Non-equilibrium structures induced by ion irradiation in Ni₄Mo

We have studied the stability of equilibrium and metastable ordered phases (called LRO and SRO, respectively) in Ni₄Mo during 500 keV Ne or 250 keV He irradiations. Some irradiations were performed in situ, allowing thus to follow the evolution of the samples, which were characterized by transmission electron microscopy (TEM). Dynamical equilibrium phase diagrams are built, by varying the irradiation flux, temperature 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 ordered phases at a very fine scale (≤ 2 nm). Comparison with results already reported after electron irradiation is made, stressing the role played by displacement cascades in our results. Furthermore we observed alignment of dislocation loops along the [001] c-axis of the quadratic LRO phase.