Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25–980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. 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. Abrupt decrease of 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 is 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.