Metal-gas equilibrium in the niobium-nitrogen terminal solid solution

Abstract

The pressure-temperature-composition equilibrium is studied in the metal-rich portion of the niobium-nitrogen system. In the terminal solid solution, Sieverts' Law is found to be obeyed for all compositions studied. ΔH N , the heat of solution of nitrogen in niobium is given as −46 kcal per g-atom of nitrogen; ΔS N° , the entropy of solution at 1 atom per cent solute is given as −14.7 E.U. per g-atom of nitrogen, in good agreement with the theoretical configurational entropy of solution. An empirical expression for the equilibrium concentration of nitrogen in solution is given as x = 6.2 × 10 −4(p N 2 ) 1 2 exp( 46,000 RT ) where x is atom per cent nitrogen and p N 2 is in mm of Hg. Nb 2N is found to be the nitride in equilibrium with the solid solution. The standard enthalpy and entropy of formation of Nb 2N above 1500°C are indicated as −67 kcal per g-atom of nitrogen and −22.3 E.U. per g-atom of nitrogen respectively. The terminal solubility of nitrogen in niobium with respect to Nb 2N is measured from 300°C to 2230°C and found, with one exception, to be in general agreement with earlier studies. An expression for the terminal solubility above 1150°C is x = 720 exp ( −20,000 RT ) . It is concluded that nitrogen exhibits nearly ideal behavior as a solute in niobium.