Incipient melting of grain boundaries in Ni3Al alloys containing zirconium

Incipient melting due to thermal treatments has been investigated in three Ni3Al alloys containing various amounts of zirconium. The alloys were designated as continuous cast IC-50 sheet (Ni77.82Al21.73Zr0.34B0.1), investment cast IC-50 rod (Ni75.80Al23.47Zr0.54B0.19) and investment cast IC-218 billet (Ni74.48Al16.98Cr8.02Zr0.51B0.10). Due to enrichment of Zr, which can form eutectic alloys with Ni, the inter-dendritic regions and grain boundaries in these alloys melted at temperatures lower than that at which the bulk melts. The incipient melting temperature of the grain boundary was determined by microstructural analysis after quick heating and quenching from various temperatures. The grain boundaries in as-cast IC-50 sheet, IC-50 rod and IC-218 ingot started to melt at 1250, 1150 and 1150 °C, respectively. To accommodate stresses due to quenching, the molten layer at the boundaries often opened up, producing cracks. The higher the nominal Zr content in the alloy, the lower the temperature at which the incipient melting took place. An annealing at 1100 °C for 5 h increased the incipient melting temperature of IC-50 sheet from 1250 to 1300 °C, probably due to homogenization of Zr. However, annealing IC-218 at 1100 °C for 23 h lowered it from 1150 to 1125 °C due to release of Zr from Zr-rich eutectic structures.