Grain boundary precipitation of tantalum and NiAl in superelastic FeNiCoAlTaB alloy

Abstract

The precipitation behavior of tantalum (Ta) and NiAl along grain boundaries has been investigated in polycrystalline Fe-28.5Ni-17.5Co-11.5Al-2.5Ta-0.05B (NCATB) (at%) alloy in order to understand their role in limiting its superelastic behavior. It is found that Ta precipitates at grain boundary triple-junctions first, then along grain boundaries, and acts as nuclei for β-NiAl phase formation. Small amounts of boron is also found distributed adjacent to the Ta precipitates. Consequently, the martensitic transformation associated with the superelastic response is promoted along grain boundaries by the Ta and β-NiAl precipitated there. Furthermore, increasing aging temperature facilitates the precipitation of Ta and β-NiAl along grain boundaries. The mean cell width of precipitates along grain boundaries significantly affects the fracture mode and ductility of NCATB specimens. Both the fraction of low-angle boundaries and the intensity of recrystallization texture are significantly increased with increasing reduction in thickness of cold-rolled specimens over 95%, which is effective for the suppression of Ta and β-NiAl formation as their formation occurs primarily on high angle grain boundaries. As a result, the superelastic behavior of the FeNiCoAlTaB alloy is closely linked to the reduction in thickness, which increases the ratio of low angle to high angle boundaries, and hence the propensity for grain boundary failure associated with the grain boundary precipitates and grain boundary initiated martensitic transformation.