Dynamic tensile mechanisms and constitutive relationship in CrFeNi medium entropy alloys at room and cryogenic temperatures

Harsh service conditions in aerospace, defense and military, and other fields are calling for materials with excellent mechanical properties to undergo extreme deformation (such as elevated and cryogenic temperatures and high strain rates) without sustaining damage while retaining high strength. Outstanding mechanical properties of high or medium entropy alloys (MEAs) render them potential candidates. In this paper, as the temperature decreases, a breakthrough of the strength-ductility tradeoff is achieved in a CrFeNi MEA with partially recrystallized face-centered cubic phases, together with body-centered cubic (BCC) precipitates. At a strain rate of $3000\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, the yield strength (YS) is increased from 920 MPa at 298 K to 1320 MPa at 77 K, while the uniform elongation (UE) increases by 28.5%. This phenomenon also occurs at quasistatic tension. Under low-temperature loading, nanotwins are popularly activated due to the decreased stacking fault energy with decreasing temperatures. A positive strain-rate dependent YS arises owing to the contributions from the short-range dislocation obstacles, BCC phases, and refined grains. High-density dislocations and BCC phases result in the degeneration of UE, especially with the increase of strain rates. Strain-rate- and temperature-dependent constitutive models were successfully established to predict the deformation behaviors of CrFeNi MEAs under such a wide range of strain rates at room and cryogenic temperatures.