In situobservations of partial dislocation climb in Ni3Al

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Abstract
Changes in the configuration of screw and mixed dislocations in Ni3Al have been monitored continuously during annealing in the hot stage of a high-voltage electron microscope. Memces in the configurations were observed depending on whether the experiments were performed below or above the threshold voltage V 1 for displacement damage. Below V t, initially straight dislocations in or close to screw orientation slowly increased their length and segmented into non-screw components during heating at 750°C. This behaviour is attributed to a resulting reduction in energy as the newly oriented segments climb dissociate into a more favourable plane where the APB energy is lower than the initial {001} glide dissociation plane. Above v t, radiation-produced interstitials induce asymmetrical climb of the 1/2〈110〉 superpartial screw dislocations during irradiation at 650°C. One partial remains essentially unchanged as the other climbs into a helical configuration in a direction away from the position of the original screw dislocation. Energy balance considerations show that the single rather than double helix configuration will be favoured if the associated antiphase boundary energy is low. It is suggested that the low energy antiphase boundary is produced when helix turns are formed by the condensation of interstitial nickel atoms in an off-stoichiometric layer. The occasional observation of residual displacement fringe contrast supports this view.