Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys |
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Authors: | G E Abrosimova A S Aronin I I Zver’kova A F Gurov Yu V Kir’yanov |
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Institution: | (1) Institute of Solid-State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia |
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Abstract: | Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the
production, nanocrystalline 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 °C leads to the creation of
a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition
of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect 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 parameters of the FCC nanocrystallites depend on the
alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse
from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure
is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal
annealing time due to enrichment by boron and molybdenum. As the structure 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-Petch law, is explained
by hardening of the amorphous matrix due to changes in its chemical composition.
Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998) |
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