Effect of Nitrogen on Creep Characteristics of αFe-W and αFe-Mo Solid Solutions

The effects of nitrogen on the creep characteristics of substitutional alpha solid solutions, the Fe--W and Fe--Mo systems (Fe--0.6W, Fe--0.6W--0.14N, Fe--0.9Mo, Fe--0.9Mo--0.15N, and Fe--0.2N and pure iron) have been investigated at 1100K. Three stress regions in the power-law creep regime are observed in these solid solutions, except the Fe--Mo--N solid solution. In Region L, N strengthens the Fe--W solid solution, while it weakens the Fe--Mo solid solution. The effect of N on the creep strength of the Fe--W solid solution is negligibly small in Regions M and H. In the Fe--Mo solid solution, however, the transition stress between Regions M and H increases with the addition of N and the steady-state creep-rate decreases in Region H. The effects of N on the creep characteristics depend on the substitutional solute in these solid solutions. The difference in the effects of N can be attributed to the difference in the interaction between interstitial and substitutional elements. In the Fe--Mo solid solution, N interacts strongly with molybdenum and so-called "IS effect" causes the increase of transition stress between Regions M and H in this system. In contrast, the interaction between N and tungsten is weaker than that of N and Mo, so that little changes in the transition stress are observed in the Fe--W solid solution. It becomes obvious that the addition of interstitial elements affects creep characteristics in the temperature range > 0.5 Tm (Tm: melting point), but accumulation of experimental results and detailed considerations are needed to understand the rate-controlling mechanism in Region L.