Fusible alloy. A fusible alloy is a metal alloy capable of being easily fused, i.e. easily meltable, at relatively low temperatures. Fusible alloys are commonly, but not necessarily, eutectic alloys.

Sometimes the term "fusible alloy" is used to describe alloys with a melting point below . Fusible alloys in this sense are used for solder.

Introduction
Fusible alloys are typically made from low melting metals.
There are 14 low melting metallic elements that are stable for practical handling. These are in 2 distinct groups:
The 5 alkali metals have 1 s electron and melt between +181 (Li) and +28 (Cs) Celsius;
The 9 poor metals have 10 d electrons and from none (Zn, Cd, Hg) to three (Bi) p electrons, they melt between -38 (Hg) and +419 (Zn) Celsius.
From a practical view, low-melting alloys can be divided into the following categories:
Mercury-containing alloys
Only alkali metal-containing alloys
Gallium-containing alloys (but neither alkali metal nor mercury)
Only bismuth, lead, tin, cadmium, zinc, indium, and sometimes thallium-containing alloys
Other alloys (rarely used)
A practical reason here is that the chemical behaviour of alkali metals is very distinct from poor metals. Of the 9 poor metals Hg (mp -38 C) and Ga (mp +29 C) have each their distinct practical issues, and the remaining 7 poor metals from In (mp +156 C) to Zn (mp +419 C) can be viewed together.
Of elements which might be viewed as related but do not share the distinct properties of poor metals:
Po is estimated to melt at 254 C and might be poor metal by properties but is too radioactive (longest halflife 125 years) for practical use;
At same reasoning as Po;
Sb melts at 630 C and is regarded as semimetal rather than poor metal;
Te is also regarded as semimetal not poor metal;
of other metals, next lowest melting point is Pu, but its melting point at 640 Celsius leaves a 220 degree gap between Zn and Pu, thus making the "poor metals" from In to Zn a natural group.

Some reasonably well-known fusible alloys are Wood's metal, Field's metal, Rose metal, Galinstan, and NaK.

Applications
Melted fusible alloys can be used as coolants as they are stable under heating and can give much higher thermal conductivity than most other coolants; particularly with alloys made with a high thermal conductivity metal such as indium or sodium. Metals with low neutron cross-section are used for cooling nuclear reactors.

Such alloys are used for making the fusible plugs inserted in the furnace crowns of steam boilers, as a safeguard in the event of the water level being allowed to fall too low. When this happens the plug, being no longer covered with water, is heated to such a temperature that it melts and allows the contents of the boiler to escape into the furnace. In automatic fire sprinklers the orifices of each sprinkler is closed with a plug that is held in place by fusible metal, which melts and liberates the water when, owing to an outbreak of fire in the room, the temperature rises above a predetermined limit.

Bismuth on solidification expands by about 3.3% by volume. Alloys with at least half of bismuth  display this property too. This can be used for mounting of small parts, e.g. for machining, as they will be tightly held.

Low-melting alloys and metallic elements

Well-known alloys

Other alloys

Starting with a table of component elements and selected binary and multiple systems ordered by melting point:

