Microstructure evolution and fracture mechanism of a Fe–Ni–Cr superalloy during various strain rates tensile deformation at elevated temperatures

The high-temperature deformation behavior and fracture mechanism of Fe–Ni-Cr superalloy under high strain rates from 1 to 100 s −1 were studied by hot tensile experiments at a temperature range of 900–1150 °C. The results manifest that the high strain rate has significant influence on the microstructure evolution and local instability fracture behavior of the superalloy. At high strain rates, the flow behavior of the alloy shows obvious dependence on the strain rate and temperature. With the increase of strain rate, a higher ratio of low-angle and low-energy grain boundaries can be obtained while keeping fine and uniform equiaxed grain microstructure, yet the control window of hot deformation microstructure of the alloy is obviously narrowed. After deforming at temperatures of 950–1000 °C and strain rates of 10–50 s −1 , the fine equiaxed grains with average grain size less than 10 μm can be controlled at high deformation rates. Under high strain rates and higher deformation temperature, it has been found that the plastic strain induced by recrystallization nucleation and growth at the original high angle grain boundary (HAGB) of the alloy is not enough to coordinate the rotation of the original grain and the sliding of the original HAGB during deformation, resulting in intergranular cracking. This is the fundamental reason for the abnormal change of thermal deformation power dissipation behavior, higher strain rate sensitivity, flow instability and fracture when the alloy is hot deformed at high strain rates.