The Effect of Molybdenum Powder Particle Size on the Properties of Molybdenum-Rhenium Alloys
Release time:
2026-11-09
Abstract: Molybdenum-rhenium alloys exhibit good ductility and machinability at room temperature and are widely used in various industries. The properties of molybdenum-rhenium alloys prepared via powder metallurgy are influenced by the particle size of the raw materials. This study experimentally investigated MR35 alloys prepared by mixing molybdenum and rhenium powders of three different particle sizes. The results showed that molybdenum-rhenium alloy billets prepared by mixing medium-sized molybdenum powder (Fraser particle size of 3.6 μm) with rhenium powder exhibited the highest density, achieving 99.9% density with a deformation rate of just 60%, whereas the molybdenum-rhenium alloy prepared by mixing rhenium powder with coarse-grained molybdenum powder (Fraser particle size of 7.0 μm) exhibited the lowest density, requiring a deformation rate exceeding 80% to reach 99.9% density. There is a positive correlation between the particle size ratio of molybdenum-rhenium alloy powder ((D90 + D10)/D50) and density; the higher the ratio, the higher the density.
Pure molybdenum is a widely used refractory metal characterized by a high melting point, high thermal conductivity, and good hot working properties. It is used in high-temperature equipment, sapphire crystal growth furnaces, and as target materials for semiconductors and displays [1–4]. However, pure molybdenum has poor formability at room temperature. Research has shown [5–6] that adding rhenium to molybdenum can effectively improve its room-temperature ductility and mechanical properties, while significantly lowering its ductile-to-brittle transition temperature. Molybdenum-rhenium alloys exhibit good ductility at room temperature and have garnered attention and found applications in industries such as semiconductors, nuclear energy, and aerospace [7–10].
The preparation of molybdenum-rhenium alloys currently relies primarily on powder metallurgy processes, in which mixed alloy powders are pressed into shapes, sintered at high temperatures, and then subjected to deformation processes such as rolling and forging to obtain the desired products [11–13]. In powder metallurgy, the particle size of the powders has a significant impact on the density of the sintered billets and subsequent deformation processes. Reference [14] investigated the effects of rhenium powder particle size and morphology on microstructure and properties, while Reference [15] prepared molybdenum-rhenium alloys by mixing molybdenum and rhenium powders; however, there are no reports on the influence of molybdenum powder particle size on the densification and deformation properties of molybdenum-rhenium alloys. This study focuses on the Mo-35%Re alloy to investigate the effects of three different molybdenum powder particle sizes on the densification and rolling deformation properties of the molybdenum-rhenium alloy, and to analyze and compare the effects of molybdenum powder particle size.
1. Experiment
The test material was MR35 alloy (35% Re by mass). The Feesey particle size and particle size distribution of the raw material are shown in Table 1, and the morphology of the raw powder is shown in Figure 1. The powder was prepared with a mass ratio of M (Mo):M (Re) = 65:35. After thorough mixing using a three-dimensional mixer, the powder was pressed into billets using a four-column hydraulic press at a pressure of 160 MPa for 1 minute. The green compact was sintered for 6 h at 2,300 °C in a hydrogen atmosphere; the sintered compact had a thickness of 10 mm. The molybdenum-rhenium alloy was subjected to unidirectional hot rolling with a deformation rate of 80% [16–17] to obtain a sheet with a final thickness of 2 mm.
Powder particle size was analyzed using a WLP-216 average particle size analyzer and a Mastersizer 2000 laser particle size analyzer. a JEOL JSM-6380LV scanning electron microscope was used to observe the powder morphology; the sintered density of the molybdenum-rhenium alloy was determined using the Archimedes’ principle; a OLYMPUS GX51 metallographic microscope was used to examine the microstructure; and mechanical testing at room temperature was conducted on a SANS-CMT-5205 electronic tensile testing machine.
Table 1 Properties of Powder Raw Materials
Figure 1 Morphology of the raw material powder
2 Results and Discussion
2.1 Properties of Alloy Powders
The particle sizes of the mixtures of molybdenum powder and rhenium powder with different particle sizes are shown in Table 2. The mixture of fine molybdenum powder and rhenium powder is denoted as XMR; the mixture of medium-grain molybdenum powder and rhenium powder is denoted as ZMR; and the mixture of coarse-grain molybdenum powder and rhenium powder is denoted as CMR. As shown in Table 2, the Fei’s particle size of the alloy powders formed by mixing molybdenum and rhenium powders is lower than that of the molybdenum powder alone. Combined with Figure 2, it can be seen that the flaky rhenium powder is uniformly distributed among the polyhedral molybdenum particles and forms a “pseudo-bond” with the molybdenum powder, thereby reducing the particle size of the alloy powder [18–21].
