Influence of the M and M’ Metals on the Carbides Population in As-Cast M’-based Alloys Designed to be MC-Strengthened

Abstract

High temperature applications such as turbine blades for aeronautics or molten glass-shaping tools require the use of refractory metallic materials. Among the later ones, cast superalloys based on some transition metals and reinforced by MC carbides stay in good place and their metallurgy merits to be well known. This work consists in a general exploration of the as-cast microstructures which can be obtained after solidification and solid state cooling down to ambient temperature for a wide series of alloys for which the base element and the MC-former element both vary. For fixed contents in chromium and carbon contents, the compositions of a total of nineteen alloys were considered. These alloys are based on Ni, Co, Fe or Nb and the M content was each time chosen to favor the appearance of TiC, TaC, NbC, HfC or ZrC, as single carbide in a given alloy. After elaboration, metallographic samples were observed by electron microscopy to investigate the obtained microstructures. The obtained results show first that the MC carbides were in many cases successfully obtained at the expense of other possible carbides (for all Co-based alloys for example) but there are also several exceptions (notably for some Ni-based alloys). Second, the obtained monocarbides have a eutectic origin and they are script-liked shaped. However they are here too some exceptions, as the rare HfC obtained in a Nb-base). In general, the results obtained in this work show that the principle of dendritic matrix combined with MC carbides with a script-like morphology is not necessarily obtained: the nature of the {base element, MC-former element} combination governs the microstructure of the alloy in its as-cast state for these particular  compositions in chromium and carbon. In some cases other carbides may appear and the microstructures may be even of another type.