Macinazione ad Alta Energia del Sistema NiAl(Fe).

The intermetallic compound NiAl, with ordered B2 structure, is a potentially interesting material for structural applications at high temperatures owing to its low density, high melting point, excellent resistance against oxidation and good heat conductivity. However, brittleness at ambient temperature and low strength at high temperature limit its industrial utilization. Several strengthening mechanisms have been exploited. Among others, iron dissolution in the NiAl lattice results in higher ductility, although a drastically lower hardness, as compared to binary alloy, was measured when the iron content is sufficiently high, i.e., above 15 at.%. High-energy ball milling has been used in this work to promote the solubilization of iron into NiAl powder for an iron concentration range 10-30 wt.%. The aim of the work is to study the effect of milling on NiAl(Fe) powders with different compositions and by varying the operative conditions as time and speed of milling and materials used for vials and ball. The microstructural evolution induced by the intense mechanical deformations due to the high energy milling has been followed by x-ray diffraction and Mössbauer spectroscopy. It is seen in the Mössbauer spectra, in which a magnetic sextet of about 33 T predominates, that increasing of time and speed of milling gives rise to increasing of a non resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a re-duction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure, with partial substitution of Fe atoms in Al positions. The amount of this atomic substitution is found to depend on the time and speed of milling, as well as on the materials used for balls and vials. Subsequent annealing modifies noticeably especially the non-magnetic component in the Mössbauer spectra, which becomes a broad singlet. Both diffraction analysis and Mössbauer spectroscopy of these samples indicate that a fcc Ni(Al,Fe) solid solution is forming in samples previously milled in agate. It is observed that the grain size of the milled products remain still in the nanometric range even after thermal treatment, which adds interest to possible applications.