Rhometal interface in pseudo-core shell powders like Permalloy/Rhometal type

The nanocrystalline Ni3Fe (around Permalloy composition) powders were prepared by dry mechanical alloying. The nanocrystalline Ni3Fe and carbonyl Fe mixture powders and green compacts have been subjected to heat treatments in an argon atmosphere in order to obtain pseudo-core-shell like particles by micro-alloying in the temperature range of 400-900 °C. The large Permalloy particles are partially covered by very small Fe particles and at the interface a layer of Rhometal is formed by micro-alloying. The Permalloy particles remain in the nanocrystalline/nanostructured state after the annealing independent on the annealing temperature up to 900 °C. Structural, microstructural characterisation and local elemental chemical analysis have been performed by X-ray diffraction, scanning electron microscopy and X-ray microanalysis. The Rhometal interface was studied and evidenced by Fe and Ni concentration profile (EDX microanalysis) and X-ray diffraction. It was found that by a heat treatment up to 900 °C the interface is in the iron zone and at the heat treatment temperature of 900 °C the interface is in both Permalloy and iron zones. By XRD the bcc and fcc structures of the Rhometal have been evidenced. The interface length in iron zones is about 0.8 μm for the heat treatment at 400 °C and reaches 2.5 μm for heat treatment at 900 °C. The interface reaches 10 μm for a temperature of a heat treatment of 900 °C. The iron zones welded by Permalloy zones is evidenced the presence of the Ni atoms up to 4.7 at%. Also, in Permalloy particles are evidenced two zones with a different amount of Ni: one around Ni0.6Fe0.4 composition close to the interface and the second in the centre of the Permalloy particle with composition close to Ni3Fe - Ni3−δFe1+δ.