In-situ observation of the incipient melting of borides and its effect on the hot-workability of Ni-based superalloys

The excellent creep properties of advanced wrought Ni-based superalloys are achieved by adding grain boundary strengthening elements such as B and C. Their low solubility in the γ matrix may promote precipitation of micrometre-sized borides and carbides which potentially deteriorate hot-workability, e.g. through incipient melting. However, their incipient melting has not yet been directly linked to a decrease in hot-workability in Ni-based superalloys. Here, we study the susceptibility of two Ni-based superalloy variants with main differences in B and C contents (high B & C variant with>triple B and C contents) to incipient melting and its effect on hot-workability. We show that incipient melting causes severe intergranular cracking at low strains during compression at 1200 °C in the high B & C variant only. Compression to higher strains separates grain boundaries fully, partially pulverising the macroscopic specimen. Metallographic specimens already show incipient melting at heating rates they experience when put into a pre-heated furnace. During subsequent cooling, the melt re-solidifies and forms lamellar structures consisting of various phases rich in Mo-Cr-B, Zr, and Ti, the constituents of M3B2 and MC. In-situ observations using a customised high temperature confocal laser scanning microscope set-up unambiguously confirmed the origin of these lamellar structures. We demonstrate for the first time directly the constituent liquation of M3B2. This liquation is assumed to be aided by the partial dissolution of adjacent MC carbides into the melt. These results will help to improve thermo-mechanical simulations and microstructural control during industrial processing of advanced wrought superalloy parts.