A nanocrystalline (NC) sample was prepared by quenching a Hf11Ni89 alloy wi
th the melt-pinning technique A randomly oriented HfNi5, nanophase with an
average grain size of about 10 nm was formed by this process A an extension
of our previous work on the as-quenched state of this NC Hf-Ni alloy, here
we use DSC and XRD to report on a thermal stability study of samples pre-a
nnealed at 340 degrees C for different periods of time t(a) up to 510 min.
Thc pre-annealing was performed under conditions: that did not result in a
change of the grain size. Similarly to the as-quenched state of the NC HfNi
5 alloy, two exothermal peaks appear in the DSC curve for the pre-annealed
NC samples on heating at a constant heating rate. The two peaks may also be
attributed to a grain-growth process of the HfNi5 nanophase prior to a pre
cipitation process of a Ni solid solution phase. Activation energies for th
e two processes were calculated by using the Kissinger equation, It was obt
ained that a pre-annealing treatment decreases the thermal stability of the
NC sample, which was characterized by a reduction of the temperatures and
the activation energies for thr grain-growth and the Ni precipitation proce
sses. The DSC analysis revealed that the total excess energy of this highly
non-equilibrium alloy and, specifically, the grain-boundary energy, decrea
se with increasing pre-annealing time. These changes in the parameters char
acterizing the thermal stability of the NC HfNi5 alloy are discussed on the
basis of observed modifications of the microstructure with the help of ato
mic diffusion processes and, also, of magnetic measurements on pre-annealed
samples described elsewhere. (C) 1999 Acta Metallurgica Inc.