Yielding and deformation behavior of the single crystal superalloy PWA 1480

Abstract

Interrupted tensile tests were conducted to fixed plastic strain levels on (001) oriented single crystals of the nickel-base superalloy PWA 1480. Testing was done in the range from 20 to 1093 °C, at strain rates of 0.5 and 50 pct/min. The yield strength was constant from 20 to 760 °C, above which the strength dropped rapidly and became a strong function of strain rate. The data could be represented very well by an Arrhenius-type equation, which resulted in three distinct temperature regimes. The deformation substructures could also be grouped in the same three regimes, indicating that there was a fundamental relationship between the deformation mechanisms and the activation energies. At low temperatures, the activation energy for yielding was zero, and the deformation was dominated by γ′ shearing by pairs of 111a/2(110) dislocations. At high temperatures, the true activation energy for yielding was calculated to be 500 kJ/mol, which is indicative of a diffusion-controlled process, and deformation was dominated by γ′ by-pass. Intermediate temperatures exhibited transitional behavior. No currently available precipitation hardening model could adequately describe the behavior observed in the low temperature regime, due to the observation that penetration into the precipitate was not rate-limiting at all temperatures. In the high temperature regime, the functional form of the Brown-Ham by-pass model fit the data fairly well. The results of this study also demonstrated that the initial deformation mechanism was frequently different from that which would be inferred by examination of specimens which had been tested to failure.