Loss of strength in Ni₃Al at elevated temperatures

Abstract

Stress decrease above the stress peak temperature (750 K) is studied in 〈123〉 single crystals of Ni₃(Al, 3 at.% Hf ). Two thermally activated deformation mechanisms are evidenced on the basis of stress relaxation and strain rate change experiments. From 500 to 1070 K, the continuity of the activation volume/temperature curves reveals a single mechanism of activation enthalpy 3.8 eV/atom and volume 90  b ³ at 810 K with an athermal stress of 330 MPa. Over the very same temperature interval, impurity or solute diffusion towards dislocation cores is evidenced through serrated yielding, peculiar shapes of stress–strain curves while changing the rate of straining and stress relaxation experiments. This complicates the identification of the deformation mechanism, which is likely connected with cube glide. From 1070 to 1270 K, the high-temperature mechanism has an activation enthalpy and volume of 4.8 eV/atom and 20  b ³, respectively, at 1250 K.

Acknowledgements

The authors would like to express their gratitude to Fonds National Suisse for supporting this research. BV thanks EPFL for leave-of-absence to perform this research in Lausanne.