Effect of TCP phases on tensile properties of engineering alloys: Insights gained from a CrMnFeCoNi high-entropy alloy

The precipitation of topologically close-packed (TCP) phases affects the mechanical behavior of engineering alloys, such as superalloys, steels, and high-entropy alloys (HEAs), in different ways. (1) The TCP-precipitates are much harder and more brittle than the matrix. They act as reinforcements and may serve as crack initiation sites, ultimately causing embrittlement. (2) Their precipitation depletes the matrix of certain solid-solution strengthening elements, thereby altering the matrix properties. (3) TCP-precipitation is usually accompanied by profound microstructural changes that affect alloy properties and are difficult to control. As often reviewed, there still is a lack of conclusive evidence regarding the magnitude of these three effects, due to the complexity of isolating them. To disentangle these confounding effects, we studied a model Cr 26 Mn 20 Fe 20 Co 20 Ni 14 HEA, for which the matrix properties (physical/mechanical) had previously been investigated over a broad compositional range. Additionally, we engineered substantial TCP (σ-phase) fractions while keeping all the other microstructural parameters constant. For the first time, this work provides a comprehensive and transferable quantification of the effect of σ-content on tensile behavior, deformation mechanisms, and embrittlement at room and cryogenic temperatures. Additional fractography investigations revealed that the σ-precipitates crack during deformation, while precipitate/matrix debonding is rarely observed. In-situ observations during tensile straining allowed to quantify how damage accumulation affects work hardening and ultimately leads to failure. Interestingly, non-percolating σ-precipitates do not cause failure prior to necking at RT. However, they do so at cryogenic temperatures, where solute depletion counteracts TCP-strengthening, alters stacking fault energy, and changes twinning propensity.