What is Molybdenum
Molybdenum is a kind of transition element, and its oxidation state is easy to change. It plays the role of electron transfer in oxidation reduction-reactions. In the form of oxidation, molybdenum is likely to be in the +6-Valence state, although it can revert to a +5-Valence state during electronic transfer. Molybdenum is not only a trace element necessary for humans, animals and plants, but also an important element in steel and alloy. Metal molybdenum and molybdenum iron are the commonly used molybdenum-containing furnace materials. At times, molybdenum oxide concentrates can also be used for direct smelting reduction of molybdenum-containing alloy grades. Molybdenum reserves are estimated to be 19 million tons in the Earth's crust with 8.6 million tons recoverable.
| I | O | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 1 H |
II | III | IV | V | VI | VII | 2 He |
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| 2 | 3 Li |
4 Be |
5 B |
6 C |
7 N |
8 O |
9 F |
10 Ne |
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| 3 | 11 Na |
12 Mg |
III | IV | V | VI | VII | VIII | I | II | 13 Al |
14 Si |
15 P |
16 S |
17 Cl |
18 Ar |
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| 4 | 19 K |
20 Ca |
21 Sc |
22 Ti |
23 V |
24 Cr |
25 Mn |
26 Fe |
27 Co |
28 Ni |
29 Cu |
30 Zn |
31 Ga |
32 Gc |
33 As |
34 Se |
35 Br |
36 Kr |
| 5 | 37 Rb |
38 Sr |
39 Y |
40 Zr |
41 Nb |
42 Mo |
43 Tc |
44 Ru |
45 Rh |
46 Pd |
47 Ag |
48 Cd |
49 In |
50 Sn |
51 Sb |
52 Te |
53 I |
54 Xe |
| 6 | 55 Cs |
56 Ba |
57-71 La-Lu |
72 Hf |
73 Ta |
74 W |
75 Re |
76 Os |
77 Ir |
78 Pt |
79 Au |
80 Hg |
81 Tl |
82 Pb |
83 Bi |
84 Po |
85 At |
86 Rn |
| 7 | 87 Fr |
88 Ra |
89-103 Ac-Lr |
104 Rf |
105 Db |
106 Sg |
107 Bh |
108 Hs |
109 Mt |
110 Ds |
111 Rg |
112 Uub |
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| La-Lu | 57 La |
58 Ce |
59 Pr |
60 Nd |
61 Pm |
62 Sm |
63 Eu |
64 Gd |
65 Tb |
66 Dy |
67 Ho |
68 Er |
69 Tm |
70 Yb |
71 Lu |
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| Ac-Lr | 89 Ac |
90 Th |
91 Pa |
92 U |
93 Np |
94 Pu |
95 Am |
96 Cm |
97 Bk |
98 Cf |
99 Es |
100 Fm |
101 Md |
102 No |
103 Lr |
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- Item Name: Molybdenum
- Element Symbol: Mo
- Atomic Number: 42
- Atomic Weight: 95.95
- Atomic Radius: 139pm
- Density: 10.2g/cm3
- Melting Point: 2620°C
- Boiling Point: 5560°C
- Electronic Layout: [Kr]4d55s1
The Atomic Structure of Molybdenum
Physical Properties
Molybdenum is a silver-white metal. It has a high melting point and ranks sixth among natural elemental substances. It is called refractory metal. The density of molybdenum is 10.23 g/cm3, which is about half of tungsten (tungsten density 19.36 g/cm3). The thermal expansion coefficient of molybdenum is very low, the thermal conductivity is high, and the resistivity is low. Molybdenum has a relatively high hardness, with a Mohs hardness of 5 to 5.5.
Chemical Properties
The chemical properties of molybdenum are relatively stable. Molybdenum is stable in air or water at room temperature or not too high temperature. When molybdenum is heated in the air, the color begins to change from white to dark gray; when the temperature rises to 520°C, molybdenum starts to be slowly oxidized to produce Mo2O3; when the temperature rises above 600°C, molybdenum is rapidly oxidized to MoO3. When molybdenum is heated to 700~800°C in water vapor, MoO2 will start to be generated. If it is further heated, MoO2 will continue to be oxidized to MoO3. Molybdenum can ignite spontaneously in pure oxygen to produce MoO3.
Molybdenum does not react with hydrofluoric acid, hydrochloric acid, dilute nitric acid, dilute sulfuric acid and alkaline solutions at room temperature. Molybdenum is only soluble in concentrated nitric acid, aqua regia, hot and concentrated sulfuric acid, and boiling hydrochloric acid.
Introduction to the Crystal Structure of Molybdenum
The crystal structure of molybdenum is body-centered cubic, and no isomerization transformation has been found so far. The figure shows the crystal structure of molybdenum.
The Crystal Structure of Molybdenum
The Role of Molybdenum in Superalloys
Molybdenum is a very common alloying element, and it also plays a very important role in superalloys. Several main functions of molybdenum in superalloys will be introduced below.
Anti-reduction Effect of Molybdenum
In the previous article, we mentioned that an important role of chromium is oxidation resistance.
On the contrary, molybdenum in the alloy can effectively improve the reduction resistance of the alloy. Therefore, many alloys will add some molybdenum to further improve the overall corrosion resistance of the alloy.
This characteristic of molybdenum is most widely used in Hastelloy B series (Hastelloy B, Hastelloy B-2, Hastelloy B-3). This type of alloy contains almost 70% nickel and 30% molybdenum. The extremely high molybdenum content makes this alloy play a role that other alloys cannot replace in projects in reducing environments.
Solid Solution Strengthening Effect of Molybdenum
Another very important role of molybdenum in alloys is solid solution strengthening. Since the atomic radius of molybdenum is much larger than that of nickel, its solid solution strengthening effect is very significant. Among them, most of the molybdenum is solid-dissolved in the γ matrix, and a small part of the molybdenum is solid-dissolved in the γ' phase (about 1/4). The following figure shows the relationship between alloy yield strength and molybdenum content.
The Effect of Molybdenum Content on the Yield Strength of Alloys
Hastelloy is mentioned above. One of the characteristics of this alloy series is that only solid solution strengthening is used to increase the strength of the alloy. Therefore, almost all Hastelloy alloys contain molybdenum. This is an important component to ensure the strength of Hastelloy.
Precipitation Strengthening Effect of Molybdenum
The addition of molybdenum will cause a large amount of M6C carbides to be formed in the alloy. These carbides are dispersed among the crystals and can play a certain precipitation strengthening effect.
Grain Refinement Effect of Molybdenum
Finally, molybdenum can also refine the grains, thereby further improving the properties of the alloy. When the molybdenum content is 6% to 7%, the grain size is No. 4 to 5. When the molybdenum content is 8% to 9%, the grain size is No. 7 to 8. This is another reason why molybdenum can improve the yield strength.
Conclusion
Molybdenum is a common alloying element, which can make the alloy have excellent corrosion resistance in a reducing environment. The larger atomic radius also makes it have a significant solid solution strengthening effect. In addition, it can also play a role in precipitation strengthening and grain refinement.
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