Nanocrystalline Ni-3.6 at.% P and its transformation sequence studied by atom-probe field-ion microscopy

The transformation sequence of electroless plated nanocrystalline Ni-3.6 at .% P layers upon different heat treatments is studied by means of different ial scanning calorimetry (DSC), X-ray diffraction (XRD) and atom-probe fiel d-ion microscopy (APFIM). APFIM reveals P segregation at the grain boundari es in the as-plated nanocrystalline alloy. DSC shows two heat releases upon isochronic heat treatment. During the first heat release, starting at abou t 136 degrees C for a heating rate of 20 degrees C/min, structural relaxati on occurs first, followed by slight crystal growth and segregation enhancem ent, as shown by XRD and APFIM. Nucleation of the equilibrium phase Ni3P st arts in the transition lo the second heat release. This second heat release , with a sharp onset at 417 degrees C for heating at a rate of 20 degrees C /min, is related to the major part of Ni3P-phase formation and substantial grain growth. The transformation sequence is compared with the one observed on amorphous Ni-P alloys and discussed in terms of a thermodynamic model. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All righ ts reserved.