Fig. 1 Initial microstructure of fine-grained specimen in a OM, b SEM, c TEM micrograph; coarse-grained specimen in d OM, e SEM, f TEM micrograph. The inset in c shows the EDS mapping of Nb-rich precipitates

Fig. 2 Temperature dependence of the engineering stress-strain curves for the tested specimens

Fig. 3 a UTS and YS, b elongation and Z reduction of the investigated specimens at different test temperatures

Fig. 4 Microstructure of the FG-specimens after tensile test at a RT, b 400 °C, c 600 °C, d 700 °C, e 800 °C, f 900 °C; TA indicates tensile axis. The insets in d, e show the microstructure at higher magnification

Fig. 5 Microstructure of the CG-specimens after tensile test at a RT, b 400 °C, c 600 °C, d 700 °C, e 800 °C, f 900 °C; TA indicates tensile axis

Fig. 6 TEM micrographs of FG-specimens after tensile test at a RT, b 400 °C, c 600 °C, d 700 °C, e 900 °C

Fig. 7 TEM micrographs of CG-specimens after tensile test at a RT, b 400 °C, c 600 °C, d 700 °C, e 900 °C

Fig. 8 SEM microstructures of FG-specimens after tensile test at a 700 °C, b 800 °C, c 900 °C; microstructures of CG-specimens after tensile test at d 700 °C, e 800 °C, f 900 °C. The insets in d, e, f show the precipitates at higher magnification

Fig. 9 a SEM-EDS concentration profile across the sigma precipitate in FG-specimen after test at 800 °C, b EDS results of the precipitates in FG-specimen after test at 900 °C, c STEM-HAADF image and corresponding map results of precipitates in CG-specimen after test at 900 °C

Fig. 10 Fracture morphology of FG-specimens after tensile test at a 600 °C, b 700 °C, c 800 °C, d 900 °C; CG-specimens after tensile test at e 600 °C, f 700 °C, g 800 °C, h 900 °C. The insets in g, h show the corresponding figures of cracks in longitudinal section