Unique transitions in uniform elongation and deformation mechanisms of a refractory medium-entropy alloy at cryogenic temperatures

Refractory high-entropy alloys (RHEAs) and medium-entropy alloys (RMEAs) are potential candidates for high-temperature applications; dislocations play crucial roles in the plastic deformation of these alloys at both room and elevated temperatures. However, there is a significant deficiency in the understanding of their temperature-dependent microstructure-mechanical property correlations at low temperatures, which is crucial for evaluating their performance and ensuring service safety under variable-temperature extreme conditions. This study investigated the mechanical properties and deformation mechanisms of a non-equiatomic Ti_48.9Zr_32.0Nb_12.6Ta_6.5 RMEA at ambient and cryogenic temperatures. Tensile testing revealed intriguing temperature-dependent behaviors: as the temperature decreased, yield strength increased, while uniform elongation (UE) followed an abnormal U-shaped trend. The RMEA exhibited good UE at 293 K (10.9 %), but UE dropped sharply to 185 K (2.2 %). However, UE peaked at 77 K (17.2 %) along with the highest ultimate tensile strength. These indicated a transition in the deformation mechanisms. Microstructural analysis showed that considerable strain hardening at 293 K was owing to abundant dislocation interactions as well as {112}<111> twins. At 185 K, strain hardening weakened due to suppressed dislocation activity, whereas kinking prevented the ductile-to-brittle transition despite limited UE. The strong strain hardening and enhanced UE at 77 K were attributed to the twinning-induced plasticity effect from {332}<113> deformation twins. In conclusion, this study highlights the anomalous temperature-dependent mechanical behavior of this RMEA and the corresponding evolution of deformation mechanisms. The findings provide key insights into the alloy design and optimizing the performance of RHEAs/RMEAs for applications in cryogenic and variable-temperature environments.