Volume 23, Issue 2 (March 1995)
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 heat-treated conditions. Detailed fractographic studies were carried out to determine the micromechanism of crack growth in tensile loading. Round cylindrical tension specimens were prepared from the single crystal nickel-based superalloy CMSX-4 in  orientation. The test specimens had the  growth direction parallel to the loading axis in tension. The specimens were given two different heat treatments to produce two different γ′ precipitate sizes, i.e., 0.3 μm and 0.5 μm, respectively. The tensile properties of these materials were evaluated at room and elevated temperatures in ambient atmosphere as per ASTM E 21, Test Methods for Elevated Temperature Tension Tests of Metallic Materials.
The results of this investigation indicate that CMSX-4 undergoes dynamic strain aging in the temperature range of 260° to 875°C. This dynamic strain aging appears to take place by the trapping of moving dislocations by the substitutional solute atoms (like rhenium, tantalum, tungsten, cobalt, and molybdenum) during the high-temperature deformation process. Fractographs at lower temperatures (T ≤ 800°C) exhibit an unusual type of cleavage fracture with several operative slip systems, while at higher temperatures (T ≥ 1094°C) the crack growth process is characterized by a relatively smooth fracture surface, with the presence of dimples created by ductile failure (typical of many nickel-based superalloys).