Investigation of annealing behavior of nanocrystalline NiAl

Nanocrystalline NiAl prepared by mechanical alloying and the vacuum heat pressing method has been studied to investigate its thermal stability. Isothermal annealing was conducted at 1000 degrees C. The results show that a significant grain growth takes place from 30 nm at the initial state to 56 nm by annealing for 30 h, then maintains this value regardless of annealing time up to 100 h. Microhardness test results show that the microhardness of the annealed specimens reduces significantly with annealing time up to 5 h and then increases slightly with further annealing due to off-stoichiometry. in addition, positron annihilation spectroscopy has been used to investigate the interfacial defects in the nanocrystalline NiAl specimens. The results show that there are three kinds of defects in the interfacial region of the nano-NiAl alloy, which are monovacancy-sized free volumes, microvoids and large voids. During the present annealing process, the sizes of the three defects have no significant change, but the amount of the defects, especially for monovacancy-sized free volumes and microvoids varies clearly with annealing time. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.