ALBERT

Notes: Abstract An attempt has been made to characterize a new, complicated Al-20Si-7.5Ni-3Cu-1Mg alloy powder produced by air atomization as a means of rapid solidification and its structural evolutions during continuous heating, in order to provide basic information for further investigations on its deformation behaviour and properties. The characterization consisted of size measurements, morphological observations, structural and thermal analyses of bulk powder, and microstructural examinations of individual powder particles. It was observed that the powder had a wide size distribution and irregular shapes, which were closely related to its varying internal structures. X-ray diffractometry (XRD) showed little shift of the diffraction line from the aluminium matrix of the powder, but a significant broadening, which has been attributed partly to the non-uniformity of supersaturation in the matrix of the powder and partly to the strains caused by the silicon crystals in the material. A differential scanning calorimetry (DSC) analysis revealed complex decomposition behaviour of the meta-stable aluminium matrix and transformations of nickel-bearing intermetallic compounds when heat was applied to the powder. XRD also showed that the meta-stable compounds formed in the powder did not match any known phases, and that they were transformed into Al3Ni, Al3(NiCu)2 and Al7Cu4Ni intermetallic dispersoids upon heating. The analyses also indicated that, due to the addition of nickel, some copper-containing phases, initially desired to create precipitation strengthening effects, no longer existed. This would diminish the ageing response of the alloy and probably change its category to be non-heat treatable — an important modification that has not yet been recognized by the alloy designers and users. Examinations on the powder particle sections showed variations in microstructure with powder particle size. Transitions in solidification mode within powder particles in accordance with local conditions of undercooling and heat extraction were also observed. The significant inhomogeneities in the microstructure of the powder have raised a problem to which special attention should be paid in both powder production and subsequent processing.