Tantalum and its Alloys
Tantalum was first separated from Nb in the year 1802. Its melting point of 2996oC is only less than those of W and Re .In spite its high melting point Ta is simply workable at room temperature as its ductile-brittle transition temperature is extremely low. Due to its high corrosion resistance Ta is utilized in chemical process industry. The major use of Ta is in electronic industry as capacitor. Its high cost is main hindrance in its extensive use.
At approx 150oC tantalum forms a thin but stable oxide film on the surface. This film obstructs further oxidation and therefore renders material resistant to corrosion. Though, Ta is attacked by sulfur trioxide and hydrofluoric acid. In such heat exchanges where at least one medium is corrosive Ta is preferably utilized. Tantalum bayonet heaters are utilized in sulfuric and nitric acid concentrators.
The capability of tantalum to compose solid solution with W and Nb results in alloys with improved strength, but their susceptibly to oxidation is improved. Hence they might be used above 480oC with protective coating only. Tantalum alloys are utilized from 1370oC to 1980oC. The common alloys are: Ta-40Nb (290 MPa at 260oC), Ta-10W (344 MPa at 1315oC), Ta-8W-2Hf. The first & third of above alloys are utilized as high temperature component in space craft and propulsion engines. Ta-10W alloy also offers the advantage of room temperature/fabricability.