Intermediate temperature embrittlement and Portevin-Le Châtelier effect of Inconel 625 alloy caused by carbides

Abstract

The phenomena of intermediate temperature embrittlement (ITE) and Portevin-Le Châtelier (PLC) effect are common in nickel-based superalloys, which seriously restrict the machinability and serviceability of the alloys. The ITE and PLC effect of Inconel 625 alloy caused by carbides and hot ductility were studied by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscopy (TEM), and Gleeble 3500 thermal simulation equipment. The results show that hot ductility has a strong dependence on temperatures of Inconel 625 alloy. During the heating process, ductility initially decreases at 600 ℃ and reaches a minimum at 800 ℃ as the temperature increases, which was due to the formation of microcracks. Dynamic recrystallization can be observed at 900 ℃, and then ductility begins to drop until 1212 ℃ due to material melting. Meanwhile, the variation trend of the cooling process is consistent with the heating process. ITE exists in both the heating process and the cooling process, occurring at 600–900 ℃ and 600–1000 ℃ respectively of Inconel 625 alloy. The microcracks caused by the precipitation of carbides (NbC, (Nb, Ti)C, and M23C6) at grain boundaries led to the formation of ITE. The size, shape, distribution position, and type of carbides have different degrees of influence on the formation of microcracks. In addition, the serrated flow was observed throughout the test temperature range of Inconel 625 alloy. With the increase of the temperature, the type and amplitude of the serration changed, which is caused by the interaction between carbides (solute atoms) and dislocations.