Identification of phases in gas-atomised droplets by combination of neutron and X-ray diffraction techniques with atom probe tomography

Powders of Al_68.5Ni_31.5 alloy have been produced by gas atomisation and sieved in different grain size families. The resulting families have been analysed by combined 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 fraction of the identified phases (Al₃Ni₂, Al₃Ni and Al) has been estimated from a profile refinement with the FULLPROF computer codes. An additional phase was observed but could not be identified in the diffraction patterns. Starting from grains less than 5 μm in diameter, samples have been shaped by annular focused ion beam into needles that were suitable for atom probe investigations. The structure and morphologies observed by different techniques are compared and discussed. It has also been possible to estimate the crystallite sizes and the strains corresponding to the different phases present in the powders from the refinement of the ND patterns. In addition to Al₃Ni₂ and Al₃Ni, a phase of composition close to the nominal one of the alloy was observed in the atom probe measurements. This phase could be one of the decagonal ones referred to in the literature. Small particles of composition close to Al₈₂Ni₁₈ are attributed to the metastable Al₉Ni₂ phase. The achieved conclusions demonstrate the complementarity of X-ray and neutron diffraction techniques and atom probe tomography to analyse complex structures.