Effect of Nb addition on Fe-7Cr-Nb and Fe-15Cr-Nb metastable alloy formation and their thermal stability

Thermal stability of any structural nanomaterial is extremely important for its prospective high temperature applications. Here, we have investigated the influence of Nb content (up to lat%) on the thermal stability of mechanically alloyed nanocrystalline Fe-Cr alloys for its possible nuclear first wall reactor application. The alloys were prepared by mechanical milling for 25 h in a high purity argon atmosphere (purity < 10 ppm O-2) at room temperature. The milling was performed in hardened steel grinding media using SPEX 8000M high energy mill and the powder to ball mass ratio was kept at 1:10. Evolution of disordered Fe-Cr-Nb solid solutions has been analyzed through estimating free energy (Gibbs) change as per Toop's model, variation of precise lattice parameter and TEM-SAED study. The Fe- 15%Cr-Nb alloys showed effective thermal stability due to the addition of only 0.25%Nb. The crystallite size of the stabilize alloy was similar to 58 nm after annealing at 1000 degrees C. TEM grain size analysis also corroborated the same. The corresponding microhardness value was found to be quite high (7.8 GPa) which also correlated the stabilizing effect. The grain boundary segregation by oversize Nb atoms along with solid solution hardening by Cr is identified to play the pivotal role for such attractive thermal stability.