A metastable allotropic transformation in Nb induced by planetary ball milling

The present investigation concerns planetary ball milling of elemental Nb in hardened steel and WC-coated media. Continued milling for about 20 h reduces the grain size below 10 nm with a concomitant lattice expansion. The final product obtained after 30–40 h milling is a pure f.c.c. phase. A detailed micro structural/compositional characterization by X-ray diffraction, resistivity measurements and wet chemical analysis proves that the said f.c.c. phase is neither NbN nor NbC. Instead, it is suggested that elemental Nb undergoes an irreversible b.c.c.→f.c.c. allotropic transformation when the grain size reduces to below 10 nm during high energy ball milling. Furthermore, this f.c.c. phase possibly undergoes an impurity stabilized ordering transformation on annealing at 900°C for 2 h. Finally, the role of impurity in causing this allotropic change is examined.