Deformation Mechanisms and Processing Maps for High Entropy Alloys (Presentation of Processing Maps in Terms of...

In this review paper, the hot compressive deformation mechanisms and processing maps of high-entropy alloys (HEAs) with different chemical compositions and crystal structures are analyzed. The stress exponent (n₁) values measured from the series of compression tests for the HEAs performed at different temperatures and strain rates are distributed between 3 and 35, and they are most populated between 3 and 7. Power law breakdown (PLB) is found to typically occur at T/T ≤ 0.6 (where T is the testing temperature and T is the melting temperature). In AlCrMnFeCoNi (x = 0-1) and AlCrFeCoNi (x = 0-1) HEAs, n₁ tends to decrease as the concentration of Al increases, suggesting that Al acts as a solute atom that exerts a drag force on dislocation slip motion at high temperatures. The values of activation energy for plastic flow (Q_c) for the HEAs are most populated in the range between 300 and 400 kJ/mol. These values are close to the activation energy of the tracer diffusivity of elements in the HEAs ranging between 240 and 408 kJ/mol. The power dissipation efficiency η of the HEAs is shown to follow a single equation, which is uniquely related to n₁. Flow instability for the HEAs is shown to occur near n₁ = 7, implying that the onset of flow instability occurs at the transition from power law creep to PLB. Processing maps for the HEAs are demonstrated to be represented by plotting η as a function of the Zener-Hollomon parameter (Z = expQcRT, where R is the gas constant). Flow stability prevails at Z ≤ 10¹² s^-1, while flow instability does at Z ≥ 3 × 10¹⁴ s^-1.