DYNAMIC STRAIN-AGING IN A NEW SINGLE-CRYSTAL NICKEL-BASED SUPERALLOY (CMSX-4)

Tensile behavior of a new single crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures in two different hear-treated conditions. Detailed fractographic studies were carried out to determine the micromechanism of crack growth in t ensile loading. Round cylindrical tension specimens were prepared from the single crystal nickel-based superalloy CMSX-4 in [001] orientatio n. The test specimens had the [001] growth direction parallel to the l oading axis in tension. The specimens were given two different heat tr eatments to produce two different gamma' precipitate sizes, i.e., 0.3 mu m and 0.5 mu m, respectively. The tensile properties of these mater ials were evaluated at room and elevated temperatures in ambient atmos phere as per ASTM E 21, Test Methods for Elevated Temperature Tension Tests of Metallic Materials. The results of this investigation indicat e that CMSX-4 undergoes dynamic strain aging in the temperature range of 260 degrees to 875 degrees C. This dynamic strain aging appears to take place by the trapping of moving dislocations by the substitutiona l solute atoms (like rhenium, tantalum, tungsten, cobalt, and molybden um) during the high-temperature deformation process. Fractographs at l ower temperatures (T less than or equal to 800 degrees C) exhibit an u nusual type of cleavage fracture with several operative slip systems, while at higher temperatures (T greater than or equal to 1094 degrees C) the crack growth process is characterized by a relatively smooth fr acture surface, with the presence of dimples created by ductile failur e (typical of many nickel-based superalloys).