EBSD Study of Microstructural Evolution in a Nickel-Base Superalloy during Two-Pass Hot Compressive Deformation

Generally, the high-temperature deformation characteristics and microstructural evolution of alloys are studied by isothermal compressive experiments at stable strain rates. But, the strain rate is variant during the practice industrial production of components. In this work, isothermal two-pass hot compression experiments with stepped strain rates are performed to analyze the microstructural evolution of a nickel-base superalloy with phase. Results reveal that the mean grain size decreases, but the percentage of undissolved phases increases, as the strain rate of the first pass (SROFP) is increased. However, the mean grain size and the percentage of undissolved phases decreases with the increase of inter-pass time (IPT) or the true strain of the first pass (TSOFP). Meanwhile, the increased deformation temperature easily enlarges the mean grain size, but obviously decreases the percentage of undissolved phases. In addition, the evolution of sigma 3(n) boundaries not only results from the new twinning mechanism, but also sigma 3(n) regeneration mechanism. sigma 3(n) regeneration mechanism becomes predominant with decreasing SROFP or increasing IPT/TSOFP. Besides, new twinning mechanism plays a major role on sigma 3(n) boundaries evolution as the temperature is increased from 950 to 980 degrees C, and then become weaken with further increasing the deformation temperature.