Structural characterisation of Al–Ni powders produced by gas atomisation

Aluminium–nickel powders of different compositions on the Al-rich side of the phase diagram have been produced by gas atomisation and sieved in different grain size ranges. The resulting families have been analysed by neutron and X-ray diffraction in order to investigate the structure and identify the existing phases at the surface and in the bulk of the grains. The weight fractions of the three main phases (Al3Ni2, Al3Ni and Al) as determined from profile refinements with the FULLPROF computer codes are found to vary with grain size. There is no indication for the existence of the B2 AlNi phase in any of the investigated powders. One or several additional metastable phases were found to exist in an amount that increased with decreasing size in grains with diameters less than about 200 μm. The combined X-ray and neutron diffraction results indicate that the cooling rates are too large for the peritectic reaction at 854 °C to proceed fully resulting in a deficit of the Al3Ni phase in the gas-atomised powders especially at the surface.