Fracture behavior of superalloy IN738LC with various precipitate microstructures

Fracture studies on differently aged specimens of IN738LC, tensile tested t o failure at different temperatures with two different strain rates, were c arried out using a scanning electron microscope. Specimens with fine-size g amma' precipitates (F) and duplex-size gamma' precipitates (D) exhibited ge nerally more cleavage type fracture than those with medium (M) and coarse ( C) size gamma' precipitates. {100}-faceted cleavage-type fracture was domin ant at room temperature, while dimple fracture was dominant at 650 degrees C. A different type of cleavage fracture was observed in the specimens brok en at 750 degrees C, which is attributed to secondary strengthening and cha nge in flow mechanism at this temperature under slow strain rate conditions . At 850 degrees C, F and D showed wider faceted cleavage fracture, while h i and C showed dimple-ductile fracture. Low tensile fracture toughness corr elates with cleavage fracture, while specimens exhibiting dimple fracture h ave higher ductility and toughness. Presence of few (Ta, Ti)C-type carbide particles along the grain boundaries probably enables easy intergranular cr ack propagation. Dimples, however, are postulated to be initiated by decohe sion at the interface between the gamma' precipitate and the matrix. Notabl y such decohesion is initiated only on coarsened particles, not on the fine ones. (C) 1999 Published by Elsevier Science S.A. All rights reserved.