Influences of Initial Microstructures on Portevin-Le Chatelier Effect and Mechanical Properties of a Ni-Fe-Cr-Base Superalloy

Effects of initial microstructures on the Portevin-Le Chatelier effect (PLC) and mechanical properties of a Ni-Fe-Cr-base superalloy are investigated by hot tensile deformation tests. The inverse PLC effect only occurs in the ST (solution treated) and SAT (solution plus phases aging precipitation) superalloys when the deformation temperature is above 873K. Effects of initial microstructures on the temperature range of PLC effect are obvious, that is, the narrowest temperature range for the HS (solution plus / phases aging precipitation) superalloy, while the widest temperature range for the ST superalloy. The average activation energy for type A/A+B/B serrations can be determined as 65-81kJmol(-1), which contributes to the migration of carbon atoms through pipe diffusion. Meanwhile, the average activation energy for type C serration is about 132-175kJmol(-1), which is induced by the pipe diffusion of substitutional atoms. An obvious increase in the yield strength with increasing the deformation temperature from 900 to 973K is observed for the ST and SAT superalloys. Also, the negative strain rate sensitivity is found in the PLC effect regime. The ST and HS superalloys exhibit the ductile fracture. However, a mixture of ductile transgranular and cleavage fracture is observed in the SAT superalloy.