1. 1) C. Y. Ang: Acta Met., 1 (1953), 123.
2. 2) J. W. Marx, G. S. Baker and \.{J}. M. Sivertsen: Acta Met., 1 (1953), 193.
3. 3) E. A. Gulbransen and K. F. Andrew: Trans. AIME, 188 (1950), 586.
4. 4) W. M. Albrech and W. D. Goode: Battelle Memorial Inst. Publ. BMI-1360, (1959), p. 16.
5. 5) H. W. Paxton and \.{J}. M. Sheehan: The Influence of H and N on Some Mechanical Properties of Nb, Metals Research Lab., Carnegie Inst. Technology, (1957), p. 463.