Abstract
A sample from the Gibeon meteorite, an iron-nickel meteorite with the typical two-phase Widmanstätten structure, was investigated by positron annihilation techniques. A high mean positron lifetime was observed which derives from open volume defects. The meteorite samples were then gradually heated in a high-vacuum furnace and cooled down to room temperature. Upon temperature treatment a decrease of the mean positron lifetime was observed, as well as a gradual evolution of the 2-dimensional Doppler broadening spectra towards pure iron. This leads to the conclusion that the open volume defects are formed during slow cooling in the progress of the formation of the Widmanstätten structure. Upon re-heating these defects start to dissolve and do not re-appear due to fast cooling rates. We therefore attribute the open volume defects to misfit dislocations between the Kamacite (Ni-poor) and Taenite (Ni-rich) phases.
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