What is Cobalt
Cobalt is a lustrous gray metal. It is an important raw material for the production of heat-resistant alloys, hard alloys, anti-corrosion alloys, magnetic alloys and various cobalt salts. The ancient Greeks and Romans used its compounds to make colored glass to produce a beautiful deep blue. The blue color on China's Tang Dynasty colored porcelain is also due to the presence of cobalt compounds.
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| 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: Cobalt
- Element Symbol: Co
- Atomic Number: 27
- Atomic Weight: 58.93
- Atomic Radius: 125pm
- Density: 8.9g/cm3
- Melting Point: 1495°C
- Boiling Point: 2870°C
- Electronic Layout: [Ar]3d74s2
The Atomic Structure of Cobalt
Physical Properties
Cobalt is a shiny gray metal with a melting point of 1493°C. It is relatively hard and brittle. Cobalt is ferromagnetic and is similar to iron and nickel in terms of hardness, tensile strength, machining performance, thermodynamic properties, and electrochemical behavior. Cobalt loses its magnetism when heated to 1150°C.
Chemical Properties
Cobalt does not interact with water at room temperature and is stable in humid air. It will oxidize when heated to above 300°C in air. Cobalt is a medium reactive metal. Its chemical properties are similar to iron and nickel. When heated, cobalt reacts violently with oxygen, sulfur, chlorine, bromine, etc. to form corresponding compounds. Cobalt is soluble in dilute acid and is passivated by forming an oxide film in fuming nitric acid. Cobalt is slowly eroded by hydrofluoric acid, ammonia and sodium hydroxide.
Introduction to the Crystal Structure of Cobalt
Similar to iron, the crystal structure of cobalt also has an allotropic transformation. From room temperature to high temperature, cobalt changes from a hexagonal close-packed structure (HCP) to a face-centered cubic structure (FCC). The figure below shows the two structures of cobalt:
Two Structures of Cobalt
The Role of Cobalt in Cobalt-based Superalloys
Cobalt exists as a matrix in cobalt-based superalloys. Cobalt as a matrix allows cobalt-based alloys to have a higher melting point, excellent thermal corrosion resistance, thermal fatigue performance and good welding performance. Therefore, cobalt-based alloys still have many applications in the aerospace industry. Turbine guide blades are one of the main uses of cobalt-based alloys.
In cobalt-based alloys, it is generally necessary to add about 10% to 30% nickel. This serves to stabilize the austenite. In addition, the iron in the cobalt-based alloy can also play a role in stabilizing austenite.
The Role of Cobalt in Nickel-based Superalloys
The atomic radius of cobalt is not much different from that of nickel, but cobalt can significantly reduce the stacking fault energy of the nickel matrix, thereby achieving the effect of solid solution strengthening. The figure below shows the relationship between the cobalt content and the stacking fault energy in the alloy.
The Relationship between Cobalt Content and Stacking Fault Energy
The addition of cobalt to the matrix of the nickel-based alloy can also reduce the solubility of aluminum and titanium in the matrix. Aluminum and titanium are the main precipitation strengthening elements, reducing the solubility of aluminum and titanium in the matrix can make them more involved in precipitation strengthening.
In addition, the cobalt element can also enter the precipitation strengthening phase to reduce the solubility of the precipitation phase, making the precipitation phase more stable at high temperatures.
The Role of Cobalt in Iron-based Superalloys
The role of cobalt in iron-based superalloys is similar to that of nickel-based alloys. However, the price of cobalt is relatively high, while iron-based superalloys are developed for reduce costs, which is contradictory. Generally speaking, cobalt is not added to iron-based alloys.
Other effects of cobalt
In addition to the above effects, cobalt can also play a role in improving creep strength in superalloys.
The Relationship between Cobalt Content and Creep Rupture Time
In addition, in some low-expansion superalloys (such as Inconel 783, Incoloy 903, Incoloy 907, Incoloy 909), the addition of cobalt can reduce the high-temperature thermal expansion coefficient.
903
Al
Ti
Co
Nb
Fe
Ni
907
Si
Al
Ti
Co
Nb
Fe
Ni
909
Si
Mn
Al
Ti
Cu
Co
Nb
Fe
Cr
Ni
783
Si
Al
Ti
Cu
Co
Nb
Fe
Cr
Ni
Conclusion
Cobalt as a matrix in cobalt-based superalloys can improve the high-temperature properties of the alloy.
Cobalt can not only play a role in solid solution strengthening in nickel-based alloys, but also indirectly improve the effect of precipitation strengthening.
Cobalt is generally not added to iron-based alloys.
Cobalt can also increase the creep strength of the alloy.
The addition of cobalt to low-expansion superalloys can reduce the thermal expansion coefficient of the alloy.
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