Electrical resistivity and structural changes upon relaxation and crystallisation of (Mo_(0.6)Ru_(0.4))_(100-x)B_x metallic glasses

Abstract

Crystallisation of (Mo_(0.6)Ru_(0.4))100_(-x)B_x glasses takes place in three steps: the first step corresponds to the precipitation of the sigma phase Mo_5Ru_3 which decomposes at higher temperatures, the second step is associated with the formation of an HCP solid solution of Mo in Ru, and in the last step the remaining amorphous matrix crystallises in an FCC boride. Detailed electrical resistivity measurements taken below the crystallisation temperature reveal an excess resistivity above the usual linear temperature dependence predicted by the Ziman theory. The increase in resistivity is associated with the onset of long-range compositional inhomogeneity (spinodal decomposition) and the decrease with the onset of crystallisation. The electrical behaviour for samples pre-annealed for 12 hours at various temperatures suggests that boron migration is partly responsible for this excess resistivity. The changes in the radial distribution function of (Mo_(0.6)Ru_(0.4))_(78)B_(22) during annealing at 525 ºC are also reported. The Mott s-d scattering model does not seem to be applicable to these amorphous alloys. The data suggest instead that an important contribution to the conduction comes from the d electrons.