UC Berkeley

High-entropy alloys (HEAs) have spurred great interested in the in the materials community in recent years, primarily due to the intriguing properties certain members have been shown to display and due to the novel approach to alloy design they represent. Of this family of multi-component alloy systems, the near-equiatomic five component “Cantor” alloy CrCoMnFeNi is of particular note due to the remarkable mechanical properties this alloy has been found to display which are only enhanced as the temperature is lowered into the cryogenic regime. Despite the interest in this alloy system, little work has to date been conducted that characterize the cyclic fatigue loading behavior of this alloy or of its compositional variants. Here I examine the damage-tolerant fatigue behavior of the Cantor alloy as well as the effect temperature and load ratio have in changing this behavior and the underlying mechanisms that may be responsible for the observed shifts. These testing conditions encompassed three temperature regimes: 293 K, 198 K, and 77 K; additionally, the impact of increased load ratio R was surveyed within each temperature regime. Fatigue testing in the threshold and linear portion of the Paris regime revealed a temperature dependence of the fatigue behavior of the Cantor alloy; as temperature was decreased into the cryogenic regime, the fatigue curve was found to shift toward higher ΔK, indicating a higher resistance to fatigue-crack propagation at lower temperature. Additionally, it was observed that higher load ratios impacted this resistance negatively, causing a shift towards lower ΔK with increasing R-ratio. Post-testing, a series of mechanistic studies were conducted to investigate the underlying cause for this observed shift. Analyses of crack closure measurements, crack-path morphology, and fractography provided strong evidence for roughness-induced crack closure as the dominant mechanism at play.