Alloys of Bismuth

Alloys of bismuth are not used to a great extent in industry. The low melting point of the metal, and the fact that it forms simple eutectiferous series of alloys with certain other metals, enables it to be used, however, in the manufacture of "fusible alloys." These are mainly ternary or quaternary alloys of bismuth, tin, lead and cadmium; some typical analyses are as follows:

Compositions of some "Fusible Alloys"

	Bi	Pb	Sn	Cd	M.pt. (° C.).
Newton's	50	31	19	. . .	95
Rose's	50	28	22	. . .	100
Darcet's	50	25	25	. . .	93
Wood's	50	24	14	12	71
Lipowitz'	50	27	13	10	M.pt. 70° C., softens at 60° C.

The freezing point diagrams, and in some cases the conditions of equilibrium, of a number of binary alloys of bismuth have been investigated. They may be described briefly as follows:

Two compounds are formed: Li₃Bi (M.pt. 1145° C.) and LiBi (formed by a peritectic reaction at 415° C.). The latter is dimorphous, with a transition point at 400° C. No ranges of solid solution have been observed. There are two eutectics, at 14 atoms per cent, lithium (M.pt. 243° C.), between Bi and a LiBi, and at 97.5 atoms per cent, lithium (M.pt. 175° C.), between Li and Li₃Bi. Primary crystals of LiBi are formed only between 35 and 37 atoms per cent, lithium.

Two intermetallic compounds are indicated: Na₃Bi (M.pt. 790° C.) and NaBi (decomposing at 450° C.). A eutectic is formed at 97.5 per cent, bismuth (M.pt. 218° C.). The compound NaBi crystallises with a tetragonal, body-centred lattice: a = 3.46 A., c = 4.80 A. The unit cell contains one atom of each element, the lattice containing no complexes of bismuth atoms. Complexes, such as Na₃Bi₅.xNH₃, are however formed with liquid ammonia, which, on removal of ammonia, yield mixtures of NaBi and bismuth.

Two compounds are formed: KBi₂ (M.pt. 553° C.) and K₃Bi₂ (M.pt. 673° C.). A transformation occurs in the latter at 281° C. A eutectic is formed at the bismuth end of the system, melting at 64° C., and the liquidus curve shows discontinuities at 18 per cent, bismuth (373° C.), 24 per cent, bismuth (423° C.) and 79 per cent, bismuth (281° C.). The compound KBi₂ crystallises with a cubic, face-centred lattice: a = 9.501 ± 0.005 A. There are eight molecules in the unit cell.

These metals form a simple eutectiferous series of alloys with the eutectic close to the bismuth end, melting at 268° C. X-ray analysis reveals that the lattice of each metal remains unchanged and indicates that the mutual solubility is very low. The influence of bismuth on the mechanical properties of copper is considerable, the presence of 0.05 per cent, producing brittleness. The addition of arsenic mitigates to some extent the effect of bismuth. With more than 0-005 per cent, bismuth, copper is unsuitable for wire drawing.

No compounds are formed. A eutectic is found at 2.5 per cent, silver (melting at 260° C.), and the maximum solid solubility of bismuth in silver is about 5.5 per cent. These results are confirmed by X-ray analysis.

A eutectic is formed at 83 per cent, bismuth (240° C.); the solid solubility of bismuth in gold is approximately 4.5 per cent. Although neither bismuth nor gold exhibits the phenomenon of superconductivity, the eutectic mixture becomes superconducting at -271.1° to -271.6° C. It is stated that the superconductivity is localised in the solid solution phase.

One compound is formed, Mg₃Bi₂ (M.pt. 823° C.), which exists in two modifications, α and β, the transition temperature being 700° C. for the pure compound and slightly lower for alloys containing a slight excess of magnesium. There appears to be a slight range of solid solution of magnesium in the compound. Two eutectics are formed, at 14.3 atoms per cent, bismuth (M.pt. 551° C.) and at 95.7 atoms per cent, bismuth (M.pt. 260° C.).

Two compounds are formed: CaBi₃ (decomposing at 506° C.) and Ca₃Bi₂ (M.pt. 928° C.). A third compound, CaBi, may also exist. There are two eutectics, at 0.5 per cent, calcium (M.pt. 270° C.) and at 88 per cent, calcium (M.pt. 785° C.). The heat of formation of the compound Ca₃Bi₂ at 21.5° C. is 51,600 gram-calories per mole.

A eutectic is formed at 97 per cent, bismuth (M.pt. 250° C.). Within the range of composition between 1 and 70 per cent, bismuth the melt, above 417° C., consists of two immiscible liquids.

These metals form a simple eutectiferous system with a eutectic at 40 per cent, cadmium (M.pt. 149° C.).

The two metals are mutually insoluble in the solid state; a eutectic is formed very close to the mercury end of the series (M.pt. -39°C.).

Within the range of composition between 3 and 98.7 per cent, bismuth the melt, above 650° C., consists of two immiscible liquids. The addition of small amounts of bismuth to aluminium causes a slight depression of the melting point, the maximum depression being 3.5° C. Bismuth is slightly soluble in aluminium in the solid state, but aluminium appears to be insoluble in bismuth.

In this system there is a range of composition within which the melt consists of two immiscible liquids. The maximum solid solubility of gallium in bismuth is 11 per cent.

