The positron annihilation and hall-petch relation in polycrystalline Fe 78B 13Si 9 alloys with ultrafine grains

Abstract

Nanometer-sized crystalline materials, which are sometimes called nanocrystalline materials', have been reported to exhibit different properties from ordinary crystalline materials. The possibility of affecting the physical or mechanical properties of materials by producing nanocrystalline structures constitutes a major subject of interest in materials science. The positron annihilation and the Hall-Petch behaviors in the nanocrystalline FeBSi alloy prepared using the crystallization method were studied. The results indicate that because of the unusual nature of its grain boundaries (existing a great amount of defects) the nanocrystalline FeBSi alloy has higher values of microhardness than those of coarse grained crystalline and amorphous FeBSi alloys with the same composition. Hardness increases with the reduction of grain size in the nanocrystalline FeBSi alloys, i.e. the normal Hall-Petch relationship was obtained. The variation of interface characteristics with the grain size will strongly influence structure-sensitive properties, such as H[sub v]. From the present results, a very good overall agreement between the trends of the mean lifetime value [bar [tau]] and Vickers microhardness value H[sub v] can be obtained.