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 differential scanning calorimetry (DSC), X-ray diffraction (XRD) and atom-probe field-ion microscopy (APFIM). APFIM reveals P segregation at the grain boundaries in the as-plated nanocrystalline alloy. DSC shows two heat releases upon isochronic heat treatment. During the first heat release, starting at about 136°C for a heating rate of 20°C/min, structural relaxation occurs first, followed by slight crystal growth and segregation enhancement, as shown by XRD and APFIM. Nucleation of the equilibrium phase Ni 3P starts in the transition to the second heat release. This second heat release, with a sharp onset at 417°C for heating at a rate of 20°C/min, is related to the major part of Ni 3P-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.