Tantalum and tantalum base alloys such as TA-40Nb, Ta 2.5W and TA-10W have relatively high melting point temperatures. For example, pure tantalum melts at 3020 °C, TA-40Nb melts at 2705 °C, TA-2.5W melts at 3005 °C and TA-10W melts at 3030 °C. Because of these higher melting point temperatures, thermite reactants can be selected to produce metal products that form volatile (sacrificial) alloy constituents. The volatile (sacrificial) alloy composition facilitates the liquefaction and polymerization of the metal products produced by thermal-aluminization by lowering the melting temperature of the alloy. As used in this paper, the specified components of tantalum alloys (e.g., Ta, Nb, W) are more volatile and thus can be easily reduced to the attached impurity levels of elements such as copper and iron in tantalum base alloys refined using electron beam melting. The precursor reactants used to produce volatile (or sacrificial) alloy components may be referred to as "sacrificial metal oxides".
Iron is added to tantalum as an alloying element to lower the melting temperature. For example, tantalum containing 5 % by weight of iron melts at 2500 °C compared to pure tantalum melting at 3020°C. Similarly, copper lowers the melting temperature of pure tantalum and tantalum alloys. Iron and copper are also readily formed by thermite reduction of iron oxide (III) and copper oxide, and both thermite reactions produce large amounts of heat resulting in high reaction temperatures. Iron and copper are also relatively more volatile than tantalum, niobium and tungsten and are therefore easily removed from the tantalum alloy matrix using electron beam melting.
In each embodiment, the reactant may also include thermite accelerators to generate sufficient heat to reach the reaction temperature that causes alloy formation and slag phase separation. Thermite accelerators are reactant compounds that oxidize aluminum and produce a large amount of reaction heat, but do not produce reduced metal prices that coalesce into tantalum alloy matrix. Examples of thermal accelerant reactants include, for example, potassium chlorate and barium peroxide.
