Positron-lifetime study of polycrystalline Ni-P alloys with ultrafine grains

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Abstract
Polycrystalline Ni-P alloys with ultrafine grains from a few to one hundred nanometers in size prepared by the crystallization method have been studied by using positron-lifetime spectroscopy. Two kinds of microdefect were found in the interfaces of the Ni-P samples: free volumes (the size of which is smaller than a monovacancy) and microvoids. Variations on the interfacial defects with grain size were revealed by an analysis of the lifetime results. The interface characteristics derived from the positron-lifetime results can be used to give a satisfactory interpretation of the dependence of mechanical behaviors on the grain size. Differences between the interfacial defects of the nanocrystalline materials prepared by the ultrafine-powder compaction method and by the crystallization method were also discussed.