Influence of niobium solutes on the microstructural evolution of nickel during hot working

Abstract

The effect of niobium additions on the microstructural evolution of nickel during hot working was investigated by optical and electron microscopy (SEM-based EBSD, electron backscatter diffraction). For this purpose, a series of wrought model alloys consisting of pure nickel and six Ni-xNb alloys (x = 0.01, 0.1, 1, 2, 5 and 10 wt% Nb) were prepared and tested in torsion to a true strain of 5 within the temperature range of 800 to 1000 °C and at three strain rates of 0.03, 0.1 and 0.3 s–1. The results revealed that the gradual Nb additions led to increasingly smaller grain sizes ranging from 800 to 40 μm and even up to 1 μm for the gripping area (undeformed head) and the gauge-length (deformed part) of the specimens, respectively. However, a saturation effect was observed at 5−wt% Nb in both cases. If the deformed Ni alloys containing 10-wt% Nb led to unsatisfactory results (in terms of volume fraction and recrystallized grain size) owing to low formability, however, under specific conditions of deformation, some of them gave good results comparable to those containing 5-wt% Nb. Overall, the gradual Nb additions affected, in complex ways, the kinetics and the nature of mechanisms of dynamic recrystallization (DRX) that turned out to be discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) operating separately and/or simultaneously, along with twinning as an additional mechanism.