Microstructural characterisation of metastable structures in intermetallic compounds.

Mechanical milling, rapid solidification, heavy ion bombardment are extensively used to produce metastable and non-equilibrium structures in materials. Nanograined metallic and intermetallic powders can be attained by bah-milling as a consequence of the progressive grain size reduction promoted by plastic deformations. The microstructural characterisation of such samples may face some difficulties related to the defect structures or to the changes introduced by the sample preparation techniques. In this study a few examples of microstructures, as investigated by X-ray diffraction methods, present in ball-milled intermetallic powders are discussed. Disordering induced by the increase in the concentration of some kinds of defects is discussed with particular reference to Ni3Al and FeAl alloys. Transformations to new polymorphs are investigated in the case of a TiAl base alloy.