HAYNES® 244® alloy is a high-strength, low coefficient of thermal expansion (CTE) Ni-based superalloy designed for use up to 760 \(^{\circ }\) C. This alloy offers an improvement in maximum temperature operation and mechanical properties such as tensile strength and creep life over the previous generation of low CTE alloy, HAYNES® 242®. Uniquely, it is strengthened through a body-centered orthorhombic (BCO) intermetallic phase, \(\gamma ^{\prime \prime \prime }\) , a Ni \(_{2}\) (Cr,Mo,W) precipitate. The additions of tungsten in the 244 alloy improve the thermal stability of the strengthening domains, compared to the 242 alloy, impeding diffusional effects of dislocation motion at elevated temperatures. To probe the possible deformation mechanisms that occur during high-temperature creep, creep testing in the vicinity of the \(\gamma ^{\prime \prime \prime }\) solvus was conducted to understand the interplay of \(\gamma ^{\prime \prime \prime }\) phase stability, precipitation of the \(\mu \) phase above 760 \(^{\circ }\) C, and the active creep deformation mechanisms. These mechanisms change from deformation twinning at the lower tested temperatures, to perfect dislocation shearing and noticeable grain boundary dislocation pile-up after the \(\gamma ^{\prime \prime \prime }\) has completely dissolved. During the intermediate temperature testing near the solvus, but where the \(\gamma ^{\prime \prime \prime }\) phase still remains in low volume fractions, a mixed twinning and dislocation shearing mechanism was observed. This manuscript elucidates the implications of mixed character creep deformation during high-temperature testing.
