Grain boundary melting and its effect on hot cracking in the weld heat-affected zone (HAZ) have been investigated in the investment cast billet of a two-phase (γ + γ′) nickel aluminide alloy (Ni74.48Al16.98Cr8.02Zr0.51B0.10), designated as IC-218. Due to enrichment of Zr, which can form eutectic alloys with Ni, the dendritic boundaries melted incipiently at 1150°C. Under thermal stresses during welding the molten layers often opened up producing liquation cracks at the boundaries in the HAZ. Annealing at 1100°C in argon for 23 h prior to welding reduced the incipient melting temperature to 1125°C and increased the propensity of liquation cracking in the HAZ.

Incipient melting and oxidation of grain boundaries at elevated temperatures have been investigated in the investment cast billet of a two-phase (γ + γ′) nickel aluminide alloy (Ni74.48Al16.98Cr8.02Zr0.51B0.10), designated as IC-218. Due to enrichment of Zr, which can form eutectic alloys with Ni, the inter-dendritic regions and grain boundaries melted incipiently at 1150°C. During quenching the molten layers often opened up producing cracks at the boundaries. Annealing at 1200°C in air for 570 min produced elongated ZrO2 precipitates at the grain boundaries. There was Zr depletion inside the grains in the vicinity of the precipitates. Such precipitates were scarcely observed in the specimens annealed in air at 1100°C for 23 h. Thus, the molten Ni-Zr alloy at the boundaries promoted internal oxidation.

Hydrogen embrittlement has been studied in continuous cast sheet of a Ni 3 Alalloy (NI 77.83 AL 21.73 ZR 0.34 B 0.1), known as IC-50, after introducing various amounts of hydrogen cathodically. The elongation and UTS decreased with the increasing content of hydrogen. When tensile-tested at a strain rate of 5.8 × 10−5 s−1, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of charging with 50 mA cm−2 current. The yield stress, however, did not change. When tested at a higher strain rate of 5.8 × 10−3 s−1, the embrittlement was less, but the yield stress increased with the hydrogen content. With increasing hydrogen content the fracture mode changed from dimpled to intergranular and cleavage modes.

Autogeneous gas tungsten arc welding was performed on plates (about 3 mm thick) cut from cast ingot of aNi 3 Al alloy (Ni 73.80 Al 15.82 Cr 8.33 Mo 1.68 Zr 0.52 B 0.05), known as IC-396M. The alloy was susceptible to cracking in the heat-affected zone (HAZ). The cracks in HAZ were liquation cracks and occurred along dendritic boundaries and around eutectic cells. The fusion zone (FZ) contained many fine eutectic cells (about 5-10 μm in diameter) and there were microcracks (1-5 μm in size) in the cells. Two types of eutectic structures were observed in the base metal, HAZ and FZ of IC-396M. One had web-like structure and the other had lamellar structure, the later having high enrichment with Zr. The role of Zr enrichment in dendritic boundaries and eutectic cells in causing liquation cracks will be discussed.

Weldability is a key issue for successful applications of ductile Ni3Al in structural components. Here we report the results of an investigation of gas tungsten arc welding behaviors of continuously cast thin sheet and cast ingot of a Ni3Al alloy, known as IC-50, containing boron and zirconium. Cracking in heat-affected zone (HAZ) was prevalent in both types of specimens. Both in cast sheet and ingot, liquation cracking was observed to occur along the grain boundaries in HAZ. Microstructural studies showed that continuously cast thin sheet consisted of columnar grains along the thickness. Easy cracking along the boundaries of columnar grains made cast sheets more susceptible to hot cracking than ingots. The possibility of segregation of boron and zirconium, causing liquation cracking, will be discussed.