Fracture behavior of superalloy IN738LC with various precipitate microstructures

Fracture studies on differently aged specimens of IN738LC, tensile tested to failure at different temperatures with two different strain rates, were carried out using a scanning electron microscope. Specimens with fine-size γ′ precipitates (F) and duplex-size γ′ precipitates (D) exhibited generally more cleavage type fracture than those with medium (M) and coarse (C) size γ′ precipitates. {100}-faceted cleavage-type fracture was dominant at room temperature, while dimple fracture was dominant at 650°C. A different type of cleavage fracture was observed in the specimens broken at 750°C, which is attributed to secondary strengthening and change in flow mechanism at this temperature under slow strain rate conditions. At 850°C, F and D showed wider faceted cleavage fracture, while M and C showed dimple-ductile fracture. Low tensile fracture toughness correlates with cleavage fracture, while specimens exhibiting dimple fracture have higher ductility and toughness. Presence of few (Ta, Ti)C-type carbide particles along the grain boundaries probably enables easy intergranular crack propagation. Dimples, however, are postulated to be initiated by decohesion at the interface between the γ′ precipitate and the matrix. Notably such decohesion is initiated only on coarsened particles, not on the fine ones.