Alloys of Tantalum

Tantalum yields alloys with a large number of other metals, but their mechanical properties and the systems produced have hitherto received little investigation. They are prepared by compressing the two metals and heating them to a high temperature in a good vacuum.

Aluminium alloys well with up to about 3.5 per cent, of tantalum, which has no effect, however, on the mechanical strength, ductility, and working properties of aluminium. Reduction of tantalum pentoxide by the thermite process yields hard, brittle alloys. A substance the composition of which corresponds with the formula TaAl₃ has been obtained by reducing potassium tantalum fluoride, K₂TaF₇, with aluminium filings at a high temperature. It is described as an iron-grey crystalline powder, of density 7.02, which is scarcely attacked by acids.

Copper alloys are mechanically strong and acid resisting.

Gold alloys resemble copper alloys. Gold-copper-tantalum alloys have also been made.

Iron alloys with tantalum in all proportions. Those alloys containing from 5 to 10 per cent, of iron are hard and ductile. Guillet has examined the effect of tantalum up to 1.05 per cent, on the structure and mechanical properties of steels prepared with tantalum-iron in the electric furnace. In the case of normal steels the usual pearlitic structure was present, the effect of the tantalum being to produce more regular distribution of the pearlite. Quenched steels displayed their usual structure. Mechanical tests showed only slight increases in the breaking load, the limit of elasticity, and the resistance to shock. The same effects can be readily obtained by the addition of small proportions of nickel and other metals to the steel. The influence of tantalum on the characteristics in the shearing test is also small. It appears, therefore, that the effect of tantalum on steel is too small to enable tantalum steels to attain any special importance. It has also been shown that no advantage is gained by substituting tantalum for nickel, cobalt or molybdenum in high-speed steels. According to Guertler, tantalum in cast iron forms mixed crystals with the iron and precipitates the carbon as graphite.

Iron undergoes " cementation " when heated in finely divided iron- tantalum alloy containing about 30 per cent, of tantalum; the interior consists of a solid solution, which is bounded by a brilliant external layer the thickness of which increases with rise of temperature and with increase in the duration of heating. Copper and brass have been similarly treated.

Magnesium A magnesium-tantalum alloy which contains about 3.5 per cent, of magnesium has been obtained by reducing tantalum pentoxide with magnesium powder in a stream of hydrogen.

Molybdenum alloys with tantalum in all proportions. Alloys containing from 10 to 40 per cent, tantalum have been suggested for the construction of chemical and electrical apparatus. When the molybdenum content is less than 5 per cent., the product can be drawn into wire 0.1 mm. diameter.

Nickel Addition of from 5 to 10 per cent, of tantalum to nickel considerably increases the resistance of the nickel to acids, and also its ductility. An alloy containing 30 per cent, of tantalum is not attacked by prolonged boiling with aqua-regia or other acids, and unlike tantalum it can be heated in air without undergoing oxidation; it is also very tough and can be easily rolled, hammered, and drawn, but may become brittle when strongly heated. Nickel-tantalum alloys which contain from 5 to 20 per cent, of chromium are also resistant to heat and corrosion. An alloy which contains 75 per cent, of nickel, 11 per cent, of iron, and 14 per cent, of tantalum and niobium is claimed to be suitable for electrical resistances and electrical heating apparatus.

Platinum alloys containing from 0.5 to 20 per cent, of tantalum are hard, withstand heat, as well as the action of acids and fused potassium hydrogen sulphate, and are more resistant to the action of aqua-regia than platinum. They possess the mechanical properties of platinum-iridium alloys and are less expensive; the relative quantities of tantalum and iridium required to produce the same hardness and mechanical resistance are stated to be 1:5. Platinum-tantalum alloys hence have been recommended for various purposes in place of platinum or platinum-iridium. Tantalum can also be coated with platinum, and can then be utilised in high-temperature work.

Silicon Small amounts of silicon do not affect the ductility of tantalum and increase its hardness. A substance the composition of which agrees with the formula TaSi₂ has been obtained by heating a mixture of tantalum pentoxide and silica in the presence of aluminium. It is described as a greyish-blue substance which forms four-sided prisms of density 8.8. It is stable in air, oxidises when heated in oxygen, and is insoluble in most acids; it is attacked by fused caustic soda.

Tungsten alloys with tantalum in all proportions. Alloys of tungsten and tantalum which also contain cobalt, chromium or molybdenum have also been prepared.

Zirconium alloys can be heated to whiteness without undergoing oxidation.

Sodium, potassium, mercury and silver do not alloy with tantalum even at high temperatures; attempts to prepare alloys with arsenic, antimony, lead, zinc and tellurium have also failed, but the formation of an alloy with silver, copper and tin for making a dental amalgam with mercury has recently been claimed.