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

Abstract

A nanocrystalline (NC) sample was prepared by quenching a Hf 11Ni 89 alloy with the melt-spinning technique. A randomly oriented HfNi 5 nanophase with an average grain size of about 10 nm was formed by this process. As 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-annealed at 340 °C for different periods of time t a up to 510 min. The 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 HfNi 5 nanophase prior to a precipitation process of a Ni solid solution phase. Activation energies for the two processes were calculated by using the Kissinger equation. It was obtained 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 the grain-growth and the Ni precipitation processes. The DSC analysis revealed that the total excess energy of this highly non-equilibrium alloy and, specifically, the grain-boundary energy, decrease with increasing pre-annealing time. These changes in the parameters characterizing the thermal stability of the NC HfNi 5 alloy are discussed on the basis of observed modifications of the microstructure with the help of atomic diffusion processes and, also, of magnetic measurements on pre-annealed samples described elsewhere.