Influence of pre-annealing on the thermal stability of a nanocrystalline Hf-Ni alloy

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.