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Other Alloys

Cobalt is used in a wide range of specialist alloys other than those listed in other sections. Typical examples are:

Controlled Expansion Alloys

These alloys are based on nickel/iron alloys with the 36% nickel version demonstrating the lowest expansion coefficient 1 x 10-6 per °C. The coefficient is however very temperature dependent and even small changes affect it. Other binary alloys such as 48% nickel are more stable but one sacrifices the very low values that can be obtained.

Figure 1 shows a general curve, relating Δ|L/L the expansion coefficient to temperature and this type of curve applies to all the low expansion materials.


Naturally, we would like a material where the flat part of the curve C-D is at a low level and T2 the temperature, where the curve inflexes and climbs again is at high level.

In practice, substituting cobalt for nickel goes some way towards this. The lowest expansion occurs with 6% Co substitution into the original 36% Ni alloy – Invar and we have Super-Invar.

In fact, various levels of cobalt can be used to vary ΔL/L and create a series of alloys with specific expansion coefficients (Table 1).

Table 1 - Low Expansion Cobalt Alloys

Trade or Common Name

Co

%

Fe

%

Ni

%

Cr

%

Mn

%

C

%

Coefficient of expansion 10-6 per °C
Super-Invar
3.5
62.5
34.0
-
-
0.007
0.3
Ditto
4.0
63.5
34.0
-
-
0.007
0.0
"
4.0
63.0
33.0
-
-
0.007
0.4
"
4.0
62.5
33.5
-
-
0.007
0.5
"
5.0
63.5
31.5
-
-
0.007
0.0
"
5.0
62.5
32.5
-
-
0.007
0.5
"
6.0
63.5
30.5
-
-
0.007
0.0
"
6.0
62.5
31.5
-
-
0.007
0.1
"
5.0
64.0
31.0
-
0.35
0.007
0.1
"
6.0
63.0
31.0
-
0.38
0.007
0.0
Super-Nilvar
4-6
Bal
31
-
-
-
-
Fernico
15
54
31
-
-
-
4.95
Fernichrome
25
37
30
8
-
-
9.0
Kovar
18
54
28
-
min.
min.
4.0

The original alloys – Invar, etc. – may have had their applications linked to mechanical devices where expansion was a problem – like clocks, watches, measuring devices, etc. – but the Super-Invar and Kovar now find their uses in the electronic age. They use their controlled expansion to match that of glass for glass to metal seals, and also in the electronic packaging industry where they can match various substrates to provide hermetic seals which can stand the rigours of the +55°C -55°C expansion test and also provide corrosion resistance.

Cobalt in Steels

Cobalt is not an element commonly added to alloy steels. It does have some effects but these are also obtainable by other additives at lower cost and mostly with better results – Molybdenum, nickel, etc. We have seen in other areas that cobalt does not form carbides and that in fact, it decreases hardenability (a measure of the depth to which a steel hardens on quenching). It hardens ferrite but only marginally and has only a small influence on the transformation temperature of iron.

The factors above ensure that cobalt is unlikely to ever find a use in high tonnage low alloy steel production. It does however have some niche markets in steel. In martensitic steels, cobalt has the effect of delaying tempering and this can be shown by plotting hardness against a time/temperature parameter as in Figure 1 (T = temperature, t = hours). Increasing cobalt levels produces increased hardness and steels of this type 1%C, 12%Cr, 4%Mo and 7%Co have attractive properties.



Some steels such as Jethete M120 have been developed for use at high temperature using the effect of cobalt to give high temperature strength in the range below which superalloys are more usual.

The steels where cobalt has found its home (apart from the high speed variety) are termed maraging. This name is derived from the fact that they are aged in the martensitic form. The original steels used were 20/25% nickel steels with small additions of Al, Ti and Nb. The secret of these steels was and is, that simple air-cooling is sufficient to transform the Austenite high temperature phase to Martensite, the hard unstable lower temperature form. On reheating, the temperature to return to Austenite is found to be much higher than the Martensite forming temperature of 250°C and is in fact over 500°C. Reheating (aging) at an intermediate temperature retains the Martensite but allows precipitation of various hardening phases such as Ni3Mo, Ni3Ti, FeMo and these raise the hardness to up to 900 Vickers. The early steels were found to be brittle and cobalt additions solved this problem. As usual, the role of cobalt is obscure but it enhances the properties and accelerates the process. Table 1 shows a range of typical steels.