Table 2 Particle Size of Molybdenum-Rhenium Alloy Powder
Figure 2 Morphology of molybdenum-rhenium alloy powder
2.2 Sintering Density
The sintering densities of molybdenum-rhenium alloys after high-temperature sintering in a hydrogen atmosphere are shown in Table 3. The theoretical density of MR35 is 12.44 g/cm³. The sintering density of the molybdenum-rhenium alloy mixed with coarse-grained molybdenum powder was only 90%, while those containing fine-grained and medium-grained molybdenum powders reached 95% and 97%, respectively.
Table 3 Comparison of the Properties of Molybdenum-Rhenium Alloy Powders
The alloy powders were analyzed using the formula in Reference [22], which shows that the ratio (D90-D10)/D50 is highest for ZMR and lowest for CMR. Combined with the particle size ratios in Table 3, it can be seen that the sintered density is positively correlated with the powder particle size ratio; a higher ratio results in a denser sintered green body. In the experiments, (D90+D10)/D50 was used for comparative analysis, and the trends in these ratios were consistent. These two ratios indicate that the molybdenum-rhenium alloy powders prepared in the experimental process conform to this relationship; however, whether other binary alloys or elemental powders also satisfy this relationship requires further study.
The sintering process of powders can generally be divided into stages such as particle bonding, the formation and growth of the sintering neck, and the closure and disappearance of pores [12–13, 19]. After mixing coarse-grained molybdenum powder with rhenium powder, the large particle size results in relatively large internal pores within the sintering necks. During high-temperature sintering at 2,300 °C, some of these closed pores fail to shrink or disappear effectively, leading to a lower sintering density. Medium- and fine-particle molybdenum powders have particle sizes similar to those of rhenium powder. The polyhedral molybdenum powder and flake-like rhenium powder are uniformly distributed; throughout the sintering process, sintering necks readily form and grow, and the internal pores are fully closed and reduced due to the good size matching between the medium-particle molybdenum powder and the rhenium powder. Consequently, the density is highest, while that of fine-grained molybdenum powder is slightly lower. As shown in Figure 3, there are significant differences in grain size in XMR; in CMR, the grains are larger with a small number of fine grains; whereas in ZMR, the variation in grain size is relatively small.
Figure 3 Microstructure of Sintered Molybdenum-Rhenium Alloy
2.3 Rolling Properties
2.3.1 Rolling Microstructure
After rolling deformation at an 80% strain rate, the microstructures of the molybdenum-rhenium alloys with the three different molybdenum powder particle sizes were essentially consistent: the structure exhibited a fibrous appearance in the rolling direction and a date-stone-like shape perpendicular to the rolling direction, as shown in Figure 4.
2.3.2 Relationship Between Strain Rate, Density, and Mechanical Properties
Rolling deformation induces strain hardening in molybdenum-rhenium alloys of varying densities, thereby increasing the material’s density [23–24]. The densities of the three molybdenum-rhenium alloys at different strain rates are shown in Table 4. As the strain rate increases, the density of ZMR reaches 99% at a strain rate of 60%; the density of CMR increases rapidly after 50% strain, but it does not reach 99% density until after 80% rolling strain. The ZMR alloy has a high green density and achieves an even higher density after the same amount of deformation; this allows for optimized billet sizing to achieve high density with minimal rolling deformation.
Figure 4 Metallographic Structure of Rolled ZMR Alloy Sheets
Table 4 Density of Molybdenum-Rhenium Alloys at Different Strain Rates
As shown by the room-temperature mechanical properties in Table 5, as the strain rate increases, both the tensile strength and elongation of the material increase simultaneously. This reflects that the microscopic pores in the billet are gradually closed during the deformation process, resulting in increased material density. At the same time, the degree of grain deformation increases, and the strain energy stored in the matrix grows progressively [25], leading to a simultaneous increase in tensile strength and elongation.
3 Conclusion
The particle size of molybdenum powder has a significant impact on the sintering density of molybdenum-rhenium alloys and their subsequent deformation processing. By mixing molybdenum powder and rhenium powder in three different particle size distributions, this study investigated the relationship between sintering density and particle size distribution, as well as the effect of different deformation rates on mechanical properties. The following conclusions were drawn:
(1) The particle size of the alloy powder obtained by mixing molybdenum powder of different particle sizes with rhenium powder is smaller than that of the molybdenum powder, while its particle size distribution lies between those of the molybdenum and rhenium powders;
(2) The ZMR alloy powder exhibits the highest (D90–D10)/D50 ratio and the highest sintered density; the (D90+D10)/D50 ratio also follows the same pattern;
(3) Green bodies with high sintering density achieve 99% density at a 60% deformation rate, while those with low sintering density require an 80% deformation rate to reach the same density; tensile strength and elongation increase in tandem with the deformation rate.
References: Powder Metallurgy Industry; “Effect of Molybdenum Powder Particle Size on the Properties of Molybdenum-Rhenium Alloys”; Zhang Danhua, Wang Guangda, Xiong Ning
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