Microstructural Investigation of Grain Growth in Cryomilled Inconel 625 Powder

Abstract

The grain growth behavior of cryomilled nanocrystalline Inconel 625 powders was investigated during isothermal heat treatment at 600 ~ 900 °C for 1 ~ 4 hours. The grain size of cryomilled Inconel 625 remained under 250 nm following heat treatment in the temperature range of 600 ~ 900 °C up to 4 hours, which represents an improved grain stability compared to that of conventional Inconel 625 and cryomilled pure Ni. Microstructural studies, using TEM, revealed the existence of oxide particles after cryomilling. The pinning effects on grain stability by oxide particles were quantitatively analyzed. The grain stability of cryomilled Inconel 625 powders at 900 °C was noted to be better than that at lower temperatures. This behavior was attributed to the presence of two types of precipitates found at this temperature, which were identified as spherical NbC and rod shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain boundary pinning mechanism. The preferred nucleation sites of those precipitates was noted to be at grain boundaries, thereby augmenting the grain boundary pinning effect.