The effect of Al/Ti ratio on the evolution of precipitates and their effects on mechanical properties for Ni35(CoCrFe)55AlxTi10-x high entropy alloys

L1(2)-structured Ni3(Al, Ti) precipitates are promising strengthening particles in face-centered-cubic high entropy alloys (HEAs). However, if the Al/Ti ratio (R) is not carefully controlled, brittle intermetallic compounds can form, especially at the grain boundaries (GBs). In this study, a series of Ni-35(CoCrFe)(55)AlxTi10-x HEAs with different R values, R = x/(10-x), were prepared, and the evolution of precipitates and their effects on the mechanical properties were investigated. The dispersed spherical precipitates within the grains act as an effective barrier to dislocation slip, producing high strength. However, increasing R (increasing Al) reduces the volume fraction of the precipitates within the grains, resulting in a decrease of yield strength. Both experimental measurements and first-principles calculations show that Al, Ti and Ni segregate to the GBs. This leads to the formation of L12 precipitates at the GB that, upon aging, coarsen to layer-like structures. When R >= 3 the local supersaturation of Al and Ti also leads to formation of a Heusler phase at the GBs, which reduces the ductility. However, when R > 3.5 the volume fraction of the Heusler phase decreases, and the plasticity increases again. These findings provide important guidance and inspire new strategies for the microstructural control in precipitate-strengthened HEAs. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the evolution of Ni3(Al, Ti) precipitates and their effects on the mechanical properties in high entropy alloys. It is found that spherical precipitates within the grains act as effective barriers to dislocation slip, resulting in high strength. However, increasing the Al content reduces the volume fraction of the precipitates, leading to a decrease in yield strength. Al, Ti, and Ni segregate to the grain boundaries, forming L12 precipitates and Heusler phases, which affect the ductility of the alloy.