In this system maxima occur on the liquidus curve at 10 per cent, bismuth (307° C.) and at 62 per cent, bismuth (220° C.); the latter corresponds to the compound Tl₃Bi₅. Eutectics are found at 5 per cent, bismuth (M.pt. 300° C.), at 46 per cent, bismuth (M.pt. 180° C.), and at 78 per cent, bismuth (M.pt. 200° C.). There are three ranges of solid solution: a between 0 and 4 per cent, bismuth, β between 5.5 and 32 per cent, bismuth, and γ between 57 and 65 per cent, bismuth. The last contains the compound Tl₃Bi₅. This compound becomes superconducting at -266.8° C.

In this system, within the range of composition between 2 and approximately 100 per cent, silicon the melt, above 1415° C., consists of two immiscible liquids. A eutectic is formed at 0.8 per cent, silicon (M.pt. 264° C.). Solid solution occurs to only a very slight extent.

The following compounds are formed: Ce₃Bi (decomposing at 1395° C.), Ce₄Bi₃ (M.pt. 1630° C.), CeBi (decomposing at 1520° C.) and CeBi₂ (decomposing at 870° C.). Eutectics are found at 5.3 per cent, bismuth (M.pt. 757° C.) and at approximately 100 per cent, bismuth (M.pt. 270° C.). Discontinuities occur on the liquidus curve at 33 per cent, bismuth (1359° C.), at 69 per cent, bismuth (1520° C.) and at 82 per cent, bismuth (870° C.).

A eutectic occurs at 58 per cent, tin (M.pt. 135° C.), and there is probably a euteetoid at 96 per cent, tin (forming at 95° C.). The solid solubility of tin in bismuth and of bismuth in tin is probably less than 1 per cent, in each case. The eutectic mixture becomes superconducting at -269.24° C.

There are no intermetallic compounds. A eutectic is formed at 58 per cent, bismuth (M.pt. 125° C.). The solid solubility of bismuth in lead is 37 per cent, at 125° C. and 16 per cent, at 0° C. The solid solubility of lead in bismuth is 2.7 per cent, at 125° C. and slightly less at 0° C. The eutectic mixture becomes superconducting at a temperature slightly above the boiling point of helium. The hardening effect of bismuth on lead is less than that of antimony.

These metals form a continuous series of solid solutions. The liquidus curve lies wholly between the melting points of the component metals, and the solidus is horizontal within the range of composition lying between 40 and 100 per cent, bismuth. X-ray analysis indicates that these alloys crystallise with a face-centred, rhombohedral structure; the lattice edge varies almost linearly with the composition.

Two compounds are formed, BiSe (decomposing at 602° C.) and Bi₂Se₃ (M.pt. 688° C.). Above 608° C. and within the range of composition between 50 and 92 per cent, selenium the melt consists of two immiscible liquids. Eutectics are formed at approximately pure bismuth and pure selenium.

One compound only, Bi₂Te₃ (M.pt. 573° C.), is formed. Eutectics occur at 15 per cent, bismuth (M.pt. 370° C.) and at 98.8 per cent, bismuth (M.pt. 253° C.). The compound Bi₂Te₃ forms solid solutions with both of the elements.

A eutectic is formed at 1.3 per cent, manganese (M.pt. 263° C.). Above 1240° C. and within the range of composition from 42 to approximately 100 per cent, manganese the melt consists of two immiscible liquids. Discontinuities occur on the liquidus curve at 8.3 per cent, manganese (453° C.) and at 42 per cent, manganese (1240° C.).

These metals are mutually insoluble in each other in the solid state, and they do not combine with one another. The addition of bismuth to cast-iron produces softening and a reduction of strength; the softening is accompanied by decomposition of the carbides.

A eutectic is formed at 97 per cent, bismuth (M.pt. 260° C.). Above 1350° C. and within the range of composition from 9 to 95 per cent, bismuth the melt consists of two immiscible liquids.

Two compounds are formed, NiBi₃ (decomposing at 560° C.) and NiBi (decomposing at 720° C.). There is a eutectic at 0.7 per cent, nickel. Discontinuities occur on the liquidus curve at 4 per cent, nickel (560° C.) and 12 per cent, nickel (720° C.). The solid solubility of bismuth in nickel is 5 per cent.; this solid solution is magnetic below the temperature range from 360° to 396° C.

Three compounds are formed, RhBi₄, RhBi₂ and RhBi. At 433° C. the compound RhBi₄ is decomposed and RhBi₂ formed. There are three eutectics, at 7 per cent, rhodium (M.pt. 260° C.), at 19.8 per cent, rhodium (M.pt. 772° C.), and at 31.5 per cent, rhodium (M.pt. 995° C.).

A few ternary alloy systems containing bismuth as one of the components have also been investigated. Among them may be mentioned the systems bismuth-zinc-cadmium, bismuth-tin-zinc, and bismuth-tin-lead. In the last system a ternary eutectic is formed containing 52 per cent, bismuth, 16 per cent, tin and 32 per cent, lead; it melts at 96° C. It has also been suggested that a ternary compound, Bi₂SnPb, is formed as a result of a reaction in the solid phase. This compound is stable below 76° C., but above that temperature it decomposes, forming three solid solutions.

The quaternary system bismuth-cadmium-tin-lead has also been investigated. A quaternary eutectic is formed containing 49.5 per cent, bismuth, 10.10 per cent, cadmium, 13.13 per cent, tin and 27.27 per cent. lead. It melts at 70° C.