Conventional Superalloys Research Articles

The effect of temperature cycling on creep-rupture of eutectic and conventional superalloys

Results are reported on the effect of temperature cycling on rupture strength of two representative fiber-strengthened eutectic composites and several non-composite superalloys. The tests involved both prior cycling and subsequent rupture and also temperature cycling under load. An initial increase and subsequent decrease in rupture life was observed with increasing number of cycles or decreasing cycle period with the former type of test. This was shown also to occur in the absence of fibers. A similar response could be induced by room-temperature prestrain. The results are therefore interpreted in terms of the effect on rupture life of cyclic straining, most likely resulting from thermal expansion mismatch. The temperature-cycling rupture tests showed a strong effect of cycle period on life, independent of the presence of fibers. A linear damage analysis gave a reasonable prediction of the average TCR rupture life although it could not account for the period dependence. A creep analysis predicted the temperature-cycling creep strain quite well from isothermal data. Using a new mean-strain transfer rule in the analysis, it also predicted a cycle period effect at long periods which was in agreement with experiment. There were no qualitative differences in the response to temperature cycling between the eutectic and conventional superalloys studied.

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides.

Stress Rupture Behavior of a Dispersion Strengthened Superalloy

A dispersion strengthened nickel-base superalloy, designated IN-853, has been made by a new process called “Mechanical Alloying.” This provides a long sought combination of properties typical of dispersion strengthened and precipitation hardened materials. The alloy has flat rupture curves over a wide temperature range. Rupture stress/temperature curves for the alloy show a transition separating the low temperature regime where precipitation hardening controls the strength, and the high temperature range where dispersion strengthening predominates. The slope of a Larson-Miller plot of stress rupture test data also decreases at high values of that parameter. At high temperatures rupture stress is less sensitive to temperature changes than is the case with conventional nickel-base superalloys. At a fixed stress level the rupture life of the dispersion strengthened superalloy is more sensitive to temperature changes.