M2C and M6C carbide precipitation in Ni-Mo-Cr based superalloys containing silicon

The effect of silicon additions on the primary carbides and second grain boundary carbides has been investigated in as-cast and wrought Ni-Mo-Cr based alloys, respectively. With the increase of Si content in the as-cast alloys, the lamellar M2C eutectic carbides at grain boundaries were replaced by skeleton-shape M6C eutectic carbides in interdendritic regions. The wrought alloys with different Si contents were thermal exposed at 750 degrees C for 1, 500 and 2000 h, respectively. It can be observed that the initial M2C carbides formed at grain boundaries were replaced by M6C carbides in the low-Si alloys (0.051 and 0.188 wt.%) with the increase of exposure time. Only M6C carbides precipitated at grain boundaries in the alloy with 0.562wt.% Si, which coarsened during the thermal exposure. Similar trends also existed in the high-Si alloy except an abnormal formation of M2C carbides after 2000 h' exposure. The effect of Si additions on the types of carbides was attributed to competitive nucleation between M2C carbides and M6C carbides based on classical nucleation theory. (C) 2016 Elsevier Ltd. All rights reserved.