Irradiation induced order-disorder transformations in nickel-molybdenum alloys

Random displacements of atoms and replacement collision sequences are important mechanisms for inducing disorder in an ordered alloy under irradiation. At the same time, enhanced diffusion due to irradiation-induced point defects causes order to be reinstated at temperatures where defects may become mobile. There are numerous alloys undergoing a phase change from a cubic phase to a less symmetrical one. The ordering-disordering transformation of Ni 4Mo and Ni3Mo alloys can be used to understand the fundamental process of radiation damage. At the same time, these radiation experiments provide an insight into ordering mechanisms.; Upon heating, an alloy of a given composition of nickel-molybdenum undergoes an order-disorder phase transformation. When this is done in a furnace, the alloy will return to its room-temperature phase with coarse microstructure. However, when the heating is very local as in the case of an irradiated sample, the bulk acts as a heat sink and the resulting microstructure will be very fine.; The objective of the study was to introduce low flux irradiation in order to transform coarse-grained alloy to a fine grain structure. X-ray diffraction spectroscopy (XRD), Rutherford backscattering spectrometry (RBS), orientation imaging microscopy (OIM) and hardness measurements were employed in the process of following the phase transformation in Ni-Mo under light and heavy ion irradiations. It was hypothesized that if we control the energy density of the bombarding beam so that we obtain gradual increase in the local temperature, two different effects result: (a) In the lower dose: Ni4 MO → Ni3Mo + FCC solid solution and (b) In the higher dose Ni 4Mo → FCC+ amorphous solid solution. The research work also followed the behavior of Ni3Mo alloy under identical irradiation/annealing conditions and observed a similar behavior. This hypothesis was substantiated by the formation of fine grain structure with long range order (LRO) after irradiation followed by low temperature annealing (100--450° C). In principle, we can end up with the desired grain size depending on the post-irradiation annealing treatment.