UC Davis

Constant stress and stress reduction creep experiments were conducted on several dispersion-strengthened (DS) alloys - GlidCop Al-15 and Al-60 (Cu-Al2O3), GRCop-84 (Cu-Cr2Nb), and FVS0812 (Al-Al13(Fe,V)3Si) - and a single phase FCC multi-principal element alloy (MPEA), CrMnFeCoNi, to characterize their creep properties and determine their rate-controlling deformation mechanisms at elevated temperatures. The analysis involves the measurement of the stress exponents and apparent activation energies of creep by constant stress creep experiments, and the determination of the operational activation area, constant structure activation energies, and microstructural strengths by stress reduction experiments. Thereafter, extensive scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization of creep microstructure was performed for proper interpretation of the mechanical insights. Finally, physics-based constitutive equations of creep were established based on the output from the creep experiments to describe and predict the creep behavior of these alloys.

The stress exponents and apparent activation energies have previously been measured for DS alloys, and their abnormally high values cannot be rationalized by mechanisms that operate in metals and alloys without dispersoids, thus indicating the importance of dislocation-particle interactions. In the current study, the operational activation areas determined for GlidCop Al-15 and Al-60, GRCop-84, and FVS0812 revealed their rate-controlling deformation mechanisms to be thermally active dislocation detachment from particles, local climb over particles, and interdislocation interactions, respectively, which were also supported by corresponding TEM characterization. These results indicate that a unified model that describes creep in all DS alloys does not exist, and the respective deformation mechanisms must be well understood in different DS alloys and modeled accordingly.

A uniform stress exponent of 3.7±0.1 was determined for CrMnFeCoNi tested at constant stresses from 1023 to 1173 K, demonstrating no change of dominating deformation mechanisms across all tested stresses. The apparent activation energy turned out to be lower than that of self-diffusion for all five elements in the high entropy matrix and decrease with increasing stress, revealing a stress-assisted, thermally activated behavior. Electron backscatter diffraction (EBSD) and TEM characterization showed no subgrain boundary formation during steady-state creep deformation in CrMnFeCoNi. Instead, the dislocation substructure features high-density arrays of curved and entangled dislocations, illustrating combined control of forest dislocations and concentrated solid solution. These mechanisms were quantitatively verified by the stress reduction creep experiments conducted at 1073 K. The measured activation areas were quantitatively separated by a Haasen plot, and their respective values are appropriate for the two determined mechanisms. The elevated-temperature deformation mode in CrMnFeCoNi appears to be the same as that at room and cryogenic temperatures, suggesting the possibility of a unified framework to describe plastic deformation in this MPEA from cryogenic to elevated temperatures.