PRODUCTION, STRUCTURE, AND MICROHARDNESS OF NANOCRYSTALLINE NI-MO-B ALLOYS

Radiography, differential scanning calorimetry, luminescence and high- resolution electron microscopy are used to study the production, nanoc rystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 degrees C leads to the creation of a granular phase consisting of FCC nanocry stallites with average grain sizes of 15-25 nm, depending on the chemi cal composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 degrees C has no appreciable effe ct on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice par ameters of the FCC nanocrystallites depend on the alloy composition an d the duration of their isothermal anneal. Within this latter time, mo lybdenum and boron atoms diffuse from the FCC solid-solution lattice i nto the surrounding amorphous matrix. The stability of the nanocrystal line structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isotherm al annealing time due to enrichment by boron and moIybdenum. As the st ructure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Fetch law, is explained by hardening of the amorp hous matrix due to changes in its chemical composition. (C) 1998 Ameri can Institute of Physics.