We have begun a series of experiments to study the defect structure produced by ion irradiation in A15 superconductors. Low temperature irradiations and microscopy are performed in the high voltage electron microscope (HVEM) accelerator facility at Argonne National Laboratory, Argonne, IL. The first transmission electron microscope (TEM) images of the martensitic structure in niobium three tin (Nb₃Sn) were recorded at various temperatures between 12 and 30 K. The transition to the superconducting state was observed by a slight deflection of the electron beam due to a perturbation of the magnetic field of the objective lens at the sample. The primary interest is in irradiation effects due to fast neutrons from a fusion reactor, and therefore, ion irradiation conditions that are somewhat comparable to this were selected. These consisted of 50 keV Xe⁺ irradiations to simulate neutron produced cascades in a near surface region (<20nm) and 1.5 MeV Kr⁺ to produce cascade damage through a greater sample thickness (0.5μm). Defect images were obtained as a function of ion dose at 13 K using both fundamental and superlattice dark field reflections in two-beam conditions.

For 50 keV Xe⁺ irradiations at 13 K, the defect yields, which are black-white contrast features in fundamental reflections or black dot contrast features in superlattice reflections, were quite low at low ion doses (2 × 10¹¹ cm^-2), where individual defect cascades are well separated. At higher ion doses (3 × 10¹² cm^-2), when significant cascade overlap is present, defect density and contrast became greater than expected. Annealing to room temperature produced a large increase in visible defect density. Irradiation by 1.5 MeV Kr⁺ at 15 K removed the martensitic structure by an ion dose of 5 × 10¹² cm^-2, Further irradiation to 3.5 × 10¹³ cm^-2 produced evidence for the initial stage of amorphization.