Then organized by practical group and alphabetic symbols of components:
Most of the pairwise phase diagrams of 2 component metal systems have data available for analysis, like at https://himikatus.ru/art/phase-diagr1/diagrams.php
Taking the pairwise alloys of the 7 poor metals other than Hg and Ga, and ordering the pairs (total 21) by alphabetic of these elements Bi, Cd, In, Pb, Sn, Tl, Zn are as follows:
Bi-Cd https://himikatus.ru/art/phase-diagr1/Bi-Cd.php simple eutectic (Bi at 271 C, Cd at 321, eutectic at 146)
Bi-In https://himikatus.ru/art/phase-diagr1/Bi-In.php has ordered phases, eutectic at +72 - in table above
Bi-Pb https://himikatus.ru/art/phase-diagr1/Bi-Pb.php eutectic at +125 - in table above
Bi-Sn https://himikatus.ru/art/phase-diagr1/Bi-Sn.php eutectic at +139 - in table above
Bi-Tl https://himikatus.ru/art/phase-diagr1/Bi-Tl.php an intermetallic alloy and the lower melting eutectic at +188
Bi-Zn https://himikatus.ru/art/phase-diagr1/Bi-Zn.php eutectic at +255
Cd-In https://himikatus.ru/art/phase-diagr1/Cd-In.php eutectic at +128
Cd-Pb https://himikatus.ru/art/phase-diagr1/Cd-Pb.php eutectic at +248
Cd-Sn https://himikatus.ru/art/phase-diagr1/Cd-Sn.php eutectic at +176
Cd-Tl https://himikatus.ru/art/phase-diagr1/Cd-Tl.php eutectic at +204
Cd-Zn https://himikatus.ru/art/phase-diagr1/Cd-Zn.php eutectic at +266
In-Pb https://himikatus.ru/art/phase-diagr1/In-Pb.php is NOT eutectic because Pb solid solution in In only raises melting point
In-Sn https://himikatus.ru/art/phase-diagr1/In-Sn.php eutectic at +120
In-Tl https://himikatus.ru/art/phase-diagr1/In-Tl.php also NOT eutectic because Tl solid solution in In raises melting point
In-Zn https://himikatus.ru/art/phase-diagr1/In-Zn.php eutectic at +143
Pb-Sn https://himikatus.ru/art/phase-diagr1/Pb-Sn.php eutectic at +183 - in table above
Pb-Tl https://himikatus.ru/art/phase-diagr1/Pb-Tl.php also NOT eutectic because the solid solution is higher melting than components
Pb-Zn https://himikatus.ru/art/phase-diagr1/Pb-Zn.php eutectic at +318
Sn-Tl https://himikatus.ru/art/phase-diagr1/Sn-Tl.php eutectic at +168
Sn-Zn https://himikatus.ru/art/phase-diagr1/Sn-Zn.php eutectic at +198 - in table above
Tl-Zn https://himikatus.ru/art/phase-diagr1/Tl-Zn.php eutectic at +292
Considering the binary systems between alkali metals: Li only has appreciable solubility in pair
Li-Na https://himikatus.ru/art/phase-diagr1/Li-Na.php eutectic at +92
The other three alkali metals:
K-Li https://himikatus.ru/art/phase-diagr1/K-Li.php
Li-Rb https://himikatus.ru/art/phase-diagr1/Li-Rb.php
Cs-Li https://himikatus.ru/art/phase-diagr1/Cs-Li.php
practically do not dissolve Li even when liquid and therefore their melting points are not lowered by presence of Li
Na is in liquid phase miscible with all three heavier alkali metals, but on freezing forms intermetallic compounds and eutectics:
K-Na https://himikatus.ru/art/phase-diagr1/K-Na.php eutectic at -12,6 - in table above
Na-Rb https://himikatus.ru/art/phase-diagr1/Na-Rb.php eutectic at -4,5
Cs-Na https://himikatus.ru/art/phase-diagr1/Cs-Na.php eutectic at -31,8
The 3 binary systems between the three heavier alkali metals are all miscible in solid at melting point, but all form poor solid solutions that have melting point minima. This is distinct from eutectic: at eutectic point, two solid phases coexist, and close to eutectic point, the liquidus temperature rises rapidly as just one separates, whereas at poor solid solution melting point minimum, there is a single solid phase, and away from the minimum the liquidus temperature rises only slowly.
K-Rb https://himikatus.ru/art/phase-diagr1/K-Rb.php solid solution minimum mp +34
Cs-K https://himikatus.ru/art/phase-diagr1/Cs-K.php solid solution minimum mp -38 - in table above
Cs-Rb https://himikatus.ru/art/phase-diagr1/Cs-Rb.php solid solution minimum mp +10

See also
Liquid metal
List of elements by melting point

References

Further reading

Weast, R.C., "CRC Handbook of Chemistry and Physics", 55th ed, CRC Press, Cleveland, 1974, p. F-22

External links
Fusible (Low Temp) Alloys
Fusible Alloys. Archived from the original on 2012-10-12.
Jenson, W.B. "Ask the Historian - Onion's fusible alloy" 

Coolants