A study of phase stability in invar Fe-Ni alloys obtained by non-conventional methods

It is known that thermodynamic equilibrium in Fe-Ni alloys, in the invar composition at temperatures below 450 deg. C, is difficult to achieve because of the slow diffusion rate at low temperatures. One of the ways in which we can study phase transformation which may be responsible for invar behavior is to investigate: (i) materials of similar composition obtained by non-conventional methods, known to allow the enhancement of diffusion at temperatures where atomic mobility is nil on the laboratory time scale; (ii) materials which have been treated for very long periods of time (geological time scale) in the same temperature range, such as in meteorites. In this context we have studied the phase stability of Fe-Ni phases in mechanically alloyed powders, in ion-beam mixed multilayers and in meteorites