Structural evolution in niobium beryllides during mechanical alloying

Abstract

Mechanical alloying of 1Nb+12Be and 2Nb+17Be powder mixtures has been conducted. Both of the powders, in a premixed condition, exhibited agglomeration and compositional inhomogeneity. Upon MA for 6 h, broadening of Nb peaks was observed, whereas most Be peaks lost their identity. Significant lattice expansion ( ε=∼2.5 %), manifested by peak position shift, was observed in the Nb unit cell in the 1Nb+12Be powders, but no measurable peak position change was noted in the 2Nb+17Be powders. In both cases, contamination from the WC tooling became noticeable. On the basis of AES analyses, the lattice expansion of Nb was caused by the incorporation of Be, which has a wide range of solid solubility in Nb. As the duration of MA increased, increasing degrees of amorphization of the powders were generated; concurrently, contamination from WC also became more extensive. While extensive MA improves the composition uniformity of the 1Nb+12Be mixtures, macroscopic inhomogeneity is still present in the 2Nb+17Be mixtures. In both cases, the average particle size decreased from a few tens of microns (in the premixed conditions) to submicron levels. Complete amorphization was achieved after MA for 24 h; no significant change was noted in the XRD spectra from the powders after MA for increased periods of time. Vacuum annealing of the 72 h, MA powders resulted in massive crystallization; the 1Nb+12Be powders turned into a mixture of NbBe 12 and Nb 3Be 2, and the 2Nb+17Be powders evolved into mainly Nb 2Be 17 and some unidentified minor second phases.