The grain size refinement of metal-ceramic composites into nanometer
size range below 100 nm is of interest because of the expectation that
the nanoscale refinement will enhance mechanical properties, such as
strength and hardness, possibly without loss of ductility. In the
present study, Fe-TiN and Ni-TiN nanocrystalline powders were prepared
by ball-milling Fe-Ti and Ni-Ti powder mixtures in nitrogen gas, and
milled powders were consolidated by dynamic compaction by using a
propellant gun under shock pressures of 38.6 GPa and above. The
dynamic compaction was performed at room temperature and 850 K for
Fe-TiN and Ni-TiN alloys, respectively. The average density measured
by an Archimedean method was 92 pct and 97 to 98 pct of theoretical
densities for the Fe-TiN and Ni-TiN bulk materials, respectively.
Specimens with various grain sizes were prepared by annealing the
materials at temperatures higher than 973 K for 1.8 x 104 s.
By dynamically consolidating Fe-63 vol. % TiN and Ni-17, 44, 64 vol. %
TiN powder alloys prepared by mechanical alloying in nitrogen gas,
bulk materials having nanostructures with grain sizes of 5 to 14 nm
can be produced. The nanostructures were stable up to about 1000 K,
and grain growth occurred at higher temperatures. The hardness of
nanocrystalline bulk materials of Ni-17, 44 and 64 vol. % TiN
increased with decreasing grain size and reaches maximums of HV= 12 ~
14 GPa at critical grain sizes of 10 ~ 15 nm, respectively. As the
grain size decreased below the critical value, the hardness slightly
decreased. The Hall-Petch slope of the Ni-TiN alloys was comparable to
that for nickel of conventional grain sizes.
The high temperature hardness was also measured at a temperature range
from room temperature to 1000 K. Softening occurred from low
temperatures of about 500 K with the grain sizes below 20 nm, and from
higher temperatures with larger grain sizes. The indentation creep
behavior at intermediate temperatures below about 0.5 Tm well confirms
to a rate equation of transient creep. The activation energies for
creep of the materials with grain sizes from 10 to 14 nm were close to
those for grain boundary diffusion for iron and nickel, and they
somewhat increase with increasing grain size to 80 nm.
The dynamic consolidation was performed by using an apparatus in the
Institute for Materials Research, Tohoku University. The authors wish
to gratefully acknowledge Professor Y. Syono and Drs. K. Fukuoka and
T. Atou in the Institute for their cooperation in making the
consolidation.
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