Table 1 - Composition and Properties of Maraging Steels

Composition, wt.%
UTS
Elong.
Hardness
Grade
Ni
Co
Mo
Ti
Al
103 psi
MN/m2
%
Rc
18Ni (200)
18
8.3
3.25
0.2
0.10
210
1450
13
43
18Ni (250)
18
7.5
4.8
0.4 0.10 255 1750 13 50
18Ni (300)
18
9.0
4.8
0.6 0.10 285 1960 11 54
18Ni (350)
17.5
12.5
3.8
1.7
0.15 355 2450 9 58
13Ni (400)
13
15.5
10.8
0.2
- 390 2690 5 59

Their properties are not the highest possible but they score in that they can be air-cooled without distortion, machined without difficulty and finally, develop their properties with a relatively simple low temperature aging process.

Maraging steels have found many uses in the aerospace and military industries where their strength coupled with workability has got them the job over possibly stronger materials.

Typical applications are landing gears, arrestor hooks, torque shafts, rocket motor casings, gun barrels, bolts, fasteners, extrusion arms, etc., etc. There are the areas where cobalt steels are best.

Electronic Alloys

Electronic technology can be divided into integrated circuits (packages of components on a microchip); discrete semi-conductors; vacuum tubes; magnetic tapes, resistors; transformers; capacitors.

Cobalt has a place in some of these and also in newer recording techniques such as thin film technology.

Integrated Circuits

An integrated circuit (IC) is a device which consists of a number of circuit elements formed in the surface of a chip of semi-conductor material (usually silicon). Modern ICs contain millions of circuit elements on a chip a few millimetres square.

Cobalt is used in a few ways in conjunction with ICs.

Contacts

Interconnections on chips are usually made with aluminium. However, smaller and smaller circuits have brought metal silicides into use because of their low resistivity, high thermal stability and good bonding properties. Pt/Si is generally used but CoSi2 has been patented for this use.

Metal Leads

Gold is the common metal for marking mechanical electrical contacts. The gold used however is co-deposited with (15%) cobalt and this promotes far superior wear properties. This may not sound important in circuitry but cycling occurs and in fact failure can be caused by friction.

Packages

Whilst not strictly ICs, these are components packed onto a circuit board (PCB). The board can be plastic, beryllia, alumina or metal – copper, copper/tungsten, copper/Invar/copper, molybdenum, etc. – and cobalt is back in the picture.

The thermal coefficient of expansion (TCE) of these alloys can be matched to a substrate and used to box in components by gluing ring frames over the circuits or brazing and finally sealing the whole unit to protect it.

Semi-Conductors

Cobalt has been used in the manufacture of semi-conductors devices containing gold or platinum. Cobalt can be diffused through the substrate and then gold or platinum is diffused in at a temperature sufficient to replace the cobalt but somewhat lower than would otherwise be required.

Miscellaneous Alloys

Cobalt is used in other alloys, Co/Pt magnets, 36%Ni12%Cr spring alloys which can be varied to provide given temperature expansion coefficients against elastic moduli.

One use left until last is the role of cobalt in cancer treatment and flaw detection with the 60Co isotope. Cobalt-59 is irradiated in a reactor for a long period and some of the metal is converted to 60Co. This isotope has a half-life of 5.3 years and emits g-rays as it decays. These rays can be used in portable machines in lieu of Y-rays for flaw detection. They have the advantage of portability and greater penetration over X-rays though they do not provide quite the same definition in photographic terms.

The rays can also be targeted at cancer cells and used to destroy them. This is the basis of radiation therapy in cancer treatment. Food also has its life prolonged by irradiation after packing.

 

    
 
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