Diffusion induced grain boundary migration and recrystallization in boron-free Ni 3Al intermetallics

Abstract

The Ni[sub 3]Al intermetallics, due to their anomalous mechanical behavior and the effect of boron addition on ductility and grain boundary (GB) structure, has received extensive attention in the past decade. In a previous work, diffusion induced grain boundary migration (DIGM) and recrystallization (DIR) in a boron-doped Ni[sub 3]Al(Cu) system were found for the first time, in which an intermetallic compound of Ni[sub 76]Al[sub 24] + 0.22 at. % B was electroplated with Cu and isothermally diffusion-annealed over a temperature range where the volume diffusion occurred in front of the migrating GB. The DIGM velocity, the activation energy for GB migration by DIGM, and the driving force for DIGM in this system were investigated as well. As has been mentioned in the literature, however, boron segregation to grain boundary in Ni[sub 76]Al[sub 24], which is associated with the ductility improvement, might produce a narrow disordered phase at the GB. Recently, this question has been argued, since some researchers did not observe this disordered layer at the GB. On this basis, the DIGM and DIR experiments in boron-free Ni[sub 3]Al should be performed for the purpose of comparison with the results for boron-doped Ni[sub 3]Al. In the present study, two boron-freemore » alloys (Ni[sub 75]Al[sub 25] and Ni[sub 76]Al[sub 24]) were selected. The main objectives were to determine whether or not the phenomena of DIGM and DIR still occur in boron-free Ni[sub 3]Al when Cu and Zn atoms were diffused into them, and, if they exhibited, to examine the chemical changes in the alloyed zones caused by DIGM and DIR.« less