles were simultaneously doped into Mo alloys using ball-milling and subsequent low temperature sintering techniques. The results showed that single Y₂O₃ particles addition could effectively refine Mo grains (10.42 μm → 3.12 μm). Accompanied by further TiC addition, the Mo grains are further refined to 1.36 μm. Actually, Y₂O₃ and TiC can form smaller Y–Ti–O–C quaternary phase (230 nm) at GBs compared to single Y₂O₃ (420 nm), so as to Y–Ti–O–C particles could effectively hinder grain movement. Thus, one can see that the co-doping of oxides and carbides can greatly decrease the size of oxide particles at GBs for traditional ball-milled ODS-Mo. In addition to Y–Ti–O–C at GBs, Y₂O₃, TiO_x, and TiC_x nanoparticles (<100 nm) also exist within Mo grains. The appearance of TiO_x indicates that active Ti within TiC will adsorb oxygen impurities of Mo alloy to form a new strengthening phase, thus purifying and strengthening the Mo matrix. Compared to pure Mo (HV_0.2 (245 ± 31)) and Mo–Y₂O₃ alloys (HV_0.2 (370 ± 26)), the Mo–Y₂O₃–TiC alloys possess the highest hardness (HV_0.2 (425 ± 25)).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 52171044 and 51804218) and the Innovation and Entrepreneurship Training Program for College Students in Fujian Province, China (No. S202111312029).

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.