Characteristics of GTA fusion zones and heat affected zones in superalloy 713C

In this paper, metallographic examinations, characterising microstructural changes in the 713C superalloy subjected to remelting by GTA method, are presented. In the fusion zone, precipitation of M23C6 or M6C carbides based on chromium and molybdenum was observed. Eutectic mixtures of (γ-gg′)-MxCy type with highly developed morphology were also perceived. It was found that, in the matrix areas with non-homogeneous chemical composition, the eutectic reaction γ-γ′ can occur at the temperature close to that of the precipitation of the MxCy carbides. The presence of silicon in the carbide phases can be conducive to lowering their solidification point by creating low-melting compound NbSi. Both in the fusion zone (FZ) and in the heat-affected zone (HAZ), the secondary precipitates of the Ni3(AlTi)-γ′ phase, varying in size from 50 to 100 nm, were found. The lattice mismatch factor of the γ and γ′ particles was +0.48 % to +0.71 %, which is characteristic of the coherent precipitates of the Ni3Al phase enriched with titanium. No dislocations or stacking faults were observed in the microstructure of the FZ. In the HAZ, some primary undissolved γ′ precipitates, with a part of aluminium probably replaced with niobium were observed, which raised their melting point.