NONEQUILIBRIUM STRUCTURES INDUCED BY ION IRRADIATION IN NI4MO

We have studied the stability of equilibrium and metastable ordered ph ases (called LRO and SRO, respectively) in Ni4Mo during 500 keV Ne or 250 keV He irradiations. Some irradiations were performed in situ, all owing thus to follow the evolution of the samples, which were characte rized by transmission electron microscopy (TEM). Dynamical equilibrium phase diagrams are built, by varying the irradiation flux, temperatur e and dose. At a fixed ion flux, there exists a temperature range were a ''mixed'' ordered state is stabilized, and remains stable up to the maximum doses reached (1.7 dpa). This state, which cannot be obtained by electron irradiation, consists in the coexistence of the two order ed phases at a very fine scale (less-than-or-equal-to 2 nm). Compariso n with results already reported after electron irradiation is made, st ressing the role played by displacement cascades in our results. Furth ermore we observed alignment of dislocation loops along the [001] c-ax is of the quadratic LRO phase.