The tensile behaviors of polycrystalline Ni-base superalloys were studied at 25 to 980C. An anomalous increase in yield strength was observed in precipitation-hardened superalloys at intermediate temperatures. An alloy with a high γ′ volume fraction exhibited a marked increase in yield strength at intermediate temperatures. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. An abrupt decrease in ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation was not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.

Anomalous Deformation Behavior and Twin Formation of Ni-Base Superalloys at the Intermediate Temperatures. I.S.Kim, B.G.Choi, H.U.Hong, Y.S.Yoo, C.Y.Jo: Materials Science and Engineering A, 2011, 528[24], 7149-55