Microstructure transformations and cracking in the matrix of γ–γ′ superalloy Inconel 713C melted with electron beam

The paper presents the results of metallographic examination of microstructural changes in the matrix of γ–γ′ superalloy Inconel 713C, subject to electron beam fusion. The obtained fusions simulate the phenomena that occur in the weld and the heat-affected zone during welding. Changes of γ′ phase morphology in the fusion zone, in HAZ and in the areas adjacent to grain boundaries that can affect crack initiation were determined. Changed morphology in the recrystallized γ′ zone can result from its reduced melting point due to impoverishment in niobium and titanium as well as to changed aluminium concentration or accelerated diffusion of alloying elements. The cracks found in HAZ propagate mainly through grain boundaries in the microporosity areas or, possibly, the areas left by molten carbide phases. However, no secondary phases with lower melting point or eutectics like (γ–γ′) (MxCy + Zr + Si) were found in the neighbourhood of cracks. The cracks present in the fusion zone were initiated at high temperature and propagate in the areas of interdendritic voids. The cracks were caused by the high solidification rate in the fusion zone and low thermal conductivity of the alloy.